Mass spectrometric and charge density studies of organometallic clusters photoionized by gigawatt laser pulses.

MASS SPECTROMETRIC AND CHARGE DENSITY STUDIES OF ORGANOMETALLIC CLUSTERS PHOTOIONIZED BY GIGAWATT LASER PULSES Purav Badani,1 Soumitra Das,1 Pramod Sharma,1 and Rajesh Kumar Vatsa1* 1

Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India 2 Department of Chemistry, University of Mumbai, Mumbai 400098, India Received 9 September 2014; revised 29 January 2015; accepted 17 February 2015 Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/mas.21469

Clusters on exposure to nanosecond laser pulses of gigawatt intensity exhibit a variety of photo-chemical processes such as fragmentation, intracluster reaction, ionization, Coulomb explosion, etc. Present article summarizes the experimental results obtained in our laboratory utilizing time-of-flight mass spectrometer which deal with one such aspect of cluster photochemistry related to generation of multiply charged atomic ions upon excessive ionization of cluster constituents (Coulomb explosion) at low intensity laser field (109 W/cm2). To understand the mechanism of laser–cluster interaction, laser as well as cluster parameters were varied. Mass spectrometric studies were carried out at different laser wavelength as well as varying the nature of cluster constituents, backup pressure, nozzle diameter, etc. In addition, charge density measurements were also preformed to get information about the total number of ions generated upon laser–cluster interaction as a function of laser wavelength. In case of pure molecular clusters, the charge state of atomic ions as well as charge density was observed to enhance with increasing laser wavelength, signifying efficient coupling of the cluster medium with nanosecond laser pulse at longer wavelength. While in case of clusters doped with species having comparatively lower ionization energy, the efficiency of laser–cluster interaction was less, in contrast to studies carried out using femtosecond lasers. Results obtained in the present work have been rationalized on the basis of proposed threestage cluster ionization mechanism, that is, multiphoton ionization ignited-inverse Bremsstrahlung heating and electron ionization. # 2015 Wiley Periodicals, Inc. Rapid Commun. Mass Spectrom. 9999: XX–XX, 2015 Keywords: Mass Spectrometry; Charge Density; Organometallic Clusters; Laser-Cluster interaction; Coulomb explosion

I. INTRODUCTION Laser–matter interaction has been a topic of vide interest for last few decades (Leith & Upatnieks, 1962; Letokhov, 1977; Christensen, 1982; Britnell et al., 2013). When low intensity light interacts with matter, it induces a linear optical response in terms of vibration, rotation and/or electronic excitation. Induc-

Present address of Purav Badani is Department of Chemistry, University of Mumbai, Mumbai 400098, India Correspondence to: R.K. Vatsa, Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India. E-mail: [email protected]

Mass Spectrometry Reviews # 2015 Wiley Periodicals, Inc.

ing non-linear responses such as multi photon absorption/ ionization and optical field ionization (tunneling ionization and barrier suppression ionization) have become possible due to advent of high intensity laser light (Peticolas, 1967; DiMauro, Freeman, & Kulander, 2000). Mode of ionization during such interaction processes essentially depends on the intensity of laser pulses and the electric field associated with it. Multiphoton ionization (MPI) dominates at laser intensity 1013 W/cm2, optical field ionization (OFI) begins to play important role (Nakashima et al., 2000). The boundary between the tunneling and multiphoton ionization regimes is parameterized by the dimensionless Keldysh nonadiabaticity parameter (g) (Keldysh, 1965) which is given by rffiffiffiffiffiffiffiffiffi vL E0 g¼ ¼ ð1Þ vtun 2U P where vL is the angular frequency of the optical field, vtun the tunneling rate at the peak of the optical field, E0 the field-free binding energy of the electron, and Up is the ponderomotive potential. The boundary between the tunneling and multiphoton regimes is generally considered to be g 1. For, g > 1, multiphoton ionization dominates while for g < 1, tunneling ionization is predominant (Sheehy, 2001; Krishnan et al., 2014). In addition to the laser intensity, temporal profile of laser pulse and wavelength also play a crucial role in determining fragments ions formed during such interaction processes (Rajeev et al., 2013). Apart from the laser conditions, the form of matter being irradiated also plays a crucial role in determining the extent of ionization during such interaction process. Ideally, solids with high local electron density provide an efficient medium for coupling of optical energy with the matter (Mathur & Rajgara, 2010; Mathur et al., 2010). Alternatively, gas phase clusters which are aggregates of atoms/molecules are used as medium for laser–matter interaction studies. These clusters possess solid/ liquid like density and allow the experimentalist to simulate the isolated nanoscale condensed phase conditions using sophisticated experimental tools that are accessible for gas phase experiments (McCarter et al., 1999; Feigerle, Bililign, & Miller, 2000). In this regards, time-of-flight mass spectrometer has been widely used to probe different aspects of laser–cluster interaction. Gas phase clusters upon interacting with laser pulses exhibit several processes such as fragmentation, intra-cluster reaction, valence and/or core shell ionization etc. Coulomb explosion of molecules/clusters is one of such phenomena which

&

BADANI ET AL.

have been investigated extensively under femtosecond and picosecond laser pulses (Ditmire et al., 1999; Ford et al., 1999; Jha, Mathur, & Krishnamurthy, 2005; Islam et al., 2006; Card et al., 2002). Also a large numbers of reviews have appeared in literature, which are specifically dedicated to cluster dynamics under intense laser field in the range of 1014–1018 W/cm2 (Nakashima et al., 2000; Mathur, 2004; Fennel et al., 2010; Saalmann & Rost, 2011). Section 2 provides a brief summary of such experimental and theoretical work carried out by various groups to investigate interaction of clusters with ultrafast laser pulses. On the contrary, nanosecond laser pulse induced multiple ionization leading to Coulomb explosion in case of gas phase clusters using laser pulses of intensity 109–1011 W/cm2, which is the topic of the present review, has not been investigated widely. Systematic studies in this field gained momentum over the last decade, following unusual experimental findings reported by Luo et al. (2005) and Sharma et al. (2006). Section 3 describes experimental details pertinent to nanosecond laser– cluster interaction studies, while the experimental findings on nanosecond laser induced Coulomb explosion of pure and doped cluster system are described in Sections 4 and 5 respectively. A probable primitive mechanism by Li and coworkers (Wang et al., 2008) has been proposed to explain different facets of nanosecond induced Coulomb explosion of clusters. The article concludes with summary of the work portrayed in section 6.

II. COULOMB EXPLOSION OF CLUSTERS Clusters efficiently couple with the laser radiation thereby extracting significant amount of energy from the optical field. This results in extensive stripping of electrons from the cluster constituents which leads to buildup of excessive positive charge on cluster. The cohesive energy within the cluster tries to keep the constituents together, while the electrostatic repulsion, arising from positive charges, forces the cluster to expand. At a stage when the repulsive Coulombic energy of cluster overcomes the total cohesive energy, the cluster disintegrates violently. This is manifested in terms of generation of multiply charged atomic ions with large kinetic energies. This phenomenon is referred as Coulomb explosion (Islam, Saalmann, & Rost, 2006). Coulomb explosion of gas phase clusters has been widely investigated over past several decades. For instance, Sattler et al. observed multiple ionization and subsequent Coulomb explosion in (Pb)n, (Xe)n, and (NaI)n clusters upon ionization using an electron impact ion source. This group suggested that the critical cluster size is required for clusters to exhibit multiple ionization (Sattler et al., 1981). Saunders et al. suggested the applicability of liquid-drop model in explaining the phenomena of Coulomb explosion in metal clusters (Saunders & Dam, 1991; Saunders, 1992). Castleman and co-workers (Purnell et al., 1994) reported extensive ionization and generation of multiply charged atomic species (Iþ17 and Arþ8), upon interaction of (HI)n(Ar)m clusters with intense femtosecond laser pulses. Energy analysis revealed that these multiply charged atomic ions fly with large kinetic energies. In another set of experiments, Castleman and coworkers (Blumling, Sayres, & Castleman, 2011; Sayres, Ross, & Castleman, 2011) ablated the metal rod using laser. The resultant plume along with carrier gas (O2 or CH4) was concomitantly passed through a pulsed valve to generate metal oxide/carbide clusters. The clusters so formed were irradiated with femto2

second laser pulses (pulse width 100 fsec) resulting in generation of highly charged atomic ions of metal (such as Taþ11, Nbþ11, Vþ9, etc.) upon Coulomb explosion of clusters. Ditmire et al. (1999) observed generation of energetic ions (upto 2 MeV) as well as neutrons upon Coulomb explosion of deuterium clusters using intense (1015 W/cm2) femtosecond laser pulses. Due to ultrafast temporal profile of femtosecond laser pulses, ions generated after initial ionization practically remain immovable thereby holding the initial geometry of cluster unaltered. This facilitates in maintaining the higher density of clusters during the entire ionization event which is necessary for efficient transfer of optical energy into the cluster medium. Interaction of cluster with intense laser pulses also leads to production of X-ray and high harmonic generation. In order to further explore different facets of laser induced Coulomb explosion phenomena in clusters, Mathur and coworkers (Kumarappan, Krishnamurty, & Mathur, 2001, 2002) ionized inert gas clusters (Ar40000) with femtosecond laser pulses. Their group observed significant variation in cluster expansion dynamics and ion energization process upon interacting inert gas (Ar) clusters with few-cycle (12 fsec) pulses as compared to many-cycle (100 fsec) laser pulses. Under identical laser intensities, ion energization was observed to be higher using many-cycle pulses as compared to few-cycles pulses (Mathur & Rajgara, 2010; Mathur et al., 2010). Additionally, significant enhancement in charge state of atomic ions was observed when Ar clusters were doped with low ionization energy molecule (H2O) (Jha, Mathur, & Krishnamurthy, 2006a). These authors further noted that efficient interaction of doped clusters as compared to pure clusters was manifested in terms of an order of magnitude enhancement in X-ray yields (Jha, Mathur, & Krishnamurthy, 2005) and generation of hotter electrons (Jha & Krishnamurthy, 2008). The mechanism by which clusters exhibit multiple ionization and subsequent Coulomb explosion, under the influence of laser field, is quite complex. However, some of the proposed ionization models which independently explain certain facets of intense femtosecond laser pulses induced Coulomb explosion of clusters are described below. Rose-Petruck, Schafer, and Barty (1997) proposed an Ionization Ignition Model (IIM) according to which during initial ionization event, several ion cores are created within the clusters. These ion cores interact with the electric field of ultrafast laser pulse and generate inhomogeneous electric field within the cluster. This field suppresses the ionization barrier allowing additional electrons to be ejected thereby leading to an increase in charge state or generation of new charge centers. The new ion cores created in this fashion interact with laser beam, thereby increasing the effective field and lowering the ionization barrier further. This process continues until repulsive Coulomb energy from charged ions exceeds the total cohesive energy of cluster and Coulomb explosion occurs. IIM has been demonstrated to play a significant role in smaller noble gas, van der Waals and hydrogen bonded clusters. Coherent Electron Motion Model (CEMM) was introduced by Rhodes and co-workers (McPherson et al., 1993, 1994a; McPherson, Boyer, & Rhodes, 1994b; McPherson et al., 1996). This model considers that the intense field of an ultrafast laser pulse initially ionizes cluster which results in loss of several electrons. The freed electrons are treated as quasi particles, which under the influence of electric field; oscillate coherently Mass Spectrometry Reviews DOI 10.1002/mas

