Article pubs.acs.org/JPCA

Reactivity of Iron Atoms at Low Temperature Serge A. Krasnokutski* and Friedrich Huisken Laboratory Astrophysics Group of the Max Planck Institute for Astronomy at the Friedrich Schiller University Jena, Helmholtzweg 3, D-07743 Jena, Germany ABSTRACT: We have studied the reactions of iron atoms and clusters with oxygen, acetylene, and water molecules in superfluid He droplets at T = 0.37 K. For all systems, the formation of weakly bound adducts was found, but the insertion reaction of iron into existing molecular bonds could not be observed. The formation of FeOH2 and FeC2H2 complexes was evidenced by mass spectrometry. However, it was found that the reaction of iron atoms with oxygen molecules under similar conditions leads to the stabilization of an intermediate reaction product, the weakly bound linear FeOO adduct, which undergoes complete dissociation upon electron impact ionization. All reactions observed are not expected to proceed in the gas phase. The R2PI spectrum of the y5D04 ← a5D4 atomic transition of Fe solvated in helium droplets was recorded. A relatively small blue shift of ∼120 cm−1 with respect to the gas phase position was found.



INTRODUCTION Iron is the most abundant metal in the universe.1 However, similar to other refractory elements, its concentration in the interstellar medium is enormously low compared to the solar system.2 The most common explanation for this observation is the depletion of the refractory elements by the process of dust particle formation. Considering that a significant portion of the interstellar dust is believed to be grown in the interstellar medium at low temperatures,3,4 it seems important to study the chemistry of iron atoms at low temperature on the surface of dust grains. The reactivity of iron atoms at low temperature was the subject of numerous studies. Nevertheless, there is a lot of disagreement in the literature. The first report on the reaction of iron atoms with oxygen molecules at 12 K in argon matrixes claims the formation of cyclic FeO2 products.5 Later, the OFeO insertion structure was reported to be the product of the lowtemperature reaction.6,7 However, the formation of the insertion structure requires the O−O bond to be broken, which puts an energy barrier into the reaction pathway. Indeed, later studies showed that OFeO products were only formed when the matrix was irradiated by UV light or when part of the Fe atoms were in excited states.8,9 Although these publications agree in the fact that the formation of the OFeO product requires an overcoming of the energy barrier, they disagree about the reactivity of the Fe atoms in the ground state. The earlier publication8 claims that linear FeOO and cyclic FeO2 products can be formed upon the diffusion of cold reagents in solid argon. In the later study, it was found that ground state iron atoms are unreactive toward O2 in solid argon and that OFeO molecules were only produced under broad-band UV/ visible irradiation.9 No reactivity of iron atoms toward oxygen was also found in the gas phase when photoionization mass spectrometry was applied for product identification.10,11 Considering the difficulty in releasing the reaction energy of © 2014 American Chemical Society

the formed complex, the failure to observe reactions of Fe in the gas phase can be easily understood. The complex of the iron atom with acetylene is less investigated. A cryogenic matrix study revealed the formation of an FeHC2H adduct that is bonded through the hydrogen atom.12 Upon near-UV irradiation, this complex rearranges to produce the HFeC2H insertion structure. Studies of the complexation of acetylene with other metal atoms also disagree in various aspects. For example, in earlier studies it was found that the Al atom forms with acetylene a σ-bonded adduct,13,14 while later studies reported a π-bonded structure.15 The large discrepancy in the results is partly due to the difficulty in assigning correctly the vibrational bands of the measured IR spectra. In addition, the laser evaporation of metal atoms often produces a large amount of metal atoms in the excited state, which are more reactive than the ones in the ground state. When the excited atoms are deposited together with the ligands, the complexes can be formed before the metal atoms relax into their ground state. As a result, the products of chemical reactions involving metal atoms in the ground and excited states are often mixed up. In contrast, there is good agreement as far as the reaction of iron atoms with water molecules is concerned. Here, the formation of weakly bound adducts was found, with the iron being connected to the oxygen atom of the water molecule.16,17 In the present study, we applied the helium droplet isolation technique to study the reactions of iron atoms with oxygen, acetylene, and water molecules. The superfluid helium droplets provide distinct advantages for the study of chemical reactions at low temperatures compared to the conventional rare gas matrix isolation technique. Because the helium droplets are Received: January 22, 2014 Revised: March 20, 2014 Published: March 20, 2014 2612

