Article pubs.acs.org/JPCA
Time-Resolved Excited State Energetics of the Solvated Electron in Sodium-Doped Water Clusters J. P. Müller,† N. Zhavoronkov, I. V. Hertel,‡ and C. P. Schulz* Max-Born-Institute, Max-Born-Strasse 2a, 12489 Berlin, Germany ABSTRACT: The energetics and dynamics of the first electronically excited state of solvated electron in sodium-doped water clusters has been studied, by means of timeresolved electron spectra created in a pump−probe fs-laser experiment. The Na ··· (H2O)n clusters were excited by pulses at a wavelength of 795 nm, while ionization was achieved at a wavelength of 398 nm, and the overall cross-correlation fwhm was about 50 fs. Mass-resolved electron spectra were taken using photoelectron−photoion coincidence (PEPICO) spectroscopy for cluster sizes ranging from n = 1 up to 22. The electron spectra give new insights into the dynamics of the excited state of solvated electrons in Na ··· (H2O)n clusters. These dynamics are compared to known results for water cluster anions. In both cases, the observed dynamics are a combination of solvent rearrangement and internal energy conversion.
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INTRODUCTION
have been investigated thoroughly by the Neumark group (see, e.g., refs 10 and 11). Alternatively, solvent clusters doped with a single alkali atom are used to study the formation and dynamics of the solvated electrons, a pathway pursued by our group for many years.12−16 In these clusters the valence electron is separated from the alkali ion core as soon as the cluster contains more than four solvent molecules. Theoretical calculations have shown that the thus formed electron is delocalized within the water cluster.17−19 The proximity of the alkali ion to the separated electron strongly influences the excitation energies, but spectroscopic studies13,14 have demonstrated that the absorption bands start to resemble that of the solvated electron as the size of the cluster increases. The dynamics of the excited state of sodium-doped water clusters Na···(H2O)n have been investigated by femtosecond pump−probe experiments, which revealed that the excited states are short-lived.16 The two color pump−probe scheme allows one to follow the population of the first electronically excited à state of sodium water clusters after the initial excitation from the X̃ ground state. The energetics and dynamics of the à state have been discussed in detail in previous publications.14−16 Since the present work is based on these studies, we briefly summarize the key results. The absorption band for the à ← X̃ transition in Na··· (H2O), the monomer, is narrow (≈ 0.15 eV fwhm) with a maximum at 1.7 eV. Toward larger cluster sizes it shifts toward lower energies with a maximum at ≈1.2 eV for n = 3,4 and its width becomes broader (up to 1 eV) fwhm. For even larger
1
Fifty years after its discovery the solvated electron in liquid water is still a challenging system. [According to ref 2, a solvated electron is ”a fully relaxed electron within the liquid in which all solvent degrees of freedom are in thermal equilibrium”.] It is unique in as far as it is the smallest possible radical, and as such it is of relevance for a broad range of chemical and biological processes. It has been shown, e.g., experimentally, that the solvated electron plays an important role in the mechanisms of radiation damage of biological matter,3 and quite recently, the solvated electron was found even in the yellow photoactive protein after photoionization:4 it was identified by its characteristic, broad absorption band, which extends over the whole visible spectral range with a maximum at 720 nm and has a finite lifetime. This spectrum reflects a transition from the electronic ground state to the first excited state and subsequent radiationless decay.1,5 The excitation and de-excitation dynamics of the solvated electron in neat water has been studied intensely by femtosecond transient absorption spectroscopy.6−9 Three different time scales have been observed, ranging from a few tens of femtoseconds to picoseconds. They have been attributed to reorientation of the solvent molecules, decay of the excited state wave function, and relaxation processes within the solvent, but this assignment is still under debate. An alternative approach to study the interaction of the solvated electron with its solvent environment uses solvent clusters. In these model systems, the buildup of the solvation of the electron and its dynamics can be studied as a function of the number of solvent molecules. Two kinds of solvent clusters are studied experimentally as well as theoretically already for many years. One possibility is to exploit negatively charged clusters of water and other solvents. The photodynamics of such systems © 2014 American Chemical Society
Special Issue: A. W. Castleman, Jr. Festschrift Received: March 4, 2014 Revised: June 17, 2014 Published: June 17, 2014 8517
dx.doi.org/10.1021/jp502238c | J. Phys. Chem. A 2014, 118, 8517−8524
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ground state, E* the energy deposited into the excited à state, and E+# the ro-vibrational energy remaining in the ion after the ionization process. The kinetic energy E of the photoelectron is thus given by
clusters this trend reverses and the maximum shifts slowly toward higher energies.14 Thus, fs pulses from a Titanium-Sapphire laser with 1.56 eV photon energy (NIR) are well suited to excite the à state for cluster sizes up to at least n = 40. The ionization potential (IP) of Na ··· (H2O)n drops from 4.3 eV for n = 1 to 3.2 eV for n = 4 and remains almost constant for all larger clusters.12,20 Therefore, ultraviolet (UV) pulses of 3.12 eV photon energy are able to ionize excited clusters of all sizes. [Recently, Na··· (H2O)n isomers with an IP of 2.8 eV for n ≥ 15 have been identified,20 which can be ionized directly by the UV pulse. In our two-color pump−probe experiment,16 no such direct ionization has been observed. This may be attributed to a different isomer distribution in the cluster beam.] It was found, that the Na···(H2O)+n ion signal decays exponentially with pump−probe delay Δt. This is attributed to a finite lifetime τ of the excited cluster Na*···(H2O)n in the à state. With increasing cluster size τ drops dramatically, from 1.2 ps for the dimer to less than 300 fs for the quadrumer. For cluster sizes n ≥ 10, it reaches values τ ≤ 120 fs. These short lifetimes have been attributed to a very efficient internal conversion (IC). Alternatively, one may envision conical intersections to be involved in the process. In any case, the electronic excitation energy of the initially populated à state is transferred into ro-vibrational excitation of the X̃ ground state. Typically, this will lead to a change of the electronic transition moment and a loss of Franck−Condon overlap to the ionic ground state, as schematically illustrated in Figure 1. What is denoted there as reaction coordinate could, e.g., be a characteristic solvent−solvent distance or angle.