PHOTOIONISATION OF ORGANOMETALLIC CLUSTERS

with electric field of laser. This leads to heating of cluster and further ionization occurs due to inelastic electron scattering. This model was originally developed to explain the production of X-rays observed in large clusters. Charge Resonance Enhanced Ionization (CREI) model (Zuo, Chelkowski, & Bandrauk, 1993) emphasizes on the role of electron motion within the ionized cluster which resonates with laser frequency eventually leading to large absorption of energy during such laser–cluster interaction. This mechanism has been used to explain results of heterocyclic molecules (Card et al., 2002) and HI-Ar van der Waals clusters (Purnell et al., 1994). According to this model, resonance occurs if the laser pulse is long enough, so that the cluster expands significantly during the pulse. Initially the eigen frequency of the ion background is much larger than the laser frequency due to the high charge density of ions. The cluster expansion leads to the decrease in the charge density of ions, hence the eigen frequency also goes down. Therefore, at some point the eigen frequency matches with the laser frequency. Then, the ion plasma resonance occur which results in efficient absorption of energy. Till 2004, Coulomb explosion studies on clusters had been reported using high intensity (I 1015 W/cm2) femtosecond laser pulses (Benedek, Martin, & Pacchioni, 1987; Kreibig & Vollmer, 1995; Dunning & Hulet, 1996). However, in 2005, researchers from China (Luo et al., 2005) observed multiple ionization and subsequent Coulomb explosion in methyl iodide clusters using surprisingly low intensity (I 1010 W/cm2, l ¼ 532 nm) nanosecond laser pulses. In an independent study, similar experimental observations were also reported by our research group (Sharma et al., 2006). An “energy pooling” mechanism was proposed to account for generation of multiply charged atomic ions at such low intensity laser field. This mechanism emphasized that resonantly excited intermediate vibronic energy levels play a dominant role in efficient coupling of laser energy with cluster medium. However, subsequent studies on low intensity Coulomb explosion of clusters revealed the presence of energetic electrons (Zhao et al., 2012) and an increase in charge state of atomic ions with increasing laser wavelength over a wide range spanning from visible to I.R. region of electromagnetic spectrum (Badani et al., 2012). These experimental observations could not be rationalized on the basis of earlier proposed “energy pooling” mechanism (Sharma et al., 2006). Hence, further studies were directed to explore the ionization processes and establish the mechanism by which clusters exhibits multiple ionization and subsequent Coulomb explosion under such low intensity laser fields. For this purpose, experiments were performed on pure (Badani et al., 2011, 2014a; Das et al., 2011a,b, 2012) as well as doped cluster systems (Badani et al., 2012, 2013, 2014a). Here in, homogenous clusters comprising of single component are referred as pure clusters. While, heterogeneous clusters in which one of the component is in large excess as compared to the other are referred as doped clusters (Haberland, 1994). In this review, the results on organometallic clusters such as tetramethyl silane [Si(CH3)4]n, tetramethyl tin [Sn(CH3)4]n and iron pentacarbonyl [Fe(CO)5]n have been used which have immense technological applications. These organometallic clusters were generated by supersonic expansion technique, at room-temperature. Resultant clusters were irradiated with nanosecond laser pulses at selected laser wavelengths ranging from UV to IR region. In these studies, the extent of ionization was measured using two Mass Spectrometry Reviews DOI 10.1002/mas

&

different methods, namely time-of-flight mass spectrometry and total charge density measurements using parallel plate assembly. The former gives qualitative picture about different charge states of atomic ion that are produced during laser–cluster interaction whereas the latter gives quantitative idea about the total number of ions generated as a result of laser–cluster interaction under given experimental conditions.

III. EXPERIMENTAL DETAILS Clusters were generated via supersonic expansion of sample vapors seeded in room temperature Ar, at a backup pressure of 1–6 bar. Schematic diagram of the instrumental setup used for laser–cluster interaction studies is shown below (Fig. 1). A pulse valve with 800 mm nozzle diameter and 350 msec pulse duration was used for generation of cluster. The supersonic jet produced in this way was skimmed at a distance of 5 cm from the nozzle. Clusters were ionized 17 cm downstream from the skimmer by 266, 355, or 532 nm output of a pulsed Nd:YAG nanosecond laser (Quanta System, Italy; GIANT G790-10; FWHM ¼ 10 nsec). The ions generated as a result of laser–cluster interactions were accelerated and guided into a 100 cm field-free region of home built time-of-flight mass spectrometer based on Wiley– McLaren ion optics and detected using a Channel Electron Multiplier (CEM). The mass resolution of the instrument is 300. Signal from the CEM was recorded on a digital storage oscilloscope. Typically 500 laser shots were averaged for each time-of-flight mass spectrum. For weak signals, 1,000 shots were averaged. The averaged signal was finally transferred to a computer for further processing. Comprehensive details of the experimental setup can be found in our earlier publications (Majumder et al., 1999; Das et al., 2011a; Badani et al., 2012). In order to quantify total number of ions generated within the ionization volume upon laser–cluster interaction, experiments were carried out in which the charge density was measured using parallel plate method (Das et al., 2010, 2011b). In this setup, a sufficiently high positive voltage (3,000 V) was applied to one of the plates (anode) in order to repel the ions towards the collector plate (cathode) and the ion current was measured across a suitable resistor (10 kV). It was ensured that all the experimental parameters were kept identical in the timeof-flight and charge density studies, so that the results could be correlated.

IV. PURE CLUSTER SYSTEM A. Photo-Ionization of Tetramethyl Silane Molecules and Clusters In general, aggregates of atoms/molecules (clusters) exhibit altogether different photo-ionization behavior, compared to its individual constituent (monomer). Thus, photoionization studies were carried out in tetramethyl silane (TMS) molecule and its clusters using nanosecond laser pulses at different laser wavelengths. Time-of-flight mass spectra obtained on interaction of TMS monomers with laser pulses of different wavelength (266, 355, and 532 nm) at an intensity of 5 109 W/cm2 are shown in Figure 2. Following 266 nm photo-ionization (Fig. 2a), the ion signals observed in the mass spectrum are Hþ, Cþ, CHþ, Siþ, SiHþ, and SiH2þ. The mass spectrum at 355 nm (Fig. 2b) consists of Hþ, Cþ, CHþ, Siþ. The mass spectrum recorded 3

&

BADANI ET AL.

FIGURE 1. Schematic diagram of instrumental setup used for laser-cluster interaction studies.

FIGURE 2. Mass spectra of TMS monomers at different laser wavelengths. a: 266 nm, (b) 355 nm, and (c) 532 nm.

4

Mass Spectrometry Reviews DOI 10.1002/mas


&

FIGURE 3. Mass spectra of TMS clusters at different laser wavelengths (a) 266 nm, (b) 355 nm, and (c) 532 nm. Inset shows split peaks of energetic multiply charged ions of carbon at 532 nm.

upon 532 nm photo-ionization (Fig. 2c) showed ion signals corresponding to Hþ, Cþ, CHþ, CH2þ, CH3þ, Siþ, SiHþ, SiH2þ, SiH3þ, SiCþ, SiCH3þ, H2SiCH3þ, and Si(CH3)3þ. All the ion signals observed in case of photo-ionization of Si(CH3)4 monomer could be assigned to singly charged species. In case of TMS clusters, time-of-flight mass spectra obtained on interaction with laser pulses of different wavelength (266, 355, and 532 nm) at an intensity of 5 109 W/cm2 are shown in Figure 3. Following 266 nm photo-ionization (Fig. 3a), ion signals corresponding to CHnþ (n ¼ 0–2), SiHnþ (n ¼ 0–3), and Si(CH3)3þ could be seen in the mass spectrum. At 355 nm, in addition to singly charged atomic and fragment ions, ion signal corresponding to C2þ and Si2þ could also be observed in the mass spectrum (Fig. 3b). One must note that, under these experimental conditions, due to inadequate resolution of the time-of-flight mass spectrometer it was not possible to resolve Si2þ and CH2þ ion signal. Since 2nd ionization energy of carbon (IE of Cþ ! C2þ ¼ 24.4 eV) is comparatively much higher than that of silicon (IE of Siþ ! Si2þ ¼ 16.3 eV), observation of C2þ in the mass spectra, indirectly confirms generation of Si2þ under these experimental conditions. On further increasing the laser wavelength from 355 to 532 nm (Fig. 3c), in addition to above singly charged species, multiply charged atomic ions of Sinþ (n ¼ 1–4) and Cnþ (n ¼ 1–4) were observed. Thus the charge state of atomic ions, generated upon laser–cluster interaction, was observed to increase with laser wavelength for a fixed laser intensity of 5 109 W/cm2. Generation of highly charged ions at 532 nm suggests that the total energy absorbed by the cluster from the laser pulse is higher at longer laser wavelength. On comparing the mass spectra of TMS monomers and clusters photoionized by 532 nm laser pulses, it can be seen that TMS monomers resulted in generation of only singly charged Mass Spectrometry Reviews DOI 10.1002/mas

silicon and carbon ions (Fig. 2c). While, clusters of TMS exhibit multiple ionization (Fig. 3c) thereby leading to generation of multiply charged atomic ions of Sinþ (n ¼ 1–4) and Cnþ (n ¼ 1– 4). The shape of ion peaks corresponding to these multiply charged atomic ions was found to be broad, split and asymmetric (Fig. 3c, inset). Observation of such signals in the time-of-flight mass spectrum indicates that the multiply charged ions are associated with large kinetic energies. This large kinetic energy arises due to strong Coulombic repulsive force experienced by ions within the charged cluster and hence the phenomenon is called Coulomb explosion of clusters. The kinetic energy acquired by these multiply charged atomic ions in the center-ofmass frame can be calculated based on the time separation between the backward and the forward component of the split ion peak, using time-of-flight equation. The kinetic energy of multiply charged ions is expressed as (Guilhaus, 1995; Snyder et al., 1996). Ekin ðeVÞ ¼

9:65 107 Dt2 Z 2 F 2 8m

ð2Þ

Here Dt is the arrival time difference (in nanosecond) between the forward and backward components of the ion signal for a given mass, F the static electric field (V cm1) applied for ion extraction, Z the charge of the ion, and m is the mass of the ions in m/z units. This method was used to calculate kinetic energy of the multiply charged ions of carbon and silicon which are listed in Table 1, from a set of several measurements. Typical error bars in the values are 20%, which arises due to fluctuations in laser energy and also due to uncertainty in assignment of peak value for forward and backward component. From the above table it can be seen that the multiply charged 5

&

BADANI ET AL.

TABLE 1. Ekin of multiply charged atomic ions at 532 nm using peak split method

Charge state of ion Kinetic energy (eV)

C+

C2+

C3+

C4+

Si+

Si2+

Si3+

Si4+

23

122

191

487

21

121

149

*

Could not be measured due to low ion yield.

atomic ions acquire kinetic energy as high as 500 eV. Further, it can be seen that the kinetic energy of multiply charged atomic ion obeyed following order: C4þ > C3þ > C2þ > Cþ and Si3þ > Si2þ > Siþ.

B. Role of Clusters in Multiple Ionization and Coulomb Explosion Phenomenon Results obtained in above study suggest that during the interaction of a low intensity laser pulse, clusters undergo multiple ionization which eventually leads to Coulomb explosion of the cluster. Further, it is well known that clusters exhibit size-dependent photochemistry (Sharma and Vatsa, 2008). Hence, photochemical studies have been carried out on TMS clusters as a function of cluster size. Qualitatively formation of clusters occurs due to collision of atoms/molecules. So it is obvious that clustering is enhanced by increasing back up pressure (P0), decreasing nozzle temperature (T0) and using nozzles of larger diameter (d). Due to lack of rigorous condensation theory, semi empirical scaling laws for cluster formation have been derived by Hagena (1987). In general, under a given set of expansion conditions, average cluster size is related to P0, T0, and d via Hagena’s parameter ðG Þ as

G ¼k

ðd=tan aÞ0:85 P0 T 0 2:29

ð3Þ

where k is the constant related to the bond formation and a is the half expansion angle of the nozzle. Thus by varying the experimental parameters like P0, d, and T0, it is possible to carry out ionization studies on clusters of different average size.

1. Sampling Different Cluster Size Using Delay Time Between Pulsed Valve and Laser Pulse To verify the presence of clusters in molecular beam for observation of multiply charged atomic ions in time-of-flight mass spectrum, experiments were performed to sample both clustered and unclustered portions of the gas pulse that interact with the focused laser beam. Since clusters are generated in the expansion chamber and photoionized in the ionization chamber at a distance of 22 cm from the nozzle, clusters of different size arrive in the ionization region at different time. Thus, by varying the delay time between the opening of pulsed valve and the laser pulse, time-of-flight mass spectra of TMS clusters were recorded at 532 nm and illustrated in Figure 4. The figure depicts variation in multiply charged atomic ion signals, as the laser pulse probes selectively different regions of the pulsed molecular beam. 6

When the laser beam interacts with the gas pulse on the leading or trailing edge, where monomers are predominant, only singly charged atomic and fragment ions like Cþ, Siþ, Si(CH3)þ, and H2Si(CH3)þ were observed. In contrast to this, when the predominantly clustered portion of beam was sampled, multiply charged atomic ions [Sinþ (n ¼ 1–4) and Cnþ (n ¼ 1–4)] were observed.