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flying in a “molecular beam” through the experimental setup, the reactants can be incorporated into the droplets at different locations. Considering the facts that the temperature of the droplets is well-known (T = 0.37 K)18 and constant and that all species picked up by the droplets adopt their temperature on a subnanosecond time scale,19 this method allows to avoid the complication of gas phase reactions and of reactions between species in excited states. In addition to a pure spectroscopic analysis, the chemical reactions can be probed by mass spectrometry and calorimetric techniques.20,21

Hz (Continuum, model NY 81). The dye laser was operated with a mixture of Rhodamine 6G and DCM dyes, and its output was doubled in a KDP crystal, thus providing a wavelength range from 290 to 320 nm. The absorption lines of gas-phase Fe atoms were used for the wavelength calibration. After having passed the spectroscopy chamber, the helium droplet beam was introduced into a differentially pumped quadrupole mass spectrometer equipped with an electron bombardment ionizer. It was used to monitor the doping of the helium droplets, to identify the reaction products, and to control the intensity of the helium droplet beam. Molecular geometries and vibrational frequencies of the possible reaction products were determined using the B3LYP hybrid functional and the 6-311+G(d,p) basis set implemented in the Gaussian09 package.24 In the past, the results obtained with this combination of method and basis set have shown good agreement with the experimental findings for weakly bound metal−organic complexes.25−28



EXPERIMENTAL SECTION The experiments have been carried out in the experimental setup reported earlier.22,23 Large helium clusters were produced by supersonic expansion of pure helium gas at high pressure (p = 20 bar) through a 5 μm diameter pinhole nozzle. The average number of He atoms (NHe) per droplet is evaluated according to the empirical equation NHe = 852 393.161 exp(−0.3591T), where T is the temperature of the nozzle in K. To derive this relationship, we have fitted the experimental data of Toennies and Vilesov.19 After skimming, the helium droplets enter the reaction chamber, where they are sequentially doped with the reactants. Commercially available oxygen (Air Liquide 99.999%) and acetylene (Air Liquide unspecified) were used without further purification. For the incorporation of Fe atoms, we used two different techniques. At first, we sublimated the metal from the surface of a resistively heated iron wire (technical low carbon iron), which was coiled around the helium droplet beam. Later we used a zirconia crucible filled with iron powder (Roth 99.5%) that was installed below the helium droplet beam. The last method, which was found to provide more stable operation, was employed for all R2PI and mass spectroscopy measurements presented here. The evaporation from the wire was only used in the calorimetric experiments, which require a relatively short time for the execution so that the long-term instability of the technique is not an issue. The residual gas in our vacuum chamber mainly consists of water vapor. Therefore, the pick-up of water molecules was controlled by adjusting the temperature of a cold shield cooled by liquid nitrogen. The doping of helium droplets by water molecules was monitored by mass spectrometry, which did not reveal incorporation of any other impurity besides water. Upon collision with the helium clusters, iron atoms were picked up by the helium nanodroplets and carried by them to the point of oxygen or acetylene molecule incorporation. These gases were introduced through a leak valve (Vacuum Generators, LVM series) into a small pick-up cell placed into the main chamber. In order to allow the helium droplet beam to pass through, it had two 6 mm wide circular openings. The local pressure in this pick-up cell was measured with an ion gauge (Leybold Heraeus IE-20). After having traversed the reaction chamber, the He droplet beam entered the spectroscopy chamber, where the helium droplets interacted with a pulsed tunable laser beam of 3 mm diameter in a perpendicular geometry. The photoions were mass-analyzed by a time-of-flight (TOF) mass spectrometer. The resonance-enhanced two photon ionization (R2PI) spectra of iron atoms were recorded by monitoring the ion yield on mass m = 56 amu. The laser system consisted of a tunable dye laser (Lambda Physik SCANMATE 1) pumped by the second harmonic (λ = 532 nm) of a pulsed Nd:YAG laser with a repetition rate of 10