E = hvUV − IP + E* − E +#
(2)
If we assume the initial ro-vibrational energy of the X̃ state to be negligible, the energy deposited into the excited state is determined by the absorbed NIR photon E* = hνNIR. Thus, by measuring the electron kinetic energy E, one determines the internal energy E+# of the final ionif the simple reaction scheme eq 1 holds. The maximum possible kinetic energy of the photoelectron is thus expected to be Emax = hvUV + hvNIR − IP
(3)
in the case that no ro-vibrational energy is deposited into the ion, i.e., E+# = 0. In the previous experiments, no information was obtained on E and E+#. To gain further insight into the energy redistribution process, in the present work, time- and mass-resolved photoelectron spectra are reported. While for negatively charged water clusters such data can be obtained in a relatively straightforward manner by size selection of the cluster anions prior to the interaction with the laser field, for neutral clusters it is not possible to select individual masses from the broad distribution of n which emerges from the neutral cluster source. Hence, the photoelectron−photoion coincidence (PEPICO) technique has been applied in the present experiment to obtain time and mass resolved kinetic energy distributions of the photoelectrons. For femtosecond pump−probe experiments, this type of coincidence technique has been applied for the first time in our laboratory several years ago.22 In the next section the experimental setup and data acquisition scheme used is described. Thereafter the experimental results are presented and discussed.
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METHODS The sodium−water clusters used in this experiment are prepared in a pick-up source. This source has been described in detail in previous publications.16,23 In the present experiment, water clusters are generated in a continuous molecular beam by expanding water vapor from an oven at 70 °C (corresponding to ca. 310 hPa) through a conical nozzle of 50 μm diameter into vacuum. The nozzle is heated to a 10 °C higher temperature than the oven to avoid clogging of the nozzle by ice. Helium (at ca. 4000 hPa) is used as coexpanding gas to create mainly smaller clusters in an intense molecular beam. The molecular beam passes through a doping cell containing sodium vapor at a temperature of 163 °C (corresponding to a vapor pressure of 2 × 10−5 hPa). At these conditions, the water clusters do typically pick up no more than one single sodium atom. The temperature of the water oven and the doping cell are regulated by PID-controllers to allow for a stable operation of the clusters source over 24h. The temperature of the water clusters formed under these conditions will be about 70 K, as determined by Buck and coworkers for a similar cluster source.24 Due to the strong intermolecular binding energies the pick-up process will not change the temperature of the clusters considerably.25 Nevertheless, a distribution of isomers will be present in the cluster beam.
Figure 1. Very schematic and highly simplified energy diagram of the pump−probe process in Na···(H2O)n clusters.
By comparing the lifetimes of Na*···(H2O)n with that of the deuterated species Na*···(D2O)n, and applying the energy gap law,21 the high frequency symmetric and antisymmetric stretch vibrations of the water molecules have been identified as the promoting modes for the conversion process.16 Explicitly the dynamics depicted in Figure 1 may be summarized by Na···(H 2O)n + hνNIR → Na*···(H 2O)n τ Na*···(H 2O)n → Na···(H 2O)n# Na*···(H 2O)n + hνUV(Δt ) → Na +···(H 2O)n# + e−(E) (1)
with the pump (hνNIR) and probe (hνUV) photon, the ionization potential IP with respect to the lowest vibrational 8518
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The interaction region is kept at ground potential during the interaction of the laser pulses with the cluster beam. After ≈200 ns, when the electrons have left the region between the attractor and repeller electrode, the potentials of the attractor electrode and all the electrodes below are switched to negative potential. The lowest electrode has a potential of −1000 V. The same holds for the ion flight tube. The repeller remains at ground potential. The cations are accelerated by the fields toward the MCP 2 detector. The transmission of the ion spectrometer can be optimized for a certain mass range by a pair of deflector electrodes, to counteract the velocity of the clusters in the molecular beam. This optimization is crucial to allow for an efficient detection of coincidence events for a broader cluster size distribution from Na···(H2O)n cluster sizes of n = 1 up to 30. The electrical pulses created by the two MCP are digitized by an Acqiris AP240 digitizer card using the SSR firmware. The digitized waveform of the MCP signals is transferred to the computer memory and processed by a software-implemented constant fraction discriminator, enabling a precise determination of the arrival times of the electrons and ions. Using these arrival times, a histogram is generated for the time-of-flight of all electrons he,a and ions hi hitting the detectors. Finally, a list of coincidence events is compiled. If for one laser shot exactly one electron and ion is detected, this is denoted as a coincidence event. The arrival times of the electrons and ions for each event is recorded in a list. From this list, the mass selected electron spectra he,X are derived by summing up the electron counts within a certain range of ion times of flight. The key problem of this PEPICO experiment with pulsed lasers is avoiding false coincidences. Typically, in experiments with pulsed lasers, many ionization events occur during one laser pulse. This must be avoided in the present experiment, by keeping the target density and the laser intensity low enough. To compensate for this, one aims at a high pulse repetition rate and usually has to accept long data acquisition times. Quantitatively, from the equations in ref 22, one obtains the average ionization probability n,̅ the detection efficiencies for electrons ξe and ions ξi, and the ratio between false and all (f) registered coincidences rf = w(f) 11 /w11. False coincidences w11 arise from detecting a coincidence event in which the electron and the ion originate from different ionization events. This rate should be kept at a reasonably low level while at the same time the counting statistics demands that many events are registered. The average ionization rate (per laser pulse) n̅ was kept at values below 0.5 in order to keep rf sufficiently low. Figure 3 shows as an example a coincidence spectrum taken at a delay Δt = 55 fs. The two-dimensional histogram shows the time-of-flight of all electrons and ions detected in coincidence. The yield of all detected ions is plotted as a function of their time-of-flight on the left of Figure 3, and on top the integrated spectrum of all detected electrons is shown. In the time-of-flight mass spectrum cluster, ions up to n = 30 are visible. In addition, Na+ and Na+2 ions and a trace of K+ ions are present in the mass spectrum, which originate from the sodium oven. The electron signals of these masses have been used to calibrate the energy scale of the electron spectra. With this calibration, the electron time-of-flight values te are converted to the kinetic energy of the electrons
In the following chamber, the doped clusters are excited by femtosecond laser pulses of 1.56 eV photon energy (NIR) and ionized by pulses of 3.12 eV photon energy (UV). The laser pulses were obtained using a commercial CPA laser system (Spectra Physics, Spitfire) providing femtosecond pulses of 35 fs duration (fwhm). The pulses have been characterized using SPIDER. The main laser beam is split into two parts to create the pump and probe laser pulses. The pulses in the pump beam may be delayed by a delay stage with a time resolution of 0.67 fs (0.1 μm positioning accuracy). The pulses in the probe beam are frequency doubled in a thin BBO-crystal (100 μm thickness) in order to keep the pulse duration of the UV pulses short. Since the temporal shape of the NIR pulses is nearly Gaussian, the cross-correlation of the pump and probe pulses can be calculated and amounts to 50 fs fwhm. Pump and probe beam are focused into the ionization region in a nearly collinear geometry by spherical mirrors of 1000 mm and 1500 mm radius. The beams intersect at an angle smaller than 2°. The peak intensity of both beams in the ionization region is ca. 1.5 × 109 W cm−2 at a pulse energy of 5 μJ. The electrons and cations generated during the ionization of the clusters are detected in a newly built electron−ioncoincidence spectrometer, which is schematically shown in Figure 2. The setup was derived from an earlier design
Figure 2. Schematic setup of coincidence-spectrometer and data acquisition. The upper left shows a blow up of the ionization and extraction region. The electronics and software is sketched in the lower left (gray shaded): pulses from microchannel plates MPC 1 (electrons) and MPC 2 (ions) are registered each by fast 8-bit-analog digital converter-cards (ADC); the time-of-flight (TOF) of the particles is determined by a software constant fraction discriminators (CFD). The PC records the list of coincidence events and generates histograms as required.