2. Sampling Different Cluster Size Using Variable Backup Pressure of Carrier Gas Equation (3) shows that rate of three body collisions can be increased by increasing the back up pressure. Thus, studies have been carried out by varying the backup pressure of the carrier gas and keeping all other parameters constant. Figure 5 represents the time-of-flight mass spectra obtained at backup pressure of 1, 1.5, 2, and 3 atm. As can be seen from figure recorded at 1 atm back-up pressure, the mass spectrum obtained resembles that of TMS monomer obtained at 532 nm. In addition, signal corresponding to cluster fragment ions such as [Si(CH3)4]nSi(CH3)3þ (n ¼ 0, 1, 2) were also observed (inset of Fig. 5). Observation of cluster fragment ions in the mass spectra, along with other singly charged fragment ions suggests that at backup pressure of 1 atm, smaller TMS clusters largely exhibit multiphoton dissociation/ionization, similar to TMS monomer. Upon marginally increasing the backup pressure to 1.5 atm, the mass spectrum exhibits additional ion signals arising from multiply charged atomic ions of C2þ, Si2þ, and Si3þ. Observation of these multiply charged ions in the mass spectrum suggest that additional ionization processes have been initiated in the larger cluster. On further increasing the backup pressure to 2 atm, the state of multiply charged ions observed in the mass spectra increases up to Si4þ and C4þ. Further increase of the backup pressure above 2 atm did not result in generation of higher charged states, though there was a significant enhancement in the overall yield of ions (particularly þ4 state) as depicted by the mass spectrum recorded at 3 atm backup pressure. Thus, this study demonstrates the role of threshold cluster size for observation of multiply charged atomic ions at gigawatt intensity laser field.

3. Sampling Different Cluster Size Using Variable Nozzles Diameters Subsequent experiments were performed by using nozzles of different diameter [(a) 500 mm and (b) 800 mm], at fixed back up pressure (3 atm). The resultant clusters were irradiated with laser wavelength of 532 nm. The time-of-flight mass spectra (Fig. 6) revealed generation of singly charged atomic ions and fragment ions at smaller nozzle diameter (500 mm). While use of the Mass Spectrometry Reviews DOI 10.1002/mas


&

FIGURE 4. Mass spectra of TMS clusters at varying delay time between opening of pulsed valve and the laser pulse (Wavelength ¼ 532 nm).

FIGURE 5. Mass spectra of TMS cluster at varying backup pressure of carrier gas (Wavelength ¼ 532 nm) using 800 mm diameter nozzle.


7

&

BADANI ET AL.

FIGURE 6. Mass spectra of TMS clusters at different sized nozzles (a) 500 mm (b) 800 mm.

larger diameter nozzle (800 mm) resulted into appearance of multiply charged atomic ions of Cnþ (n ¼ 1–4) and Sinþ (n ¼ 1– 4), in addition to singly charged species. Here in, the effect of increasing back pressure of carrier gas (Ar) and nozzle diameter is to increase the average size of clusters generated upon supersonic expansion. Each of the above set of experiments unambiguously indicates that under gigawatt intensity laser field, presence of clusters above threshold size is an essential requirement for multiple ionization and subsequent Coulomb explosion of the cluster.

C. Effect of Cluster Constituents on Coulomb Explosion Phenomenon The time-of-flight mass spectrum obtained in the above study displayed extensive ionization and subsequent Coulomb explosion of Si(CH3)4 clusters at 532 nm. One of the probable reason for efficient interaction of TMS clusters with laser wavelength of 532 nm could be the participation of some resonantly excited state of the TMS molecule which could be easily accessible upon multiphoton excitation at 532 nm. If this be the case, then multiple ionization and subsequent Coulomb explosion of clusters, under low intensity laser field, should be molecule specific phenomenon at this wavelength. To verify this, subsequent experiments were performed by replacing sample molecule [i.e., Si(CH3)4] by Sn(CH3)4. Resultant clusters were ionized with laser pulses of 266, 355, or 532 nm. The time-offlight mass spectrum obtained on interaction of tetramethyl tin (TMT) clusters, with laser wavelength of 266 nm, is depicted in Figure 7a. The mass spectrum showed an intense ion peak 8

corresponding to Snþ. The broad nature of this ion peak is due to the presence of several isotopes (m/z ranging from 112 to 122) of Sn. In addition to Snþ, low intensity ion signals corresponding to Sn(CH3)þ could also be observed in the mass spectrum. On changing the laser wavelength from 266 to 355 nm, in addition to the above ion signal, the mass spectrum revealed generation of Sn2þ (Fig. 7b). Also cluster fragment ions, that is, Sn2þ and [(CH3)4Sn][Sn(CH3)3]þ, could be detected in the mass spectrum at this ionizing wavelength. For 532 nm photo-ionization (Fig. 7c) multiply charged atomic ions of Snnþ (n ¼ 1–5) and Cnþ (n ¼ 1–4) were also observed in the mass spectrum in addition to the singly charged species. Kinetic energy analysis of atomic ions of Snnþ (n ¼ 1–5) formed in 532 nm photoionization has been performed using retarding potential analyzer (RPA) method (Das et al., 2012). The peak splitting method, used earlier for determining the kinetic energy of the atomic ions arising from Coulomb exploding TMS clusters, could not to be employed for TMT clusters because of presence of large number of isotopes of Sn since the split peak merges with the next isotopes. Peak splitting method gives the information regarding most probable kinetic energy of ions, while retarding potential analyzer method provides maximum kinetic energy of ions. In this method, mass spectra were recorded in which the retardation field was progressively increased in steps of 50 V/cm (Fig. 8). In RPA method, only those ions reach the detector which has sufficient kinetic energy to overcome the retarding field. Kinetic energy of the multiply charged tin ions was derived based on the retardation potential up to which the ion signal for the species is observed in the mass spectra and is listed in Table 2. Mass Spectrometry Reviews DOI 10.1002/mas


&

FIGURE 7. Mass spectra of TMT clusters at different laser wavelengths. (a) 266 nm, (b) 355 nm, and (c) 532 nm.

During these experiments, since the mass spectrometer had to be operated in single focusing condition, the mass resolution was poor as evident from the time-of-flight mass spectra presented in Figure 8. Energy analysis of multiply charged atomic ions of Sn, that are generated due to interaction of TMT

clusters with laser pulses of 532 nm, suggests that the ions fly with huge amount of energy (several hundreds of eV’s) upon Coulomb explosion. The results obtained from above studies once again display the wavelength dependent photo-ionization behavior of clusters, suggesting that the phenomenon of

FIGURE 8. Mass spectra of TMT clusters at different retardation potential at 532 nm (I 5 109 W/cm2).


9

&

BADANI ET AL.

TABLE 2. Ekin of atomic ions of Snnþ ions [(n ¼ 1–5)] obtained by RPA method

Charge state of atomic ion

Kinetic energy (eV)

+

~ 150

Sn

2+

~ 400

Sn

3+

~ 750

Sn

Sn4+ Sn5+

Coulomb explosion of clusters, under low intensity laser field, is independent of cluster composition.

D. Measurement of Total Ionization Yield of Interaction Zone at Different Laser Wavelengths Although mass spectrometry is a very sensitive method for detection of ions, however it is difficult to deduce the relative efficiency of laser–cluster interaction at different laser wavelengths and at different cluster composition on the basis of charge states of the multiply charged ions observed in the mass spectra. Thus, purely based on observation of multiply charged ions, it is difficult to conclude that 532 nm radiation interacts more efficiently with the cluster. It is possible that the total number of ions (including multiply charged ions) generated in 532 nm ionization (or 355 nm) might be less than the total number of singly charged ions formed in 266 nm ionization. To throw more light on the laser–cluster interaction process, charge density measurements were carried out to quantify the total ions generated in laser–cluster interaction volume, at all the three

~ 1000 Could not be measured because of low yield of ion

wavelengths, under identical experimental conditions using the parallel plate method (Das et al., 2010). This technique was also utilized to comment on the relative efficiency of laser– cluster interaction, as a function of wavelength and cluster composition. Figure 9 depicts the representative graph of charge densities for TMS cluster system, at different laser wavelengths. The charge densities measured from the saturated region of the graph at 266, 355, and 532 nm for TMS and TMT cluster is given in Table 3. Comparing the results of a particular cluster system, at different wavelengths, it can be seen that charge density increases with laser wavelength. For instance, in case of TMS clusters (Fig. 9) measured charge density was lower at 266 and then increases slightly for 355 nm and finally at 532 nm the overall charge density was 10 times higher than that at 266 nm. It should be noted here that time-of-flight mass spectrometer showed presence of multiply charged atomic ions for 532 nm ionization. Further, at the same wavelength, the charge density is also found to be 10 times higher than at 355 and 266 nm. Thus, the two experimental techniques (i.e., mass

FIGURE 9. Charge density of TMS clusters at different laser wavelengths.

10



&

TABLE 3. Charge density of TMS and TMT clusters at different laser wavelengths

Charge density (charges/cm3) Wavelength (nm) TMS clusters 10

TMT clusters ~ 5 x 1010

266

~ 4 x 10

355

~ 5 x 1010

~ 1.5 x 1011

532

~ 5 x 1011

~ 8 x 1011

spectrometry and charge density measurements) give results which are in good qualitative agreement with each other. Comparing the charge density of different molecular cluster systems (i.e. TMS and TMT), at a particular laser wavelength (say 532 nm), it can be seen that charge density of TMT clusters is about 1.6 times higher as compared to TMS clusters. This suggests that though the ionization trend (i.e., increase in charge state of atomic ions with laser wavelength) is independent of cluster composition, however the relative efficiency of laser–cluster interaction depends on the nature of cluster constituents.

E. Plausible Mechanism for Multiple Ionization and Coulomb Explosion of Clusters

1. MPI Probability and IBS Absorption Process The results described above demonstrate efficient interaction of TMS/TMT clusters with optical pulses at longer laser wavelength (532 nm) as compared to shorter wavelength (266/ 355 nm). In addition to this, the results display prominent size dependent Coulomb explosion of clusters. As described earlier, the mode of ionization (MPI or OFI) essentially depends on the intensity of laser pulses. The boundary between the OFI and MPI regimes is parameterized by the dimensionless Keldysh nonadiabaticity parameter (g) (see Equation (1)) (Keldysh, 1965). Under the experimental conditions used in our studies, the ponderomotive energy at 532 nm is 1.3 104 eV. The IE of TMS and TMT is 9.8 and 8.8 eV respectively. Corresponding value of g is 200 (which is 1). Hence it can be safely assumed that ionization of TMS/TMT clusters is driven via multiphoton absorption. In order to verify this, laser power dependence was measured for different ions which were generated during lasercluster interaction. For ionization of TMS clusters (IE 9.8 eV), minimum number of photons required in the UV (266/355 nm) and visible region (532 nm) is 3hn and 5hn, respectively. In case of 532 nm photoionization, multiply charged ions having ionization energy as high as 64.5 eV (Cþ4) and 45.1 eV (Siþ4) were observed suggesting that large numbers of photons from the laser pulse have to be absorbed by the TMS cluster. The power dependence studies at different laser wavelengths were carried out on fragment ions of the TMS cluster by varying laser energy. ln–ln plot of ion signals versus laser energy (mJ/pulse) shows a slope of 2 for 266 and 355 nm (Fig. 10a and b) indicating two photon dependence for different ions at these wavelengths. Similarly, a power dependence of four has been observed for Si2þ, C2þ, and C3þ ions generated upon interaction of 532 nm laser pulses with TMS clusters (Fig. 10c). Mass Spectrometry Reviews DOI 10.1002/mas