RESULTS AND DISCUSSION For the pick-up of refractory metal atoms, the pick-up crosssection (σ) of a helium droplet could be much smaller than its geometrical cross-section. Therefore, the simple estimation of the doping efficiency based on the Poisson distribution,29 with the number density n of the atoms in the pick-up region of length l and σ being equal to the geometrical cross-section of the droplet, cannot be applied. Instead, for the estimation of the doping efficiency, we used two different approaches. First, we compared the intensities of the Fe1, Fe2, and Fe3 peaks in the mass spectra of helium droplets doped by iron atoms measured with the quadrupole mass spectrometer. This ratio uniquely defines the nσl product in the Poisson distribution function. A disadvantage of this method is the problem of Fe cluster fragmentation during the ionization process leading to a considerable underestimation of the doping efficiency. For the second method, we measured the intensity of the depletion peak produced by the laser tuned to the absorption frequency of the solvated iron atom. A detailed description of the depletion technique can be found in our earlier publications.23,30 In the present study, the intensity of the depletion peak was measured with the quadrupole mass spectrometer tuned to the masses m = 8 and 56 amu. On m = 8 amu (helium dimer), we measure a signal that is proportional to the total number of helium droplets present in the beam, while on m = 56 amu (Fe), we only probe the helium droplets doped with iron. Therefore, the ratio of the relative depletions (D) measured on both masses m = 8 and 56 amu (D8/D56) should give us the ratio of Fe-doped helium droplets to all helium droplets. Such measurements were performed for helium droplets produced with a nozzle temperature of 13 K, which pass over the iron oven kept at T = 1350 °C. Under these conditions, the depletion method yields a doping efficiency of 17%, while the simple mass spectrometric method (measuring the ion signals of Fe, Fe2, and Fe3) results in a somewhat smaller value (10%). The smaller value derived from the evaluation of the cluster formation can be explained by the fragmentation of the Fe clusters in the ionizer. The fragmentation causes an increase of the Fe monomer intensity at the expense of the Fe cluster peak intensities and thus results in an underestimation of the doping efficiency. However, it should be mentioned that the depletion method could also underestimate the doping efficiency. This is 2613

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This effect could be explained by a particularly low probability of charge transfer from the initially formed He+ to the solvated iron in conjunction with a higher charge transfer probability to the FeC2H2 adduct and a large dissociation probability of this complex. Thus, the ionization of the FeC2H2 complex results in the formation of Fe+. As can be seen in Figure 1, the FeC2H2 complex could also be detected as a positive peak on the mass of FeC2H+ (m = 81 amu). On the basis of these observations, we can already predict a rather weak bonding of the FeC2H2 complex. In the case of oxygen being the second reactant, the formation of FeO2 ions was not detected by mass spectrometry. The positive peaks in the (Fe + O2) − Fe differential mass spectrum are assigned to O2 and O2Hen ions. The small negative peak that can be assigned to FeOH is due to the reduction of iron water complexes after incorporation of oxygen. With increasing iron vapor pressure, the appearance of Fe2O2 and Fe3O2 peaks can be readily observed. These peaks are due to the reaction of multiple iron atoms picked up by the same droplet with an oxygen molecule. However, our mass spectrometric study does not clearly reveal the formation of a complex between a single Fe atom and the O2 molecule. In Figure 2, the R2PI spectrum of the y5D04 ← a5D4 atomic transition of Fe solvated in helium droplets is given by the

due to the fact that, after ionization of a Fe-doped helium droplet, charge transfer to the iron atom could occur. This would lead to a reduction of the ion signal on m = 8 amu (He2+) and to an increase of the contribution of undoped helium droplets to the signal on this mass. However, for the relatively large helium droplets, which were used in this experiment (⟨NHe⟩ ≈ 8500), the probability of charge transfer to the incorporated impurity is expected to be rather small.31−33 Our experimental results also suggest a low charge transfer probability to the solvated iron atoms (see forthcoming discussion on the mass spectra). Therefore, the doping efficiency obtained by the depletion method (17%) should be rather close to the real value. These experiments have shown that the pick-up cross-section of a helium droplet for iron atoms is actually much smaller than the geometrical cross-section of this droplet. We used the results of these measurements to establish the proper doping conditions so as to achieve predominantly single iron atom incorporation into the helium droplets. Differential mass spectra are useful to study chemical reactions inside helium droplets.21,22 They are obtained by subtracting the mass spectrum of helium droplets doped by a single reactant from the mass spectrum of helium droplets doped by both reactants. As a result, the differential mass spectrum shows positive and negative peaks. The positive peaks reveal the products formed by the reaction as well as the second reactant, while the negative peaks belong to the first reactant, which has been consumed in the reaction. As a result, the differential mass spectra demonstrate the effect of the incorporation of the second reactant into the helium droplets that were previously doped by the first reactant. Figure 1 shows the differential mass spectra (Fe + O2) − Fe and (Fe + C2H2) − Fe. These mass spectra demonstrate the

Figure 2. R2PI spectra of Fe atoms in the gas phase (blue curve) and solvated in helium droplets (black curve). The asterisk denotes the spectral line, which corresponds to the transition from the ground state. The arrow marks the wavelength of the excitation laser, which was used to measure the pressure dependence curves of Figure 3.