developed in our group, using a combined magnetic bottle electron and Wiley−McLaren type ion time-of-flight spectrometer.22 The magnetic field that collects the electrons is generated by four strong NeFeB magnets. They are positioned below the ionization region, within the attractor electrode. The north poles of these magnets are pointing toward the central hole of the electrode. The electrons are guided by the weak field of a coil within a μ-metal shielded flight tube, extending from the electrode above the ionization region to the microchannel plate detector (MCP 1).
E(te) = A · 8519
l2 + E0 (te − t0)2
(4)
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Figure 4. Color coded plot of the electron count rate as a function of the pump−probe pulse delay Δt and the electron kinetic energy E. The step size of the delay time was 25 fs. The measured electron counts have been smoothed by convolution with a Gaussian of 0.075 eV fwhm. Red dots indicate the delays times Δt at which the coincidence spectra have been taken, which will be discussed in the following.
Figure 3. Middle: 2D plot of the coincidence signal as a function of the electron and ion time-of-flight at a given pulse delay Δt = 55 fs. On the left, the yield hi is plotted for all ions detected as a function of their time-of-flight. Similarly, on the top the yield he, a of all electrons detected is plotted as a function of the electron time-of-flight. The color coded 2D-histogram is created from the arrival time list of the electron−ion coincidence events.
small and larger cluster sizes (IP = 4.3 eV for n = 1, while IP = 3.2 eV for n ≥ 4). The total photon energy available for ionization by the NIR and UV photons is 1.56 eV + 3.12 eV = 4.68 eV. Thus, for the smallest clusters with long life times, the electron kinetic energy will be E ≤ 0.38 eV (highest probability observed for 0.1 eV), while for clusters with n ≥ 4 and a lifetime τ ≤ 100 fs, we expect E ≤ 1.5 eV (highest probability observed for 0.8 eV). The next step is of course to really measure the electron kinetic energy distribution and time dependence for each cluster size by coincidence with the respective ion signal. As explained above, the count rate in a genuine coincidence measurement must be very small (significantly less than 1 event per laser shot, i.e., much less than 1 for a specific cluster size). Hence, since the measuring time for coincidence spectra is long, we have recorded coincidence spectra only for a few selected delay times Δt between NIR and UV pulse, as marked by red dots on the right side of Figure 4. Figure 5 shows the thus determined kinetic energy distributions of the photoelectrons for the Na···(H2O)n monomer, dimer, and trimer at all pump−probe delay times
with the constants A = 2.843 × 106 eV ns2 m−2 and l = 0.712 m, and the parameters t0 = (361 ± 25) ns and E0 = (−0.6 ± 0.1) eV. The electron spectra are created by selecting the coincidence signal in a certain ion time-of-flight range and converting each electron time-of-flight values to E using eq 4. The result is sorted into a histogram with a binning interval of 0.01 eV. Since the mass selected electron spectra contain only a few hundred electrons, the spectra are smoothed with a Gaussian of 0.045 eV fwhm, well below the resolution limit of the magnetic bottle spectrometer. [Small shifts of the spectrometer calibration due to charging effects in long-term operation are recalibrated at each experimental run by adjusting the parameter E0 with the help of the spectrum for the K+ ion (the shift is typically 150 fs. The mean kinetic energy E̅ A of the low energy part A also drops for increasing pump−probe delay but less is pronounced and on a longer time scale. The behavior for n = 8, 9, and 10 is quite similar, except that no rise of the low energy average is observed for longer delay times. The difference of the mean energies between the low and high energy parts of all cluster sizes shown in Figure 7b is about 0.6 eV. Since the value of E̅ B drops only by 0.1 eV within 100 fs, it is safe to assume that the electrons of the high energy part B and the ones from the low energy part A come from different excitation and ionization processes. In our earlier pump−probe study,16 a small long-lived contribution was identified in the ion signal. The low energy part A of the electron signal is also small and long-lived. Consequently, one may assume that both observations have the same origin. There are several possible mechanisms to explain the bimodal structure of the kinetic energy distributions of the photoelectrons with their respective decay times. The low 8522