For a pure nonlinear photo-ionization process, the power dependence for the ion signal is found to be affected by the presence of resonant excited states of the species lying below the ionization continuum, which often lead to a lower power dependency than the total number of photons which are actually required based on energetic considerations. Thus, the two photon dependence obtained at 266 and 355 nm points towards intermediacy of resonant excited state at 9.32 and 7 eV respectively in the multiphoton ionization at these two wavelengths. Similarly at 532 nm, a four-photon dependence observed for Si2þ, C2þ, and C3þ ion points towards intermediacy of resonant excited state at 9.32 eV, for generation of these multiply charged ions. Laser power dependency studies were also performed on TMT clusters (Fig. 11). The laser power dependence of 1 and 2 was measured, for different fragment ions of TMT clusters, at 266 and 355 nm respectively. While, ionization of TMT clusters at 532 nm displayed the power dependency of 3. Power dependence studies confirm that primary ionization in TMS/TMT cluster is initiated via multiphoton absorption process. However, wavelength dependent generation of multiply charged atomic ions cannot be explained solely on the basis of MPI of clusters. This is because the energy required to generate Si4þ, which is the highest charge state atomic ion detected in case of TMS clusters at 532 nm, is 45.1 eV. Generation of Si4þ via MPI, at this ionizing wavelength, requires absorption of at least 20 photons. However the observed power dependency of Si4þ was much lower (i.e., 4). Similarly in case of TMT clusters, the IE of highest observed charged state atomic ion (i.e., Sn5þ) is 77.2 eV. This requires absorption of 33 photons of 532 nm. But the measured power dependency was much lower (i.e., 3). Also laser intensity utilized in this studies, to generate Si4þ/ Sn5þ is 106 times (see Table 4) lower the intensity predicted by the formula of Augst et al. (1989, Equation 4), which considers the effect of electric field of the laser pulse for generation of multiply charged atomic ions. IðW=cm2 Þ ¼

4 109 E4z Z2

ð4Þ

where Ez is the ionization energy of isolated atomic ion and Z is the charge state of atomic ion. It must be noted here that above equation predicts the threshold intensity required for observation of different charge states of an ion. Due to higher photon energy in case of UV as compared to visible region, less number of photons are required for ionization. Hence, one would expect higher ionization efficiency of clusters at shorter laser wavelength (266 nm) as compared to longer laser wavelength 11

&

BADANI ET AL.

FIGURE 10. Laser power dependency of ions generated upon interaction of TMS clusters with different laser wavelengths. ln–ln plot of integrated area of individual ion signal with laser energy has are shown (laser energy in mJ/pulse).

(532 nm). Thus, to explain this contrast photo-ionization behavior of clusters additional processes need to be considered. A three-stage cluster ionization mechanism has been proposed by Li and coworkers (Zhang et al., 2009, 2010; Zhao et al., 2012) to explain different facets of the low laser intensity induced Coulomb explosion of clusters. This mechanism suggests that the primary step for generation of multiply charged atomic ions is the multiphoton ionization of the neutral cluster constituents (in this case TMS or TMT molecule). The electrons generated due to the ionization of atoms/molecules on the cluster surface leave the cluster immediately, thereby rendering a net positive charge on the cluster. In case of a cluster, this process is known as outer ionization and refers to the removal of electrons from the surface of cluster. On the other hand, electrons generated due to ionization of atoms/molecules present in the bulk have left the parent atom but are confined to the volume of cluster due to surface Coulombic barrier created by the outer ionization process. These are known as inner ionized/quasi free electrons. At the time of their generation, the inner ionized electrons possess kinetic energy which is given by Equation (5). ðKEÞelectron ¼ nðhnÞ IE

ð5Þ

where n is the number of photons required for ionization of neutral atom/molecule and IE is the ionization energy of atom/ molecule. Subsequently, these confined electrons, which are under the influence of Coulomb field within the cluster, keep extracting energy from the laser pulse presumably via inverse Bremsstrahlung (IBS) process through electron–ion and elec12

tron–neutral collisions. The energy absorption coefficient for electron–neutral (aen) and electron–ion (aei) inverse Bremsstrahlung is given by Equations (6) and (7) respectively (Wieneke, Bruckner, & Viol, 2010). aen ¼

1 e2 nen ne l2L : 4Pe0 Pme c3

n2 e6 aei ¼ pffiffiffi e3 3 6 3he0 hvL m2e

1e

ð6Þ

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi me 2Pk B T e

ð hvL Þ=ðk B T e Þ

ffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffi3ffi rPk T B

P

hvL

e

ð7Þ

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi where h is the refractive index of cluster medium ¼ 1 ne =nc , nc ¼ critical electron density ¼ ð4P 2 e0 me c20 Þ=ðe20 l2L Þ ¼ 0.00058 a.u. (3.94 1027/m3) (at lL ¼ 532 nm). Te is the electron energy (also referred as electron temperature), ne the electron density ¼ Q(naSZimi), Q the fraction of atoms/molecules that are ionized, na is the cluster density (which is taken to be equal to liquid density), Zi the charge state of atomic ions and mi is the number of particular element in a molecule. The quasi free electrons extract energy from the laser pulse by the above mentioned process. Once the electron energy exceeds ionization energy, secondary ionization within the cluster takes place via electron ionization, resulting in more free electrons being available for energy extraction via IBS process. Since laser pulse (10 nsec) is much longer than ionization time (of the order of



&

FIGURE 11. Laser power dependency of ions generated upon interaction of TMT clusters with different laser wavelengths. ln–ln plot of integrated area of individual ion signal with laser energy are shown (laser energy in mJ/pulse).

femtosecond), the process is repeated till the entire cluster becomes highly charged. Repetition of these energy extraction processes slowly builds up an avalanche of ionization events within the cluster. This sequence of events continues until a stage comes when Coulombic repulsive forces (due to positively charged ions in close proximity) within the cluster overcome the total cohesive energy and the highly charged cluster explodes resulting in generation of multiply charged atomic ions. The potential energy stored in the cluster is converted to kinetic energy of positive ions.

The energy extracted by the inner ionized electron via IBS process from the laser pulse is given by Equation (8) (Ditmire et al., 1996; Kumarappan, Krishnamurthy, & Mathur, 2003) dE ¼ Up n dt

ð8Þ

U p ¼ 9:33 1014 IðW=cm2 Þl2 ðmmÞ2

ð9Þ

TABLE 4. Threshold laser intensity required to generate different multiply charged atomic ions based on formula of Augst et al. (1989)

Tetramethyl Silane cluster system Ions and

Threshold laser intensity 2

Tetramethyl Tin cluster system Ions and

Threshold laser intensity

ionization energy

(W/cm )

ionization energy

(W/cm2)

Si+ (8.1 eV)

~2 x 1013

Sn+ (7.3 eV)

~1 x 1013

Si2+ (16.3 eV)

~7 x 1013

Sn2+ (14.6 eV)

~5 x 1013

Si3+ (33.4 eV)

~6 x 1014

Sn3+ (30.5 eV)

~4 x 1014

Si4+ (45.1 eV)

~1 x 1015

Sn4+ (40.7 eV)

~7 x 1014

Sn5+ (77.2 eV)

~ 6 x 1015

--------------------


13

&

BADANI ET AL.

Here n is the collision frequency of the inner ionized electrons and is of the order of 1014–1015 Hz and Up is ponderomotive energy, that is, averaged energy of the wiggling electrons inside the laser field as given by Equation (9). From Equations (8) and (9), it is clear that overall energy gained by the inner ionized electrons in the total interaction time (starting from initial ionization till the disintegration of cluster) is dictated by the product of ponderomotive energy and the total number of effective electron–ion/neutral collision frequency. Equation (9) also suggests that as the wavelength of ionization (l) increases, ponderomotive energy (Up) of electrons increases quadratically for a given laser intensity. Hence, higher level of ionization is expected at longer laser wavelengths. It is worth mentioning here that based on the above proposed mechanism; one would expect higher level of ionization and charging of clusters at 1,064 nm (which is the fundamental wavelength output of Nd:YAG laser) as compared to 532 nm. But under the experimental conditions used in these studies, no ion signal was observed at 1,064 nm. This is due to the fact that IE of TMS and TMT molecules in the cluster is expected to be 9.8 and 8.8 eV respectively. Hence, initial ionization of TMS and TMT molecules at 1,064 nm (hn ¼ 1.16 eV) via MPI process requires simultaneous absorption of 9 and 8 photons, respectively. The MPI probability of simultaneous absorption of such large number of photons at laser intensity of 109 W/cm2 is extremely low. Due to this, primary ionization process does not occur and the quasi-free electrons are not generated, ruling out subsequent extraction of energy from the laser pulse by the ionized electrons. Thus, it can be concluded that primary ionization by multiphoton absorption serves to ignite the multiple ionization stage in a cluster, eventually leading to Coulomb explosion. The above proposed three stage cluster ionization model provides a qualitative understanding about the Coulomb explosion phenomena of clusters under gigawatt intensity conditions. It also provides logical explanation for the systematic increase in the charge state of atomic ions as a function of cluster size, as shown in Figures 5 and 6. Increasing the argon backup pressure or nozzle diameter results in an increase in the three body collision rate and hence the average size of the cluster. As a result of increased size, the probability of retaining inner ionized electrons for longer duration within the cluster is more as compared to smaller clusters. This allows the quasi free electrons to keep interacting with the nanosecond laser pulse for much longer times and extract more amount of energy from the laser field before the inner ionized electrons escape from the cluster volume. Thus with increasing cluster size, an increase in charge state of the atomic ions is expected, which is in good agreement with the experimental results of this study. The three stage cluster ionization model emphasized dominant role of inner ionized electrons in governing the ionization dynamics of clusters. In order to validate the role of inner ionized electrons in governing the ionization dynamics of clusters, subsequent experiments were performed on TMS doped rare gas clusters. With reference to the doped clusters, it should be noted that in each of the experiments discussed above using Ar as carrier gas, no ion signal corresponding to Arþ was seen in the mass spectrum. While Castleman and coworkers (Ford et al., 1999) reported the observation of the ion signal corresponding to carrier gas (i.e., Arþ), upon interaction of methyl iodide clusters with high intensity laser fields. The Arþ 14

ion signal appeared in the mass spectrum only when highly charged iodine ions were generated during Coulomb explosion of methyl iodide cluster. Generation of Arþ ion was attributed to either Coulomb explosion of Ar–CH3I co-cluster or due to charge transfer from highly ionized iodine to Ar atom. However, under the gigawatt intensity experiments discussed earlier, no ion signal corresponding to carrier gas was observed upon Coulomb explosion of clusters. These observations indicate that either carrier gas does not constitute part of clusters or the charge transfer reaction is not initiated in the cluster medium. In order to identify the reason for the absence of the ion signal of carrier gas in the mass spectra, the supersonic expansion conditions were modified, by varying the relative concentration of argon and TMS. Argon with 6 atm back up pressure was allowed to flow over few drops of TMS kept in a stainless steel cell and connected to pulsed valve. Resulting clusters were ionized by the laser pulse of 532 nm. Under these conditions, the time-of-flight mass spectrum (Fig. 12) revealed generation of multiply charged atomic ions of argon, that is, Arnþ (n ¼ 1–5), in addition to multiply charged atomic ions of carbon and silicon. It is important to mention here that, under identical experimental conditions, when experiments were carried out on pure argon clusters, no ion signal could be observed. This is due to the fact that Ar clusters possess an ionization energy (IE) of TMS–Ar > Ar–Ar. Thus upon clusterization, the heat of condensation released is removed from the cluster system by Ar atoms which boil off from the cluster system during their flight from expansion zone to ionization zone, resulting in formation of pure TMS clusters for higher relative concentration of TMS. For lower relative concentration of TMS, the TMS–TMS condensation is not significant and gives rise to primarily Ar clusters which are doped by TMS. The above study suggests that generation of doped clusters is efficient when the relative concentration of molecules is lower with respect to the carrier gas. Further, observation of Arnþ (n ¼ 1, …, 5) ions upon interaction of TMS doped rare gas clusters with laser pulses could be ascribed to indiscriminate ionization of cluster constituents (Ar) by the energetic inner ionized electrons that are generated due to multiphoton ionization of dopant molecules (i.e., TMS). These experiments on doped clusters again qualitatively support the basic feature of the proposed three stage cluster ionization model.