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(2) Chen, X. Y.; Bradforth, S. E. The Ultrafast Dynamics of Photodetachment. Annu. Rev. Phys. Chem. 2008, 59, 203−231. (3) Wang, C. R.; Nguyen, J.; Lu, Q. B. Bond Breaks of Nucleotides by Dissociative Electron Transfer of Nonequilibrium Prehydrated Electrons: A New Molecular Mechanism for Reductive DNA Damage. J. Am. Chem. Soc. 2009, 131, 11320−11322. (4) Zhu, J. Y.; Paparelli, L.; Hospes, M.; Arents, J.; Kennis, J. T. M.; van Stokkum, I. H. M.; Hellingwerf, K. J.; Groot, M. L. Photoionization and Electron Radical Recombination Dynamics in Photoactive Yellow Protein Investigated by Ultrafast Spectroscopy in the Visible and Near-Infrared Spectral Region. J. Phys. Chem. B 2013, 117, 11042−11048. (5) Jou, F. Y.; Freeman, G. R. Temperature and Isotope Effects on the Shape of the Optical Absorption Spectrum of Solvated Electrons in Water. J. Phys. Chem. 1979, 83, 2383−2387. (6) Yokoyama, K.; Silva, C.; Son, D. H.; Walhout, P. K.; Barbara, P. F. Detailed Investigation of the Femtosecond Pump-Probe Spectroscopy of the Hydrated Electron. J. Phys. Chem. A 1998, 102, 6957−6966. (7) Assel, M.; Laenen, R.; Laubereau, A. Femtosecond Solvation Dynamics of Solvated Electrons in Neat Water. Chem. Phys. Lett. 2000, 317, 13−22. (8) Pshenichnikov, M. S.; Baltuska, A.; Wiersma, D. A. HydratedElectron Population Dynamics. Chem. Phys. Lett. 2004, 389, 171−175. (9) Thaller, A.; Laenen, R.; Laubereau, A. Femtosecond Spectroscopy of the Hydrated Electron: Novel Features in the Infrared. Chem. Phys. Lett. 2004, 398, 459−465. (10) Griffin, G. B.; Young, R. M.; Ehrler, O. T.; Neumark, D. M. Electronic Relaxation Dynamics in Large Anionic Water Clusters: (H2O)−n and (D2O)−n (n = 25−200). J. Chem. Phys. 2009, 131, 194302. (11) Young, R. M.; Neumark, D. M. Dynamics of Solvated Electrons in Clusters. Chem. Rev. 2012, 112, 5553−5577. (12) Hertel, I. V.; Hü glin, C.; Nitsch, C.; Schulz, C. P. Photoionization of Na(NH3)N and Na(H2O)N Clusters - A Step Towards the Liquid-Phase. Phys. Rev. Lett. 1991, 67, 1767−1770. (13) Brockhaus, P.; Hertel, I. V.; Schulz, C. P. Electronically Excited States in Size-selected Solvated Alkali Metal Atoms. III. Depletion Spectroscopy of Na(NH3)n-Clusters. J. Chem. Phys. 1999, 110, 393− 402. (14) Schulz, C. P.; Bobbert, C.; Shimosato, T.; Daigoku, K.; Miura, N.; Hashimoto, K. Electronically Excited States of Sodium−Water Clusters. J. Chem. Phys. 2003, 119, 11620−11629. (15) Schulz, C. P.; Scholz, A.; Hertel, I. V. Ultrafast Energy Redistribution in Photoexcited Sodium-Ammonia Clusters. Isr. J. Chem. 2004, 44, 19−25. (16) Liu, H. T.; Müller, J. P.; Zhavoronkov, N.; Schulz, C. P.; Hertel, I. V. Ultrafast Dynamics in Na-Doped Water Clusters and the Solvated Electron. J. Phys. Chem. A 2010, 114, 1508−1513. (17) Hashimoto, K.; Kamimoto, T. Theoretical Study of Microscopic Solvation of Lithium in Water Clusters: Neutral and Cationic Li(H2O)n (n = 1−6 and 8). J. Am. Chem. Soc. 1998, 120, 3560−3570. (18) Tsurusawa, T.; Iwata, S. Theoretical Studies of Structures and Ionization Threshold Energies of Water Cluster Complexes with a Group 1 Metal, M(H2O)n (M = Li and Na). J. Phys. Chem. A 1999, 103, 6134−6141. (19) Cwiklik, L.; Buck, U.; Kulig, W.; Kubisiak, P.; Jungwirth, P. A Sodium Atom in a Large Water Cluster: Electron Delocalization and Infrared Spectra. J. Chem. Phys. 2008, 128, 154306. (20) Forck, R. M.; Dauster, I.; Schieweck, Y.; Zeuch, T.; Buck, U.; Oncak, M.; Slavicek, P. Communications: Observation of Two Classes of Isomers of Hydrated Electrons in Sodium-water Clusters. J. Chem. Phys. 2010, 132, 221102. (21) Englman, R.; Jortner, J. Energy Gap Law for Radiationless Transitions in Large Molecules. Mol. Phys. 1970, 18, 145−164. (22) Stert, V.; Radloff, W.; Schulz, C. P.; Hertel, I. V. Ultrafast Photoelectron Spectroscopy: Femtosecond Pump-Probe Coincidence Detection of Ammonia Cluster Ions and Electrons. Eur. Phys. J. D 1999, 5, 97−106. (23) Bobbert, C.; Schulz, C. P. Solvation and Chemical Reaction of Sodium in Water Clusters. Eur. Phys. J. D 2001, 16, 95−97.
does not allow a deeper insight into the internal conversion (IC) process of the electronic excitation into vibrationally excited ground state. At this point it has to remain open whether or not solvent rearrangement and IC take place on the same time scale. Referring to eq 1: it is unclear whether the reaction in line 3 is preceded by an internal rearrangement process in the excited Na*···(H2O)n system.