2. Screening Effects Though the three stage cluster ionization model qualitatively accounts for wavelength and size dependent photo-ionization behavior of clusters, however some aspects of low intensity induced Coulomb explosion of clusters, cannot be solely Mass Spectrometry Reviews DOI 10.1002/mas


&

FIGURE 12. Mass spectrum of Si(CH3)4 doped Ar clusters at 532 nm.

explained using this model. For instance, why the relative ionization efficiency of TMT clusters is higher as compared to TMS clusters? Also, according to the three stage cluster ionization model, the inner ionized electrons, generated upon initial MPI of cluster constituents, needs to acquire energy equal to or greater than the ionization energy (IE) of neighboring species, so as to cause further ionization. Hence the maximum electron energy, generated during this interaction process, is expected to be equal to or greater than the ionization energy (IE) of highest observed charge state. However, experimentally measured maximum electron energy for different cluster systems was significantly lower than energy required for generation of highest observed charge state (see Table 5) (Sharma et al., 2006; Zhang et al., 2009, 2010; Zhao et al., 2012). Hence, in order to account for generation of high IE atomic species with low energy electrons, screening effects in clusters ought to be considered. Screening effects primarily arise due to presence of charged particles in the cluster medium. As mentioned earlier, ions and electrons are generated in cluster medium upon initial photoionization via multiphoton absorption process. For the clusters in

which distance between neighboring species is of the order of few Angstroms, each singly charged ion produces an electric field equivalent to 109 V/m on the neighboring neutral (Karras & Kosmidis, 2010). This constitutes an internal electric field in the cluster medium. However, calculating the total electric field in cluster is difficult due to presence of electrons. The high electric field due to ions is expected to lower the Coulomb barrier of neutral atoms/molecules present in its vicinity. This, in turn, would result in lowering the energy requirement for ionization of these atoms/molecules thereby facilitating generation of new charge centers. In addition the ions, inner ionized electrons favor the ionization process by decreasing the IE of charged species. The trajectory of inner ionized electrons is bent towards the ion due to Coulombic interaction. This would result in increasing the average electron density on the ion as compared to the average electron density of clusters. Due to this, the inner ionized electrons will screen the nucleus; more efficiently, thereby decreasing the binding energy of the atomic electron (Murillo & Weisheit, 1998; Micheau et al., 2005; Mulser & Bauer, 2010). Hence, due to presence of ions and inner ionized electrons, the ionization energy of cluster constituents is

TABLE 5. Experimentally measured maximum electron energy and IE of highest observed charge state, for different cluster systems

Cluster system

Pulse duration

I.E. of highest observed

Maximum electron

(ns)

charge state (eV)

energy (eV) at 532 nm

CH3I CH3I C2H5OC2H5


8 5 5

3+

47.8 (C ) 4+

44 (I ) 4+

77.4 (O )

30 40 40

15

&

BADANI ET AL.

expected to be lower as compared to their isolated monomer counterpart, which would ultimately facilitate the process of generation of multiply charged atomic ions. As described above, lowering of IE of cluster constituents arises due to ions and quasi free electrons in the cluster medium. This, in turn, facilitates inner ionization thereby favoring the generation of next higher charge state atomic ions and enhancing the efficiency of conversion of optical energy into particle energy. To understand the factors affecting lowering of IE, we calculate IE of atomic ions [Sinþ (n ¼ 2–4), Snnþ (n ¼ 2–5), and Cnþ (n ¼ 2–4)] after considering screening effects inside the TMS/TMT clusters, using Equation (10) as given by Gets and Krainov (2006) EIE ¼

Z 2eff Z eff ½3n2 lðl þ 1Þ n2 ½5n2 þ 1 3lðl þ 1Þ þ 2n2 R 4R2 12Z eff R3 ð10Þ

where EIE is the IE of atomic ions after considering p screening ffiffiffiffiffiffiffiffi effects, Zeff the effective charge of the atomic core ¼ n 2E Z , EZ the IE of isolated atomic ion, n the principal quantum number, l the azimuthal quantum number, R the Debye radius ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi T e =ð4Pne Þ, Te the electron energy (also referred as electron temperature), ne the electron density ¼ Q(naSZimi), Q the fraction of molecules that are ionized in cluster medium, na the number density of molecules in clusters, Zi the charge state of atomic ions, and mi is the number of particular element in a molecule.

All calculations presented here have been performed in atomic units (a.u.). Cluster density is considered as the liquid density. The liquid density of TMS and TMT are 0.648 and 1.29 g/cm3 respectively. Calculations have been performed for different charge state of ions of silicon, tin, and carbon, at varying electron energy, assuming 0.1% ionization (Q ¼ 0.001) in both [Si(CH3)4 and Sn(CH3)4] cluster medium. The representative calculation, of lowering in IE, for Siþ ! Si2þ, in case of TMS clusters, is illustrated in detail below. The number density of TMS clusters (corresponding to mass density of 0.648 g/ cm3) ¼ 4.435 1021 atoms/cm3 ¼ 0.00065 a.u. The ionization energy of isolated atomic ion (Ez) for Siþ ! Si2þ ¼ 16.34 eV ¼ 16.34/27.2114 a.u. ¼ 0.600 a.u. Electronic configuration of Siþ is 1s22s22p63s23p1. Therefore for Siþ, principal quantum number (n) ¼ 3 and azimuthal quantum number (l) ¼ 1. Hence, for Siþ ! Si2þ, submitting n ¼ 3, l ¼ 1, SZimi ¼ (1 12)H þ (1 1)Si þ (1 4)C ¼ 17 and ne ¼ Q(naSZimi) ¼ 0.0000108 a.u. in above equation. The calculated EIE as a function of electron energy, for different atomic ions of TMS cluster system, is shown in Figure 13. Similar methodology was used to calculate EIE of the atomic ions that were generated upon interaction of TMT clusters with laser pulses. From this figure, it can be seen that the ionization energies of the atomic ions, present in cluster medium, is dependent on electron energy. The extent of lowering of IE of multiply charge atomic ion is more pronounced at lower electron energy. This suggests that screening of nuclear charge is more efficient at lower electron energy. A more comprehensive way to compare the extent of lowering of IE of different atomic ions is in terms of percentage

FIGURE 13. Calculated EIE of [Sinþ (n ¼ 2–5) and Cnþ (n ¼ 2–4)] as a function of electron energy for TMS clusters.

16



decrease of EIE with respect to Ez. This can be calculated using Equation (11). Percentage decrease in E IE ¼

E Z EIE 100 EZ

ð11Þ

Percentage decrease in EIE of different atomic ions of Si and Sn, at varying electron energies, is plotted in Figure 14. Above calculations have been performed, considering 0.1% ionization of cluster, following multiphoton absorption. However, once the ionization due to subsequent steps (i.e., energy absorption by inner ionized electrons via IBS process followed by electron ionization) starts, number of charge centers generated in cluster would enhance manifold. This would further facilitate in lowering IE of cluster constituents and generate atomic ions with higher charge state. Due to instrumental limitations we could not measure electron energies in the present study. Earlier, electron energy has been measured for different molecular clusters such as benzene (Zhang et al., 2009), diethyl ether (Zhang et al., 2010), methyl iodide (Zhao et al., 2012) and the mean electron energy lies in the range of 25–30 eV with maxima at 60–65 eV under similar experimental conditions (Table 5). Thus, electron energy distribution in case of TMS and TMT clusters is expected to be somewhat similar under the experimental conditions used during these studies. For all these cluster systems, experimentally measured maximum electron energy was lower than IE of highest observed charged state of carbon and iodine ions. For better evaluation of TMS and TMT cluster system, EIE and percentage decrease in EIE of few atomic ions of Si and Sn, at selected mean electron energies from 5 to 20 eV is tabulated below.

&

Table 6 depicts that low energy electrons efficiently decrease the IE of atomic ions, while as the electron energy increases, EIE approaches Ez. For example, percentage decrease in IE for Siþ ! Si2þ is 14% and 7%, at 5 and 20 eV of electron energy respectively. Further, at particular electron energy, screening effects are found to be dependent on charge state as well as atomic number (Z) of the ions. For a given atomic ion, extent of lowering in IE is found to be greater for lower charge state as compared to higher charge state. For instance, percentage decrease in EIE of Siþ ! Si2þ is greater (1.3 times) as compared to Si3þ ! Si4þ. Also for a given charge state, screening effects were found to be more dominant for higher Z ions (Sn ions) as compared to lower Z ions (Si ions). Thus, above calculations suggest that due to higher atomic number of Sn, the screening effects are more dominant in case of TMT clusters as compared to TMS cluster. It is worth mentioning here that the calculations presented here were performed considering the generation of similar number of charge centers upon multiphoton ionization, but during experiments, the situation might be different. For initial ionization at 532 nm, via multiphoton absorption, the TMS and TMT clusters requires simultaneous absorption of 5 and 4 photons respectively. Hence, based on MPI probabilities, larger number of charge centers is expected to be generated for TMT clusters as compared to TMS clusters. This would result in higher ion as well as electron density in TMT clusters following multiphoton ionization, which, in turn would further enhance the screening effects in TMT clusters. Hence, the extent of lowering of ionization energies of the species would be much more pronounced in TMT clusters as compared to the TMS clusters. Thus, higher relative ionization efficiency of TMT clusters, as compared to TMS clusters, could

FIGURE 14. Percentage decrease in EIE of [Snnþ (n ¼ 2–5) and Cnþ (n ¼ 2–4)] at varying electron energy.


17

&

BADANI ET AL.

TABLE 6. IE of different atomic ions for 0.1% ionized TMS/TMT cluster system

I.E. of atomic ions at different electron energies (in eV)

Mean electron energy (eV)

Si → Si2+

Si3+ → Si4+

Sn+ → Sn2+

Sn4+ → Sn5+

5

13.9 (14.3%)

40.1 (10.9%)

11.0 (24.0%)

67.8 (12.0%)

10

14.6 (10.3%)

41.5 (7.0%)

12.0 (17.4%)

70.5 (8.6%)

15

14.9 (8.4%)

42.1 (6.4%)

12.4 (14.4%)

71.7 (7.1%)

20

15.0 (7.3%)

42.5 (5.6%)

12.7 (12.5%)

72.4 (6.1%)

16.3

45.1

14.6

77.2

I.E. of isolated atomic ion (eV)

+

Figures in parenthesis indicate percentage decrease in EIE as compared to IE of isolated atomic ion.

be attributed to greater screening effects. Also, screening which results in lowering of ionization energies of cluster constituents facilitates generation of species which have higher ionization energy, even at lower electron energy. Finally, our calculations suggest that, in addition to energy absorption by inner ionized electrons via IBS process, screening effects also play a dominant role in evolution of higher charge states of atomic ions during laser–cluster interaction.

cluster systems primarily depends upon the vapor pressure of dopant molecules. In an earlier study on water doped argon clusters, the extent of doping of argon clusters by water was measured to be 2–5% in the range of 1–8 atm Ar pressure (Jha et al., 2006b). In the experiments discussed here the dopant (iron pentacarbonyl) also has vapor pressure similar to that of water (i.e., 18 Torr at 298 K). Hence, we assume similar percentage of doping in iron pentacarbonyl doped cluster system.

V. DOPED CLUSTER SYSTEM In the previous section, advantages of using cluster over isolated molecules for generation of multiply charged atomic ions were demonstrated. The study suggested crucial role of laser wavelength for controlling the charge states of multiply charged atomic ions. Moreover, laser–cluster interaction studies also emphasis to understand the effect of cluster size in governing the ionization dynamics of clusters. Another way of influencing the ionization dynamics of clusters is by changing the constituents of the cluster target, as suggested by Castlemann and coworkers (Purnell et al., 1994) in the context of small clusters. They introduced HI molecules (having lower ionization energy than Ar) in the stream of Ar gas in a supersonic expansion source. The resultant HI doped Ar clusters were ionized with femtosecond laser pulses (I 1015 W/cm2). Enhancement in charge state of atomic ions was observed in HI doped Ar clusters as compared to pure Ar cluster which was attributed to charge transfer process in doped clusters. In view of the fact that ionization dynamics of pure and doped cluster are different, experiments were also performed on doped cluster system and the results are discussed below. Experiments were previously carried out using tetramethyl silane as dopant molecule (Badani et al., 2011). In those studies, it was demonstrated that the process of generation and growth of doped clusters is efficient when the relative concentration of dopant molecules is lower with respect to the carrier gas. However, due to high vapor pressure of tetramethyl silane (713 Torr at 298 K), decreasing the relative concentration of Si(CH3)4 in carrier gas matrix was comparatively difficult. Hence, subsequently the dopant molecule (i.e., tetramethyl silane) was replaced by iron pentacarbonyl possessing lower vapor pressure (18 Torr at 298 K), which provides better control on level of doping. The level of doping in such doped 18

A. Role of Charge Centers in Governing the Expansion Dynamics of Doped Clusters Rare gases such as argon (Ar), krypton (Kr), or xenon (Xe) are often used for generation of atomic clusters. Xenon, due to its higher polarizability and larger value of condensation parameter (k) as compared to other rare gases (Ar or Kr), is an ideal choice for generation of atomic clusters. These properties of Xe over other rare gases facilitate generation of clusters of comparatively larger size under identical expansion conditions (pressure, temperature, and nozzle diameter). Further, due to higher electron density coupled with lower IE of Xe as compared to Ar/ Kr, Xe interacts more efficiently with optical pulses. Hence, Xe gas was utilized for cluster generation. Further, Xe clusters were doped with iron pentacarbonyl molecules. The ionization energy of dopant Fe(CO)5 (IE ¼ 7.8 eV) is significantly lower as compared to Xe (IE ¼ 12.1 eV). Thus, the presence of a dopant molecule with low ionization energy provides an ease for initial ionization step during the interaction process. Fe(CO)5 doped Xe clusters were ionized with nanosecond laser pulses of gigawatt intensity. Experiments were performed at selected laser wavelengths (266, 355, 532, or 1,064 nm) ranging from UV to IR region.

1. Time-of-Flight Mass Spectrometry Studies at Different Laser Wavelengths When Fe(CO)5 doped Xe clusters were subjected to 266 nm laser pulse of intensity 1.5 109 W/cm2 (Fig. 15a), ion signals corresponding to Feþ, FeCOþ, Fe2þ, and Xeþ were observed in the time-of-flight mass spectrum. At 355 nm (Fig. 15b), ion signals corresponding to COþ, Feþ, FeCOþ, and Fe2þ were observed in the mass spectrum, under identical experimental Mass Spectrometry Reviews DOI 10.1002/mas


&

FIGURE 15. Mass spectra of Fe(CO)5 doped Xe clusters at (a) 266 nm, (b) 355 nm, (c) 532 nm (laser intensity 109 W/cm2). Inset depicts the typical ion signal corresponding to Feþ obtained in our studies. Ion signal marked with arise due to different isotopes of Xe.

condition. On irradiating Fe(CO)5 doped Xe clusters at 532 nm (Fig. 15c), apart from the above ion signals, an additional ion signal at m/z ¼ 187 (inset of Fig. 15c) corresponding to [FeXe]þ appeared in the mass spectrum. Another interesting feature observed on photo-ionizing Fe(CO)5 doped rare gas clusters at 532 nm is generation of multiply charged atomic ions (i.e., Fe2þ, Xe2þ, Xe3þ, C2þ, and O2þ). The wavelength dependent generation of multiply charged atomic ions in doped clusters is in agreement with the three stage cluster ionization model discussed earlier. Based on the above proposed mechanism, the wavelength of irradiation plays a crucial role in determining the extent of energy absorption and subsequent ionization of cluster constituents. Ponderomotive energy for laser intensity (I) 109 W/cm2, at 266 nm and 532 nm is 3.3 105 eV and 1.3 104 eV respectively. This energy is insignificant to cause any secondary ionization. Hence electrons need to interact with large number of optical cycles to acquire sufficient energy that can cause secondary ionization. As higher ponderomotive energy is acquired by electrons at 532 nm as compared to 266 nm, higher level of ionization is expected at longer laser wavelength. Based on the three stage cluster ionization model, at a particular wavelength, energy acquired by electron from optical pulses can be enhanced by increasing laser intensity (see Equation (9)). In case of 266 nm, upon increasing the laser intensity to 4 109 W/cm2, ponderomotive energy increases to 1.3 104 eV, which is equal to energy gained by electron at the laser intensity of 1 109 W/cm2 for 532 nm. Under this condition similar level of ionization is expected at both the wavelengths. However the experimental results differed signifiMass Spectrometry Reviews DOI 10.1002/mas

cantly, that is, despite increasing the laser intensity at 266 nm by a factor of 4, the highest observed charge state of atomic ions did not show any enhancement. From these results it is clear that the three stage cluster ionization model explains experimental observations only qualitatively. However to account for the overall ionization dynamics in the case of nanosecond induced Coulomb explosion of clusters, apart from energy absorption by electrons via IBS process, additional processes need to be taken into account, as suggested by these experiments.

2. Total Charge Density Measurements at Different Laser Wavelengths In addition to time-of-flight studies, charge density measurements were also carried out for Fe(CO)5 doped Xe clusters at different laser wavelengths to quantify the number of ions generated during the laser–cluster interaction process. The charge densities measured from the saturated region of the graph at 266, 355, and 532 nm were 1.6 1011, 1 1011, and 4.8 1010 charges/cm3 respectively. In the previous studies on pure tetramethyl silane and tetramethyl tin clusters, the charge densities were observed to increase with increasing laser wavelength. However, in case of doped clusters, charge densities were observed to decrease with increasing wavelength. This suggests that though ions of higher charge states are observed at 532 nm but the total ion yield is 3 times higher at 266 nm. This is due to presence of low IE dopant molecules, that is, Fe(CO)5 which enhances the efficiency of multiphoton absorption. On decreasing laser wavelength from 532 to 266 nm, multiphoton ionization (MPI) probability further increases ultimately 19

&

BADANI ET AL.

resulting in enhancement of the overall charge densities of ions generated upon laser–cluster interaction. Thus the results discussed above suggest that MPI processes are comparatively more dominant in clusters doped with low IE species as compared to pure clusters. In order to understand the role of MPI in governing the ionization dynamics of clusters, one needs to consider expansion dynamics of charged cluster interacting with laser pulse. Cluster constituents upon interacting with optical pulses; ionize via multiphoton absorption resulting in generation of ions and electrons. Electrons generated due to ionization of atom/ molecules present on the surface leave the cluster rendering it positively charged. Thus, the ionized cluster experiences net repulsive Coulombic forces. The magnitude of repulsive Coulombic force (FiC) in a cluster is given by Equation (12) (Doppner et al., 2000) F Ci ¼

1 X q2ion e2 ri rj 4Pe0 j6¼i jri rj j2 jri rj j

ð12Þ

where qion ¼ qcluster/N, qcluster ¼ charge on cluster and N is the number of atoms/molecules in cluster. These Coulombic repulsive forces tend to expand the ionized cluster while the attractive binding forces, governed by Lennard-Jones potentials (Equation (13)) (Doppner et al., 2000; Arunan, 2009), tend to decelerate the rate of cluster expansion F LJ i

12E b X ¼ r0 j6¼i

" # jri rj j 13 jri rj j 7 ri rj jri rj j r0 r0 ð13Þ

where Eb is the binding energy and r0 is the equilibrium distance between adjacent cluster constituent. In the studied mentioned earlier, since clusters are generated under identical conditions, cluster sizes as well as binding energies are expected to be similar (Scoles, 1988). These clusters of same size and composition, upon irradiation with laser pulses of different wavelengths (in UVand Visible), will generate different number of charge centers due to their different ionization probability at different wavelength. At shorter laser wavelength (266 nm) MPI probability would be higher due to the fact that lesser number of photons are needed for initial ionization as compared to longer laser wavelength (532 nm). Hence, the number of charge centers generated due to initial MPI would be larger at 266 nm as compared to 532 nm. This would result in stronger repulsive Coulombic forces at 266 nm as compared to 532 nm. Hence the rate of expansion of cluster is expected to be higher at 266 nm (based on Equation 12). For a given laser wavelength, MPI probability increases with increasing laser intensity in accordance with Equation (14). W n / sn I n

ð14Þ

where sn is the n photon absorption cross-section and I is the laser intensity. For single photon process, upon increasing the laser intensity from Ia (1 109 W/cm2) to Ib (4 20

109 W/cm2), MPI probability would increase 4 times, that is, W1b 4W1a. Thus more number of charge centers would be generated at higher laser intensity (as confirmed by charge density measurement experiments, Fig. 16) which would further accelerate the rate of cluster expansion thereby decreasing the time for which quasi free electrons interact with optical pulses to gain energy and cause secondary ionization. Hence, multiply charged atomic ions were not detected at lower laser wavelength, despite increasing laser intensity.

3. Comparison of the Photo-Ionization of Pure and Doped Xe Clusters Using high intensity laser pulses (1016 W/cm2), Jha, Mathur, and Krishnamurthy (2005) reported significant enhancement in charge state of atomic ions when clusters doped with low IE molecules were irradiated as compared to pure clusters. The enhancement in efficiency of laser–cluster interaction in doped clusters was attributed to faster rise in the electron density due to ease of ionization of dopant molecule which had lower ionization energy. Higher density of electrons, generated during initial raising portion of laser pulse, facilitates efficient coupling of laser energy into the cluster medium thereby leading to generation of larger fraction of energetic electrons causing further ionization and ultimately resulting in generation of higher charge state atomic ions. Since a significant difference in overall ionization dynamics was observed for the doped and pure cluster, it was thought to see if this is true for the case of low intensity also. For this purpose, subsequent experiments were performed on pure Xe clusters and results were compared with Fe(CO)5 doped Xe clusters. Figure 17 shows the time-of-flight mass spectra obtained on interaction of pure Xe clusters with laser pulses of different wavelengths (I 5 109 W/cm2). On comparing the time-offlight mass spectra of pure (Fig. 17) and doped (Fig. 15) Xe gas clusters, at each of the laser wavelengths, one clearly observes lowering of charge states of atomic ions in doped clusters as compared to pure rare gas clusters. This suggests that the extent of energy transfer from optical pulses into the cluster medium decrease upon doping clusters with low IE molecules. These experimental observations differed significantly as compared to those reported by Jha, Mathur, and Krishnamurthy (2005); Jha et al. (2006b), under high intensity laser fields (I 1015 W/cm2). The contrasting experimental observations under low and high intensity laser fields can be understood if one considers the dynamics of cluster expansion. In case of high intensity laser fields (1015 W/ cm2), the rate of energy extraction by inner ionized electrons via IBS is very high (60 eV per optical cycle). Due to their ultrafast temporal profile, ions generated after initial step of ionization practically remain frozen during the span of laser pulse thereby maintaining electron density at considerably higher level. This higher density of inner ionized electrons in charged cluster system facilitates efficient energy absorption from laser pulses via secondary processes, that is, IBS, leading to extensive ionization. However, at low intensity laser fields (109 W/ cm2), rate of energy extraction by inner ionized electrons via IBS is rather very low (104 eV per optical cycle). As a result, inner ionized electrons generated after initial multiphoton ionization needs to interact with large number of optical cycles to gain sufficient energy that can cause further ionization. For 10 nsec long laser pulses, after initial Mass Spectrometry Reviews DOI 10.1002/mas


&

FIGURE 16. Saturated charge densities of Fe(CO)5 doped Xe clusters obtained at 266 nm as a function of laser intensity.

FIGURE 17. Mass spectra of pure Xe clusters at (a) 266 nm, (b) 355 nm, (c) 532 nm.


21

&

BADANI ET AL.

ionization, cluster expands at a faster rate due to Coulombic repulsion, decreasing the density of inner ionized electrons drastically (Liu & Parashar, 2007). This decrease in density of inner ionized electrons would be more prominent in doped rare clusters as compared to pure clusters, due to presence of lower ionization energy of dopant molecule, that is, Fe(CO)5. This results in decreasing the efficiency of laser–cluster interaction. The inefficient laser–cluster interaction is manifested in terms of decrease in charge states of multiply charged atomic ions in rare gas clusters doped with Fe(CO)5. Thus the lowering of charge states of atomic ions in doped rare gas clusters is attributed to accelerated disintegration, upon facile initial ionization of dopant molecules. These results emphasize again the dominance of cluster expansion dynamics in the overall ionization process in laser irradiated clusters.