4. CONCLUSIONS Time resolved electron kinetic energy distributions of Na··· (H2O)n clusters with n = 1 to 22 have been measured by combining two-color femtosecond pump−probe techniques with photoelectron−photoion coincidence spectroscopy. These electron spectra are fingerprints of the energetics and dynamics of the first electronically excited à state being resonant with the pump laser. The electron energy distribution is narrow for the monomer (fwhm = 0.15 eV) and dimer (≈ 0.6 eV). For larger clusters (n ≥ 3) the electron spectra are broad and more structured. Two distinctively different parts of the spectrum can be distinguished, both representing different dynamics: The signal at lower kinetic energies (
Time-Resolved Excited State Energetics of the Solvated Electron in Sodium-Doped Water Clusters J. P. Müller,† N. Zhavoronkov, I. V. Hertel,‡ and C. P. Schulz* Max-Born-Institute, Max-Born-Strasse 2a, 12489 Berlin, Germany ABSTRACT: The energetics and dynamics of the first electronically excited state of solvated electron in sodium-doped water clusters has been studied, by means of timeresolved electron spectra created in a pump−probe fs-laser experiment. The Na ··· (H2O)n clusters were excited by pulses at a wavelength of 795 nm, while ionization was achieved at a wavelength of 398 nm, and the overall cross-correlation fwhm was about 50 fs. Mass-resolved electron spectra were taken using photoelectron−photoion coincidence (PEPICO) spectroscopy for cluster sizes ranging from n = 1 up to 22. The electron spectra give new insights into the dynamics of the excited state of solvated electrons in Na ··· (H2O)n clusters. These dynamics are compared to known results for water cluster anions. In both cases, the observed dynamics are a combination of solvent rearrangement and internal energy conversion.
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INTRODUCTION
have been investigated thoroughly by the Neumark group (see, e.g., refs 10 and 11). Alternatively, solvent clusters doped with a single alkali atom are used to study the formation and dynamics of the solvated electrons, a pathway pursued by our group for many years.12−16 In these clusters the valence electron is separated from the alkali ion core as soon as the cluster contains more than four solvent molecules. Theoretical calculations have shown that the thus formed electron is delocalized within the water cluster.17−19 The proximity of the alkali ion to the separated electron strongly influences the excitation energies, but spectroscopic studies13,14 have demonstrated that the absorption bands start to resemble that of the solvated electron as the size of the cluster increases. The dynamics of the excited state of sodium-doped water clusters Na···(H2O)n have been investigated by femtosecond pump−probe experiments, which revealed that the excited states are short-lived.16 The two color pump−probe scheme allows one to follow the population of the first electronically excited à state of sodium water clusters after the initial excitation from the X̃ ground state. The energetics and dynamics of the à state have been discussed in detail in previous publications.14−16 Since the present work is based on these studies, we briefly summarize the key results. The absorption band for the à ← X̃ transition in Na··· (H2O), the monomer, is narrow (≈ 0.15 eV fwhm) with a maximum at 1.7 eV. Toward larger cluster sizes it shifts toward lower energies with a maximum at ≈1.2 eV for n = 3,4 and its width becomes broader (up to 1 eV) fwhm. For even larger
1
Fifty years after its discovery the solvated electron in liquid water is still a challenging system. [According to ref 2, a solvated electron is ”a fully relaxed electron within the liquid in which all solvent degrees of freedom are in thermal equilibrium”.] It is unique in as far as it is the smallest possible radical, and as such it is of relevance for a broad range of chemical and biological processes. It has been shown, e.g., experimentally, that the solvated electron plays an important role in the mechanisms of radiation damage of biological matter,3 and quite recently, the solvated electron was found even in the yellow photoactive protein after photoionization:4 it was identified by its characteristic, broad absorption band, which extends over the whole visible spectral range with a maximum at 720 nm and has a finite lifetime. This spectrum reflects a transition from the electronic ground state to the first excited state and subsequent radiationless decay.1,5 The excitation and de-excitation dynamics of the solvated electron in neat water has been studied intensely by femtosecond transient absorption spectroscopy.6−9 Three different time scales have been observed, ranging from a few tens of femtoseconds to picoseconds. They have been attributed to reorientation of the solvent molecules, decay of the excited state wave function, and relaxation processes within the solvent, but this assignment is still under debate. An alternative approach to study the interaction of the solvated electron with its solvent environment uses solvent clusters. In these model systems, the buildup of the solvation of the electron and its dynamics can be studied as a function of the number of solvent molecules. Two kinds of solvent clusters are studied experimentally as well as theoretically already for many years. One possibility is to exploit negatively charged clusters of water and other solvents. The photodynamics of such systems © 2014 American Chemical Society
Special Issue: A. W. Castleman, Jr. Festschrift Received: March 4, 2014 Revised: June 17, 2014 Published: June 17, 2014 8517
dx.doi.org/10.1021/jp502238c | J. Phys. Chem. A 2014, 118, 8517−8524
The Journal of Physical Chemistry A
Article
ground state, E* the energy deposited into the excited à state, and E+# the ro-vibrational energy remaining in the ion after the ionization process. The kinetic energy E of the photoelectron is thus given by
clusters this trend reverses and the maximum shifts slowly toward higher energies.14 Thus, fs pulses from a Titanium-Sapphire laser with 1.56 eV photon energy (NIR) are well suited to excite the à state for cluster sizes up to at least n = 40. The ionization potential (IP) of Na ··· (H2O)n drops from 4.3 eV for n = 1 to 3.2 eV for n = 4 and remains almost constant for all larger clusters.12,20 Therefore, ultraviolet (UV) pulses of 3.12 eV photon energy are able to ionize excited clusters of all sizes. [Recently, Na··· (H2O)n isomers with an IP of 2.8 eV for n ≥ 15 have been identified,20 which can be ionized directly by the UV pulse. In our two-color pump−probe experiment,16 no such direct ionization has been observed. This may be attributed to a different isomer distribution in the cluster beam.] It was found, that the Na···(H2O)+n ion signal decays exponentially with pump−probe delay Δt. This is attributed to a finite lifetime τ of the excited cluster Na*···(H2O)n in the à state. With increasing cluster size τ drops dramatically, from 1.2 ps for the dimer to less than 300 fs for the quadrumer. For cluster sizes n ≥ 10, it reaches values τ ≤ 120 fs. These short lifetimes have been attributed to a very efficient internal conversion (IC). Alternatively, one may envision conical intersections to be involved in the process. In any case, the electronic excitation energy of the initially populated à state is transferred into ro-vibrational excitation of the X̃ ground state. Typically, this will lead to a change of the electronic transition moment and a loss of Franck−Condon overlap to the ionic ground state, as schematically illustrated in Figure 1. What is denoted there as reaction coordinate could, e.g., be a characteristic solvent−solvent distance or angle.