B. Role of Binding Energy in Governing Expansion Dynamics of Doped Clusters Studies described in Section 5.1 demonstrated that the expansion dynamics plays a significant role in governing the overall energy absorption by clusters from the optical fields. The experiments discussed were performed by keeping fixed dopant concentration and varying laser parameters (wavelength, intensity) and variation in expansion dynamics of clusters was attributed to the difference in the number of charged centers generated upon initial MPI step. In subsequent studies, the experiments were performed where the number of charged centers generated upon initial MPI step was held identical. For this purpose, experiments were performed at 1,064 nm, by changing the carrier gas (Xe and SF6). It is important to note here that due to high IE of Xe and SF6, they do not ionize

via multiphoton absorption process at 1,064 nm (I 5 109 W/cm2). Each of these clusters were doped with similar concentration of dopant molecule [Fe(CO)5]. The low IE dopant molecule [i.e., Fe(CO)5] ionizes easily at this wavelength. This ensures that number of charge centers generated during initial ionization step in these clusters are similar. Fe(CO)5 doped Xe clusters on interacting with laser pulses of 1,064 nm resulted in generation of multiply charged atomic ions of Xenþ(n ¼ 1–8) (Fig. 18). On changing the carrier gas from atomic system (Xe) to molecular system (SF6), the timeof-flight mass spectra (Fig. 19) revealed generation of multiply charged atomic ions of Fnþ (n ¼ 1–7). It was expected that the ionization efficiency of doped atomic cluster system with optical pulses would be higher, as compared to doped molecular cluster system, because molecular species possess several inherent dissipation channels such as dissociation, fragmentation, negative ion formation, etc. which are absent in atomic species. However the time-of-flight mass spectra revealed a contrast photo-ionization behavior for these doped cluster systems. The ionization energy of the highest observed charge state in case of Fe(CO)5 doped SF6 clusters is F7þ (IE ¼ 185 eV) which was quite high as compared to that of Fe(CO)5 doped Xe clusters [Xe8þ (IE ¼ 106 eV)]. This suggests that the efficiency of interaction of laser pulses with clusters, at a given wavelength, is of the following order: Fe(CO)5 doped SF6 clusters > Fe(CO)5 doped Xe clusters. A similar increasing trend of total ion yield was obtained in the charge density measurements suggesting that overall ionization efficiency of doped molecular clusters was higher than doped atomic clusters. In order to explain these results, one needs to consider the expansion dynamics of charged clusters (doped atomic and doped molecular), which are formed during initial MPI step,

FIGURE 18. Mass spectrum of Fe(CO)5 doped Xe clusters at 1,064 nm.

22



&

FIGURE 19. Mass spectrum of Fe(CO)5 doped SF6 clusters at 1,064 nm.

prior to its disintegration. In these experiments, since atomic and molecular clusters were doped with similar level of low IE dopant molecules [Fe(CO)5], the number of charge centers generated during initial MPI step at 1,064 nm is expected to be comparable. Hence, based on Equation (4), the expansion rate of charged clusters, due to Coulombic repulsive forces is expected to be similar in these cluster systems. However, since these clusters are made up of different constituents (i.e., Xe or SF6), the binding energies (BE) of these clusters are expected to be different. The BE of Xe–Xe and SF6–SF6 are 0.024 eV (Arunan, 2009) and 0.062 eV (Kolomiitsova et al., 2002) respectively. If one neglects the effect of dopant in altering the BE, then BE of these doped clusters follows the order: Fe(CO)5 doped SF6 clusters > Fe(CO)5 doped Xe clusters. Doppner et al. (2000) studied the effect of BE on expansion dynamics of clusters under Coulomb explosion conditions and demonstrated that for a uniformly ionized cluster carrying identical number of charge centers, the rate of cluster expansion decreases with increase in BE of clusters. Thus, Fe(CO)5 doped SF6 clusters, following multiphoton ionization, are expected to expand at a slower rate as compared to Fe(CO)5 doped Xe clusters, if one considers the effect of binding energy. This slower rate of expansion of Fe(CO)5 doped SF6 clusters would allow the inner ionized quasi free electrons to interact for longer duration with the optical pulse. This would facilitate extraction of more energy by inner ionized electrons from the optical pulse via IBS thereby enhancing the overall ionization efficiency of Fe(CO)5 doped SF6 clusters, as compared to Fe(CO)5 doped Xe clusters. Mass Spectrometry Reviews DOI 10.1002/mas

VI. CONCLUSIONS This work provides a simple and efficient method for generation of multiply charged atomic ions, under nanosecond laser conditions. Depending on the experimental requirement, the desired ions can be selectively gated and used. It has been shown that under nanosecond laser conditions, ionization efficiency of species can be significantly enhanced upon clusterization. The article discusses ionization processes that are exhibited by organometallic [such as Si(CH3)4 and Sn(CH3)4] and metal carbonyl [i.e., Fe(CO)5] cluster systems under low intensity (I 109 W/cm2) laser fields. Experiments have demonstrated generation of multiply charged atomic ions by means of multiple ionization and Coulomb explosion in clusters using laser intensity levels as low as 109 W/cm2. The laser intensity utilized to induce Coulomb explosion, is 106 times lower as compared to the conventionally used high intensity laser pulses (I 1015 W/cm2). The advantages of using nanosecond lasers, over femtosecond lasers are that they are relatively simple, lowcost and rugged, pulsed radiation sources. Results clearly demonstrate that wavelength of irradiation and the size of the cluster are crucial parameters. These two parameters can significantly affect the charge state as well as the total number density of the ions generated by the photoionization processes. The work of Yatsuhasi et al. and Dota et al. reports generation of multiply charged atomic ion (such as Si4þ and Fe6þ) upon exposure of organometallic monomers to femtosecond laser pulses of intensity 1015 W/cm2 (Yatsuhashi 23

&

BADANI ET AL.

& Nakashima, 2010; Yatsuhashi, Marukami, & Nakashima, 2011; Dota et al., 2012). This work demonstrates that even low intensity nanosecond laser pulses (109 W/cm2) can efficiently generate multiply charged atomic ions, upon irradiation of organometallic clusters. Thus, it would be interesting to investigate interaction of organometallic cluster with femtosecond laser pulses, since the maximum utilization of the femtosecond laser pulse can be achieved by allowing it to interact with the cluster instead of a monomer. This interaction will benefit from the near unity ionization probability of the femtosecond laser pulse (generating large number of caged electrons) as well as from the large number density of molecular species within the cluster, which can subsequently contribute to the overall ionization process. In contrast, though the application of a nanosecond laser is limited by the low ionization probability, it could still be a good choice since it allows sufficient interaction time which seems to compensate for the lower ionization probability. Till date, all the theoretical models that have been proposed to explain the Coulomb explosion process deal with the interaction of clusters with intense/ultra intense laser fields 1014 W/cm2. Present study shows generation of multiply charged atomic ions at gigawatt laser intensities. Although the three stage model rationalizes our observations qualitatively, the exact mechanism of laser–matter interaction operating in molecular clusters at 109 W/cm2 is not properly understood at present and more experimental and theoretical work is clearly needed before a clear picture can emerge.

REFERENCES Arunan E. 2009. Molecule matters van der Waals molecules. Resonance 14:1210–1222. Augst S, Strickland D, Meyerhofer DD, Chin SL, Eberly JH. 1989. Tunneling ionization of noble gases in a high-intensity laser field. Phys Rev Lett 63:2212–2215. Badani PM, Das S, Mundlapati VR, Sharma P, Vatsa RK. 2011. Interaction of nanosecond laser pulse with tetramethyl silane [Si(CH3)4] clusters: Generation of multiply charged silicon and carbon ions. AIP Adv 1:042164 (1–13). Badani PM, Das S, Sharma P, Vatsa RK. 2012. Effect of cluster expansion on photoionization of iron pentacarbonyl doped inert gas clusters under gigawatt intensity laser irradiation. Rapid Commun Mass Spectrom 26:2204–2210. Badani PM, Das S, Sharma P, Vatsa RK. 2013. Photoionization of atomic and molecular clusters doped with low ionization energy molecules: Effect of laser wavelength, intensity and cluster composition. Int J Mass Spectrom 348:53–58. Badani PM, Das S, Sharma P, Vatsa RK. 2014a. Evidence for charge induced dipole reaction in laser ionized van der waals clusters: A case of Fe2þ reacting with argon atoms inside a clusters. RSC Adv 4:2339– 2345. Badani PM, Das S, Sharma P, Vatsa RK. 2014b. Generation of multiply charged tin and carbon ions in low intensity Coulomb explosion of tetramethyl tin clusters: Role of screening effects. Int J Mass Spectrom 358:36–42. Benedek G, Martin TP, Pacchioni G, editors. 1987. Elemental and molecular clusters. Berlin: Springer-Verlag. pp. 12–44. Blumling DE, Sayres SG, Castleman AW Jr. 2011. Strong-field ionization and dissociation studies on small early transition metal carbide clusters via time-of-flight mass spectrometry. J Phys Chem A 115:5038–5043. Britnell L, Ribeiro RM, Eckmann A, Jalil R, Belle BD, Mishchenko A, Kim YJ, Gorbachev RV, Georgiou T, Morozov SV, Grigorenko AN, Geim AK, Casiraghi C, Castro Neto AH, Novoselov KS. 2013. Strong light-

24

matter interactions in heterostructures of atomically thin films. Science 340:1311–1314. Card DA, Wisniewski ES, Folmer DE, Castleman AW Jr. 2002. Dynamics of Coulomb explosion and kinetic energy release in clusters of heterocyclic compounds. J Chem Phys 116:3554–3567. Christensen C. 1982. Some emerging applications of lasers. Science 218:115–121. Das S, Sharma P, Majumder A, Vatsa RK. 2010. A technique for charge density measurement in laser-cluster interaction studies. J Indian Chem Soc 87:165–172. Das S, Badani PM, Sharma P, Vatsa RK. 2011a. Coulomb explosion phenomenon using gigawatt intensity laser fields: An exotic realm of laser-cluster interaction. Curr Sci 100:1008–1019. Das S, Badani PM, Sharma P, Vatsa RK, Majumder A, Das AK. 2011b. Multiphoton ionization and Coulomb explosion of C2H5Br clusters: A mass spectrometric and charge density study. Rapid Commun Mass Spectrom 25:1028–1036. Das S, Badani PM, Sharma P, Vatsa RK. 2012. Size and wavelength dependent photoionization of xenon clusters. Chem Phys Lett 552:13– 19. DiMauro LF, Freeman RR, Kulander KC. 2000. Multiphoton processes. Am Inst Phys 525:50–65. Ditmire T, Donnelly T, Rubenchik AM, Falcone RW, Perry MD. 1996. Interaction of intense laser pulses with atomic clusters. Phys Rev A53:3379–3402. Ditmire T, Zweiback J, Yanovsky VP, Cowan TE, Hays G, Wharton KB. 1999. Nuclear fusion from explosions of laser-heated deuterium clusters. Nature 398:489–492. Doppner T, Teuber S, Schumacher M, Tiggesbaumker J, Meiwes-Broer KH. 2000. Charging dynamics of metal clusters in intense laser fields. Appl Phys B 71:357–360. Dota K, Garg M, Tiwari AK, Dharmadhikari JA, Dharmadhikari AK, Mathur D. 2012. Intense two-cycle pulses induced time-dependent bond hardening in a polyatomic molecule. Phys Rev Lett 108:073602. Dunning FB, Hulet RG. 1996. Atomic, molecular and optical physics: Atoms and Molecules, Vol. 29B. California: Acedamic Press. pp. 115– 189. Feigerle CS, Bililign S, Miller JC. 2000. Nanochemistry-chemical reactions of iron and benzene within molecular clusters. J Nanoparticle Res 2:147–155. Fennel T, Meiwes-Broer KH, Tiggesbaumker J, Reinhard PG, Dinh PM, Suraud E. 2010. Laser driven non-linear cluster dynamics. Rev Mod Phys 82:1793–1842. Ford JV, Zhong Q, Poth L, Castleman AW Jr. 1999. Femtosecond laser interactions with methyl iodide clusters. 1. Coulomb explosion at 795 nm. J Chem Phys 110:6257–6267. Gets AV, Krainov VP. 2006. The ionization potentials of atomic ions in laserirradiated Ar, Kr and Xe clusters. J Phys B At Mol Opt Phys 39:1787– 1795. Guilhaus M. 1995. Principles and instrumentation in time-of-flight mass spectrometry. J Mass Spectrom 30:1519–1532. Haberland H, editor. 1994. Clusters of atoms and molecules. Berlin: Springer. pp. 56–64. Hagena OF. 1987. Condensation in free jets: Comparison of rare gases and metals. Z Phys D 4:291. Islam MR, Saalmann U, Rost JM. 2006. Kinetic energy of ions after Coulomb explosion of clusters induced by an intense laser pulse. Phys Rev A73:041201. Jha J, Krishnamurthy M. 2008. Hotter electron generation in doped clusters. J Phys B At Mol Opt Phys 41:041002 (1–6). Jha J, Mathur D, Krishnamurthy M. 2005. Enhancement in x-ray yields from heteronuclear cluster plasmas irradiated by intense laser light. J Phys B At Mol Opt Phys 38:L291–L299. Jha J, Mathur D, Krishnamurthy M. 2006a. Engineering clusters for tabletop acceleration of ions. Appl Phys Lett 88:041107 (1–3).