E = hvUV − IP + E* − E +#
(2)
If we assume the initial ro-vibrational energy of the X̃ state to be negligible, the energy deposited into the excited state is determined by the absorbed NIR photon E* = hνNIR. Thus, by measuring the electron kinetic energy E, one determines the internal energy E+# of the final ionif the simple reaction scheme eq 1 holds. The maximum possible kinetic energy of the photoelectron is thus expected to be Emax = hvUV + hvNIR − IP
(3)
in the case that no ro-vibrational energy is deposited into the ion, i.e., E+# = 0. In the previous experiments, no information was obtained on E and E+#. To gain further insight into the energy redistribution process, in the present work, time- and mass-resolved photoelectron spectra are reported. While for negatively charged water clusters such data can be obtained in a relatively straightforward manner by size selection of the cluster anions prior to the interaction with the laser field, for neutral clusters it is not possible to select individual masses from the broad distribution of n which emerges from the neutral cluster source. Hence, the photoelectron−photoion coincidence (PEPICO) technique has been applied in the present experiment to obtain time and mass resolved kinetic energy distributions of the photoelectrons. For femtosecond pump−probe experiments, this type of coincidence technique has been applied for the first time in our laboratory several years ago.22 In the next section the experimental setup and data acquisition scheme used is described. Thereafter the experimental results are presented and discussed.
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METHODS The sodium−water clusters used in this experiment are prepared in a pick-up source. This source has been described in detail in previous publications.16,23 In the present experiment, water clusters are generated in a continuous molecular beam by expanding water vapor from an oven at 70 °C (corresponding to ca. 310 hPa) through a conical nozzle of 50 μm diameter into vacuum. The nozzle is heated to a 10 °C higher temperature than the oven to avoid clogging of the nozzle by ice. Helium (at ca. 4000 hPa) is used as coexpanding gas to create mainly smaller clusters in an intense molecular beam. The molecular beam passes through a doping cell containing sodium vapor at a temperature of 163 °C (corresponding to a vapor pressure of 2 × 10−5 hPa). At these conditions, the water clusters do typically pick up no more than one single sodium atom. The temperature of the water oven and the doping cell are regulated by PID-controllers to allow for a stable operation of the clusters source over 24h. The temperature of the water clusters formed under these conditions will be about 70 K, as determined by Buck and coworkers for a similar cluster source.24 Due to the strong intermolecular binding energies the pick-up process will not change the temperature of the clusters considerably.25 Nevertheless, a distribution of isomers will be present in the cluster beam.
Figure 1. Very schematic and highly simplified energy diagram of the pump−probe process in Na···(H2O)n clusters.
By comparing the lifetimes of Na*···(H2O)n with that of the deuterated species Na*···(D2O)n, and applying the energy gap law,21 the high frequency symmetric and antisymmetric stretch vibrations of the water molecules have been identified as the promoting modes for the conversion process.16 Explicitly the dynamics depicted in Figure 1 may be summarized by Na···(H 2O)n + hνNIR → Na*···(H 2O)n τ Na*···(H 2O)n → Na···(H 2O)n# Na*···(H 2O)n + hνUV(Δt ) → Na +···(H 2O)n# + e−(E) (1)
with the pump (hνNIR) and probe (hνUV) photon, the ionization potential IP with respect to the lowest vibrational 8518
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The interaction region is kept at ground potential during the interaction of the laser pulses with the cluster beam. After ≈200 ns, when the electrons have left the region between the attractor and repeller electrode, the potentials of the attractor electrode and all the electrodes below are switched to negative potential. The lowest electrode has a potential of −1000 V. The same holds for the ion flight tube. The repeller remains at ground potential. The cations are accelerated by the fields toward the MCP 2 detector. The transmission of the ion spectrometer can be optimized for a certain mass range by a pair of deflector electrodes, to counteract the velocity of the clusters in the molecular beam. This optimization is crucial to allow for an efficient detection of coincidence events for a broader cluster size distribution from Na···(H2O)n cluster sizes of n = 1 up to 30. The electrical pulses created by the two MCP are digitized by an Acqiris AP240 digitizer card using the SSR firmware. The digitized waveform of the MCP signals is transferred to the computer memory and processed by a software-implemented constant fraction discriminator, enabling a precise determination of the arrival times of the electrons and ions. Using these arrival times, a histogram is generated for the time-of-flight of all electrons he,a and ions hi hitting the detectors. Finally, a list of coincidence events is compiled. If for one laser shot exactly one electron and ion is detected, this is denoted as a coincidence event. The arrival times of the electrons and ions for each event is recorded in a list. From this list, the mass selected electron spectra he,X are derived by summing up the electron counts within a certain range of ion times of flight. The key problem of this PEPICO experiment with pulsed lasers is avoiding false coincidences. Typically, in experiments with pulsed lasers, many ionization events occur during one laser pulse. This must be avoided in the present experiment, by keeping the target density and the laser intensity low enough. To compensate for this, one aims at a high pulse repetition rate and usually has to accept long data acquisition times. Quantitatively, from the equations in ref 22, one obtains the average ionization probability n,̅ the detection efficiencies for electrons ξe and ions ξi, and the ratio between false and all (f) registered coincidences rf = w(f) 11 /w11. False coincidences w11 arise from detecting a coincidence event in which the electron and the ion originate from different ionization events. This rate should be kept at a reasonably low level while at the same time the counting statistics demands that many events are registered. The average ionization rate (per laser pulse) n̅ was kept at values below 0.5 in order to keep rf sufficiently low. Figure 3 shows as an example a coincidence spectrum taken at a delay Δt = 55 fs. The two-dimensional histogram shows the time-of-flight of all electrons and ions detected in coincidence. The yield of all detected ions is plotted as a function of their time-of-flight on the left of Figure 3, and on top the integrated spectrum of all detected electrons is shown. In the time-of-flight mass spectrum cluster, ions up to n = 30 are visible. In addition, Na+ and Na+2 ions and a trace of K+ ions are present in the mass spectrum, which originate from the sodium oven. The electron signals of these masses have been used to calibrate the energy scale of the electron spectra. With this calibration, the electron time-of-flight values te are converted to the kinetic energy of the electrons
In the following chamber, the doped clusters are excited by femtosecond laser pulses of 1.56 eV photon energy (NIR) and ionized by pulses of 3.12 eV photon energy (UV). The laser pulses were obtained using a commercial CPA laser system (Spectra Physics, Spitfire) providing femtosecond pulses of 35 fs duration (fwhm). The pulses have been characterized using SPIDER. The main laser beam is split into two parts to create the pump and probe laser pulses. The pulses in the pump beam may be delayed by a delay stage with a time resolution of 0.67 fs (0.1 μm positioning accuracy). The pulses in the probe beam are frequency doubled in a thin BBO-crystal (100 μm thickness) in order to keep the pulse duration of the UV pulses short. Since the temporal shape of the NIR pulses is nearly Gaussian, the cross-correlation of the pump and probe pulses can be calculated and amounts to 50 fs fwhm. Pump and probe beam are focused into the ionization region in a nearly collinear geometry by spherical mirrors of 1000 mm and 1500 mm radius. The beams intersect at an angle smaller than 2°. The peak intensity of both beams in the ionization region is ca. 1.5 × 109 W cm−2 at a pulse energy of 5 μJ. The electrons and cations generated during the ionization of the clusters are detected in a newly built electron−ioncoincidence spectrometer, which is schematically shown in Figure 2. The setup was derived from an earlier design
Figure 2. Schematic setup of coincidence-spectrometer and data acquisition. The upper left shows a blow up of the ionization and extraction region. The electronics and software is sketched in the lower left (gray shaded): pulses from microchannel plates MPC 1 (electrons) and MPC 2 (ions) are registered each by fast 8-bit-analog digital converter-cards (ADC); the time-of-flight (TOF) of the particles is determined by a software constant fraction discriminators (CFD). The PC records the list of coincidence events and generates histograms as required.