Jha J, Sharma P, Nataraju V, Vatsa RK, Mathur D, Krishnamurthy M. 2006b. Characterization of doping levels in heteronuclear, gas-phase, van der Waals clusters and their energy absorption from an intense optical field. Chem Phys Lett 430:26–31. Karras G, Kosmidis C. 2010. Multielectron dissociative ionization of CH3I clusters under moderate intensity ps laser irradiation. Int J Mass Spectrom 290:133–141. Keldysh LV. 1965. Ionization in the field of a strong electromagnetic wave. Sov Phys JETP 20:1307–1314. Kolomiitsova TD, Mielke Z, Shchepkin DN, Tokhadze KG. 2002. Infrared matrix isolation spectra of SF6 dimers. Chem Phys Lett 357:181–188. Kreibig U, Vollmer M, editors. 1995. Optical properties of metal clusters. Berlin: Springer-Verlag. pp. 108–116. Krishnan SR, Gopal R, Rajeev R, Jha J, Sharma V, Mudrich M, Moshammer R, Krishnamurthy M. 2014. Photoionization of clusters in intense fewcycle near infrared femtosecond pulses. Phys Chem Chem Phys 16:8721–8730. Kumarappan V, Krishnamurty M, Mathur D. 2001. Asymmetric high energy ion emission from argon clusters in intense laser fields. Phys Rev Lett 87:085005. Kumarappan V, Krishnamurty M, Mathur D. 2002. Two dimensional effects in the hydrodynamic expansion of xenon clusters under intense laser irradiation. Phys Rev A 66:033203. Kumarappan V, Krishnamurthy M, Mathur D. 2003. Explosions of water clusters in intense laser fields. Phys Rev A 67:063207. Leith EN, Upatnieks J. 1962. Reconstructed wavefronts and communication theory. J Opt Soc Am 52:1131–1142. Letokhov VS. 1977. Laser separation of isotopes. Ann Rev Phys Chem 28:133–159. Liu CS, Parashar J. 2007. Laser self-focussing and nonlinear absorption in expanding clusters. IEEE Trans Plasma Sci 35:1089–1097. Luo X, Niu C, Kong X, Wen L, Liang F, Pei K, Wang B, Li H. 2005. Cluster assisted generation of multiply charged atomic ions in nanosecond laser ionization of seeded methyl iodide beam. Chem Phys 310:17–24. Majumder C, Jayakumar OD, Vatsa RK, Kulshreshtha SK, Mittal JP. 1999. Multiphoton ionization of acetone at 355 nm: A time-of-flight mass spectrometry study. Chem Phys Lett 304:51–59. Mathur D. 2004. Cluster dynamics in intense laser fields. Adv Multiphoton Processes Spectrosc 16:273–306. Mathur D, Rajgara FA. 2010. Ionization and Coulomb explosion of xenon clusters by intensity, few-cycle laser pulses. J Chem Phys 133:061101 (1–4). Mathur D, Rajgara FA, Holkundkar AR, Gupta NK. 2010. Strong-field ionization and Coulomb explosion of argon clusters by few-cycle laser pulses. Phys Rev A 82:025201. McCarter B, Bililign S, Feigerle CS, Miller JC. 1999. Nanochemistry: Iron cluster reactions with methyl iodide. J Phys Chem A 103:6740– 6745. McPherson A, Luk TS, Thompson BD, Boyer K, Rhodes CK. 1993. Multiphoton-induced x-ray emission and amplification from clusters. Appl Phys B 57:337–347. McPherson A, Borisov AB, Boyer K, Rhodes CK. 1994a. Multiphotoninduced x-ray emission at 4–5 keV from Xe atoms with multiple core vacancies. Nature 370:631–634. McPherson A, Boyer K, Rhodes CK. 1994b. X-ray superradiance from multiphoton excited clusters. J Phys B At Mol Opt Phys 27:L637– L642. McPherson A, Borisov AB, Boyer K, Rhodes CK. 1996. Competition between multiphoton xenon cluster excitation and plasma wave Raman scattering at 248 nm. J Phys B At Mol Opt Phys 29:L291–L298. Micheau S, Gutierrez FA, Pons B, Jouin H. 2005. Screening models for laser-cluster interactions. J Phys B At Mol Opt Phys 38:3405–3422. Mulser P, Bauer D. 2010. High power laser-matter interaction, Vol. 238. Berlin, Heidelberg: Springer Tracts in Modern Physics. pp. 19–21.


&

Murillo MS, Weisheit JC. 1998. Dense plasmas, screened interactions and atomic ionization. Phys Rep 302:1–65. Nakashima N, Shimizu S, Yatsuhashi T, Sakabe S, Izawa Y. 2000. Large molecules in high intensity laser fields. J Photochem Photobiol C Photochem Rev 1:131–143. Peticolas WL. 1967. Multiphoton spectroscopy. Ann Rev Phys Chem 18:233–260. Purnell J, Snyder EM, Wei S, Castleman AW Jr. 1994. Ultrafast laserinduced Coulomb explosion of clusters with high charge states. Chem Phys Lett 229:333–339. Rajeev R, Trivikram T, Rishad KPM, Narayanan V, Krishnakumar E, Krishnamurthy M. 2013. A compact laser-driven plasma accelerator for megaelectronvolt-energy neutral atoms. Nature Phys 9:185–190. Rose-Petruck C, Schafer KJ, Barty CPJ. 1997. Ultrafast electron dynamics and inner-shell ionization in laser driven clusters. Phys Rev A 55:1182–1190. Saalmann U, Rost JM. 2011. Atomic clusters in intense laser fields. Handb Nanophys 2:1–14. Sattler K, Muhlback J, Echt O, Pfau P, Recknagel E. 1981. Evidence for Coulomb explosion of doubly charged microclusters. Phys Rev Lett 47:160–163. Saunders WA. 1992. Metal-cluster fission and the liquid-drop model. Phys Rev A 46:7028–7041. Saunders WA, Dam N. 1991. Liquid drop model of multiply charged metal cluster fission. Z Phys D—Atoms, Mol Clusters 20:111–113. Sayres SG, Ross MW, Castleman AW Jr. 2011. Metallic behavior of transition metal oxide clusters under strong-field excitation. J Chem Phys 135:054312. Scoles G, editor. 1988. Atomic and molecular beam methods. New York: Oxford University Press. pp. 123–135. Sharma P, Vatsa RK. 2008. Photochemistry of acetone clusters: Size dependent observation of Coulomb explosion in the multiphoton ionization regime. Europhys Lett 84:43003 (1–6). Sharma P, Vatsa RK, Kulshreshtha SK, Jha J, Mathur D, Krishnamurthy M. 2006. Energy pooling in multiple ionization and Coulomb explosion of clusters by nanosecond-long, megawatt laser pulses. J Chem Phys 125:034304. Sheehy B. 2001. 10 Chemical processes in intense optical fields. Annu Rep Prog Chem Sect C Phys Chem 97:383–410. Snyder EM, Wei S, Purnell J, Buzza SA, Castleman AW Jr. 1996. Femtosecond laser-induced Coulomb explosion of ammonia clusters. Chem Phys Lett 248:1–7. Wang W, Li H, Niu D, Wen L, Zhang N. 2008. Cluster-assisted multipleionization of methyl iodide by a nanosecond laser: Wavelength dependence of multiple-charge ions. Chem Phys 352:111–116. Wieneke S, Bruckner S, Viol W. 2010. Interaction of short laser pulses with gases and ionized Gases. Croatia: InTech. pp. 156–158. Yatsuhashi T, Nakashima N. 2010. Dissociation and multiply charged silicon ejection in high abundance from hexamethyldisilane. J Phys Chem 114:11890–11895. Yatsuhashi T, Marukami E, Nakashima N. 2011. Fezþ (z ¼ 1-6) generation from ferrocene. Phys Chem Chem Phys 13:4234–4238. Zhang N, Wang W, Cang H, Wang H, Li H. 2009. Multiply ionization of benzene clusters by a nanosecond laser: Distributions of the ion charge state and the electron energy. Chem Phys Lett 469:14–18. Zhang N, Wang W, Zhao W, Han F, Li. H. 2010. Multiply ionization of diethyl ether clusters by 532 nm nanosecond laser: The influence of laser intensity and the electron energy distribution. Chem Phys 373:181–185. Zhao W, Wang W, Qu P, Hou K, Li. H. 2012. Multiple ionization of CH3I clusters by nanosecond laser: Electron energy distribution and formation mechanism of multiply charged ions. Chem Phys Lett 543:55–60. Zuo T, Chelkowski S, Bandrauk AD. 1993. Harmonic generation by the H2þ molecular ion in intense laser fields. Phys Rev A 48:3837–3844.

25

Attosecond nonlinear optics using gigawatt-scale isolated attosecond pulses.

About systematic errors in charge-density studies.

Mass Spectrometric Imaging Using Laser Ablation and Solvent Capture by Aspiration (LASCA).

Mass spectrometric sampling of a liquid surface by nanoliter droplet generation from bursting bubbles and focused acoustic pulses: application to studies of interfacial chemistry.

ionization mass spectrometric imaging.

ionization.

Mass spectrometric studies of reductive elimination from Pd(IV) complexes.

Optimal control of charge transfer for slow H+ + D collisions with shaped laser pulses.

Increasing protein charge state when using laser electrospray mass spectrometry.

Cu2+-assisted two dimensional charge-mass double focusing gel electrophoresis and mass spectrometric analysis of histone variants.

Exploring cyclopentadienone antiaromaticity: charge density studies of various tetracyclones.

Tautomerism and proton transfer in photoionized acetaldehyde and acetaldehyde-water clusters.

Studies on advanced glycation end products by recent mass spectrometric techniques.

Sunitinib: from charge-density studies to interaction with proteins.

Mass spectrometric characterization of different norandrosterone derivatives by low-cost mass spectrometric detectors using electron ionization and chemical ionization.

Three-body fragmentation of CO2 driven by intense laser pulses.

Annealing of SnO₂ thin films by ultra-short laser pulses.

Mass spectrometric metabolomic imaging of biofilms on corroding steel surfaces using laser ablation and solvent capture by aspiration.

Self-referenced characterization of femtosecond laser pulses by chirp scan.

Detection of formestane abuse by mass spectrometric techniques.

Intense isolated few-cycle attosecond XUV pulses from overdense plasmas driven by tailored laser pulses.

Breaking DNA strands by extreme-ultraviolet laser pulses in vacuum.

ionization mass spectrometric analysis.

Laser microprobe mass spectrometric identification of cyclosporin-induced intrarenal microliths in rat.