developed in our group, using a combined magnetic bottle electron and Wiley−McLaren type ion time-of-flight spectrometer.22 The magnetic field that collects the electrons is generated by four strong NeFeB magnets. They are positioned below the ionization region, within the attractor electrode. The north poles of these magnets are pointing toward the central hole of the electrode. The electrons are guided by the weak field of a coil within a μ-metal shielded flight tube, extending from the electrode above the ionization region to the microchannel plate detector (MCP 1).
E(te) = A · 8519
l2 + E0 (te − t0)2
(4)
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Figure 4. Color coded plot of the electron count rate as a function of the pump−probe pulse delay Δt and the electron kinetic energy E. The step size of the delay time was 25 fs. The measured electron counts have been smoothed by convolution with a Gaussian of 0.075 eV fwhm. Red dots indicate the delays times Δt at which the coincidence spectra have been taken, which will be discussed in the following.
Figure 3. Middle: 2D plot of the coincidence signal as a function of the electron and ion time-of-flight at a given pulse delay Δt = 55 fs. On the left, the yield hi is plotted for all ions detected as a function of their time-of-flight. Similarly, on the top the yield he, a of all electrons detected is plotted as a function of the electron time-of-flight. The color coded 2D-histogram is created from the arrival time list of the electron−ion coincidence events.
small and larger cluster sizes (IP = 4.3 eV for n = 1, while IP = 3.2 eV for n ≥ 4). The total photon energy available for ionization by the NIR and UV photons is 1.56 eV + 3.12 eV = 4.68 eV. Thus, for the smallest clusters with long life times, the electron kinetic energy will be E ≤ 0.38 eV (highest probability observed for 0.1 eV), while for clusters with n ≥ 4 and a lifetime τ ≤ 100 fs, we expect E ≤ 1.5 eV (highest probability observed for 0.8 eV). The next step is of course to really measure the electron kinetic energy distribution and time dependence for each cluster size by coincidence with the respective ion signal. As explained above, the count rate in a genuine coincidence measurement must be very small (significantly less than 1 event per laser shot, i.e., much less than 1 for a specific cluster size). Hence, since the measuring time for coincidence spectra is long, we have recorded coincidence spectra only for a few selected delay times Δt between NIR and UV pulse, as marked by red dots on the right side of Figure 4. Figure 5 shows the thus determined kinetic energy distributions of the photoelectrons for the Na···(H2O)n monomer, dimer, and trimer at all pump−probe delay times
with the constants A = 2.843 × 106 eV ns2 m−2 and l = 0.712 m, and the parameters t0 = (361 ± 25) ns and E0 = (−0.6 ± 0.1) eV. The electron spectra are created by selecting the coincidence signal in a certain ion time-of-flight range and converting each electron time-of-flight values to E using eq 4. The result is sorted into a histogram with a binning interval of 0.01 eV. Since the mass selected electron spectra contain only a few hundred electrons, the spectra are smoothed with a Gaussian of 0.045 eV fwhm, well below the resolution limit of the magnetic bottle spectrometer. [Small shifts of the spectrometer calibration due to charging effects in long-term operation are recalibrated at each experimental run by adjusting the parameter E0 with the help of the spectrum for the K+ ion (the shift is typically 150 fs. The mean kinetic energy E̅ A of the low energy part A also drops for increasing pump−probe delay but less is pronounced and on a longer time scale. The behavior for n = 8, 9, and 10 is quite similar, except that no rise of the low energy average is observed for longer delay times. The difference of the mean energies between the low and high energy parts of all cluster sizes shown in Figure 7b is about 0.6 eV. Since the value of E̅ B drops only by 0.1 eV within 100 fs, it is safe to assume that the electrons of the high energy part B and the ones from the low energy part A come from different excitation and ionization processes. In our earlier pump−probe study,16 a small long-lived contribution was identified in the ion signal. The low energy part A of the electron signal is also small and long-lived. Consequently, one may assume that both observations have the same origin. There are several possible mechanisms to explain the bimodal structure of the kinetic energy distributions of the photoelectrons with their respective decay times. The low 8522
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(2) Chen, X. Y.; Bradforth, S. E. The Ultrafast Dynamics of Photodetachment. Annu. Rev. Phys. Chem. 2008, 59, 203−231. (3) Wang, C. R.; Nguyen, J.; Lu, Q. B. Bond Breaks of Nucleotides by Dissociative Electron Transfer of Nonequilibrium Prehydrated Electrons: A New Molecular Mechanism for Reductive DNA Damage. J. Am. Chem. Soc. 2009, 131, 11320−11322. (4) Zhu, J. Y.; Paparelli, L.; Hospes, M.; Arents, J.; Kennis, J. T. M.; van Stokkum, I. H. M.; Hellingwerf, K. J.; Groot, M. L. Photoionization and Electron Radical Recombination Dynamics in Photoactive Yellow Protein Investigated by Ultrafast Spectroscopy in the Visible and Near-Infrared Spectral Region. J. Phys. Chem. B 2013, 117, 11042−11048. (5) Jou, F. Y.; Freeman, G. R. Temperature and Isotope Effects on the Shape of the Optical Absorption Spectrum of Solvated Electrons in Water. J. Phys. Chem. 1979, 83, 2383−2387. (6) Yokoyama, K.; Silva, C.; Son, D. H.; Walhout, P. K.; Barbara, P. F. Detailed Investigation of the Femtosecond Pump-Probe Spectroscopy of the Hydrated Electron. J. Phys. Chem. A 1998, 102, 6957−6966. (7) Assel, M.; Laenen, R.; Laubereau, A. Femtosecond Solvation Dynamics of Solvated Electrons in Neat Water. Chem. Phys. Lett. 2000, 317, 13−22. (8) Pshenichnikov, M. S.; Baltuska, A.; Wiersma, D. A. HydratedElectron Population Dynamics. Chem. Phys. Lett. 2004, 389, 171−175. (9) Thaller, A.; Laenen, R.; Laubereau, A. Femtosecond Spectroscopy of the Hydrated Electron: Novel Features in the Infrared. Chem. Phys. Lett. 2004, 398, 459−465. (10) Griffin, G. B.; Young, R. M.; Ehrler, O. T.; Neumark, D. M. Electronic Relaxation Dynamics in Large Anionic Water Clusters: (H2O)−n and (D2O)−n (n = 25−200). J. Chem. Phys. 2009, 131, 194302. (11) Young, R. M.; Neumark, D. M. Dynamics of Solvated Electrons in Clusters. Chem. Rev. 2012, 112, 5553−5577. (12) Hertel, I. V.; Hü glin, C.; Nitsch, C.; Schulz, C. P. Photoionization of Na(NH3)N and Na(H2O)N Clusters - A Step Towards the Liquid-Phase. Phys. Rev. Lett. 1991, 67, 1767−1770. (13) Brockhaus, P.; Hertel, I. V.; Schulz, C. P. Electronically Excited States in Size-selected Solvated Alkali Metal Atoms. III. Depletion Spectroscopy of Na(NH3)n-Clusters. J. Chem. Phys. 1999, 110, 393− 402. (14) Schulz, C. P.; Bobbert, C.; Shimosato, T.; Daigoku, K.; Miura, N.; Hashimoto, K. Electronically Excited States of Sodium−Water Clusters. J. Chem. Phys. 2003, 119, 11620−11629. (15) Schulz, C. P.; Scholz, A.; Hertel, I. V. Ultrafast Energy Redistribution in Photoexcited Sodium-Ammonia Clusters. Isr. J. Chem. 2004, 44, 19−25. (16) Liu, H. T.; Müller, J. P.; Zhavoronkov, N.; Schulz, C. P.; Hertel, I. V. Ultrafast Dynamics in Na-Doped Water Clusters and the Solvated Electron. J. Phys. Chem. A 2010, 114, 1508−1513. (17) Hashimoto, K.; Kamimoto, T. Theoretical Study of Microscopic Solvation of Lithium in Water Clusters: Neutral and Cationic Li(H2O)n (n = 1−6 and 8). J. Am. Chem. Soc. 1998, 120, 3560−3570. (18) Tsurusawa, T.; Iwata, S. Theoretical Studies of Structures and Ionization Threshold Energies of Water Cluster Complexes with a Group 1 Metal, M(H2O)n (M = Li and Na). J. Phys. Chem. A 1999, 103, 6134−6141. (19) Cwiklik, L.; Buck, U.; Kulig, W.; Kubisiak, P.; Jungwirth, P. A Sodium Atom in a Large Water Cluster: Electron Delocalization and Infrared Spectra. J. Chem. Phys. 2008, 128, 154306. (20) Forck, R. M.; Dauster, I.; Schieweck, Y.; Zeuch, T.; Buck, U.; Oncak, M.; Slavicek, P. Communications: Observation of Two Classes of Isomers of Hydrated Electrons in Sodium-water Clusters. J. Chem. Phys. 2010, 132, 221102. (21) Englman, R.; Jortner, J. Energy Gap Law for Radiationless Transitions in Large Molecules. Mol. Phys. 1970, 18, 145−164. (22) Stert, V.; Radloff, W.; Schulz, C. P.; Hertel, I. V. Ultrafast Photoelectron Spectroscopy: Femtosecond Pump-Probe Coincidence Detection of Ammonia Cluster Ions and Electrons. Eur. Phys. J. D 1999, 5, 97−106. (23) Bobbert, C.; Schulz, C. P. Solvation and Chemical Reaction of Sodium in Water Clusters. Eur. Phys. J. D 2001, 16, 95−97.
does not allow a deeper insight into the internal conversion (IC) process of the electronic excitation into vibrationally excited ground state. At this point it has to remain open whether or not solvent rearrangement and IC take place on the same time scale. Referring to eq 1: it is unclear whether the reaction in line 3 is preceded by an internal rearrangement process in the excited Na*···(H2O)n system.
4. CONCLUSIONS Time resolved electron kinetic energy distributions of Na··· (H2O)n clusters with n = 1 to 22 have been measured by combining two-color femtosecond pump−probe techniques with photoelectron−photoion coincidence spectroscopy. These electron spectra are fingerprints of the energetics and dynamics of the first electronically excited à state being resonant with the pump laser. The electron energy distribution is narrow for the monomer (fwhm = 0.15 eV) and dimer (≈ 0.6 eV). For larger clusters (n ≥ 3) the electron spectra are broad and more structured. Two distinctively different parts of the spectrum can be distinguished, both representing different dynamics: The signal at lower kinetic energies (