broad band. The position of the corresponding spectral line in the gas phase is 33095.9408 cm−1 (not shown in the figure). The blue shift from the gas phase position (∼120 cm−1) is relatively small compared to the shifts found for some other metal atoms solvated in helium droplets. We also found nearly the same blue shift for the energetically higher transition of Fe (y5D03 ← a5D4). The peak of the corresponding gas phase transition is marked by an asterisk. The relatively small value of the solvation-induced blue shift can be explained by the fact that the corresponding electronic transition does not involve electron transfer to an orbital with higher quantum number. Similar to the case of other metal atoms solvated in helium,34−36 the absorption band of the solvated iron is, with about 400 cm−1, rather broad. The absorption of iron in this spectral range can be used to monitor the number of helium droplets, which contain a single iron atom. To test the reactivity of iron atoms, we tune the laser to the wavelength of 299 nm (∼33 445 cm−1; see the arrow in Figure

Figure 1. Differential mass spectra obtained by subtracting the mass spectrum of He droplets doped with Fe from the mass spectrum of He droplets doped with both Fe and O2 (lower black curve) or Fe and C2H2 (upper red curve). Both spectra were recorded at a nozzle temperature of 11 K.

effect of the incorporation of oxygen and acetylene molecules into helium droplets predoped by iron atoms. Surprisingly, in both differential mass spectra, there are no negative peaks on the mass of iron (m = 56 amu). Moreover, the intensity of the iron peak even increases after the incorporation of acetylene molecules. This effect was studied separately by monitoring the ion signal on m = 56 amu upon varying the vapor pressure of acetylene in the second pick-up cell. Indeed, this study demonstrates an increase of the iron ion signal when the acetylene pressure is raised. 2614

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helium droplet beam caused by the pick-up of the second reactant. In this case, the attenuation of the R2PI signal should be equal to the attenuation of the helium droplet beam intensity measured with the quadrupole mass spectrometer on the mass of He2. This situation can be observed for the case of the Al + C2H2 interaction, where the aluminum R2PI signal changes in the same way as the He2 ion signal or the total pressure in the detector chamber (not shown in the figure). Therefore, the dependence of the Al R2PI signal on the C2H2 vapor pressure is exactly what one would expect if no reaction takes place. The variations of the Fe R2PI signals as a function of the C2H2 and H2O vapor pressures comply with the predicted signal change for the case of reaction (dashed red curves). Thus, the formation of FeC2H2 and FeH2O complexes inside helium droplets is confirmed. In the case of oxygen as the second reactant, the pronounced attenuation of the Fe R2PI signal strongly suggests FeO2 complex formation although the Fe R2PI signal curve slightly deviates from the dashed reaction curve. The difference between the theoretical dashed curve and the Fe R2PI line, if O2 is the reaction partner, is likely caused by a lower pick-up efficiency for the oxygen molecules. Because of the fact that the oxygen molecule has two unpaired electrons in its ground state, its sticking probability to the helium droplet could be reduced. This interpretation is supported by the close agreement between the R2PI signals of Al and Fe as a function of O2 pressure. We found that, in the case of oxygen, the pickup cross-section of helium droplets is 2.3 times smaller than their geometrical cross-section. The number of helium droplets containing no oxygen that has been calculated with this assumption is given in the middle frame of Figure 3 by the dotted (green) line. Following this discussion, we can be sure that iron atoms do react to form FeC2H2, FeO2, and FeOH2 complexes. However, the absence of the FeO2 peak in the mass spectrum (Figure 1) clearly indicates rather weak bonding of this complex. To obtain a better understanding of the geometry and bonding in these complexes, we performed quantum chemical calculations in combination with calorimetric measurements to determine the reaction energies. Figure 4 shows the energy scan along the

2) and record the R2PI signal on the mass of iron (m = 56 amu). This wavelength was selected to avoid any coincidence with the gas phase transitions of iron atoms. If iron forms a complex with a second species (C2H2, O2, or H2O), the position of the absorption band of the complex is expected to shift by more than 400 cm−1. Thus, reaction or complex formation will be accompanied by a reduction or depletion of the R2PI signal on m = 56 amu. Figure 3 shows the dependence

Figure 3. R2PI signals for Fe atoms recorded at λexc = 299 nm (see arrow in Figure 2) and Al atoms (λexc = 300.93 nm) as a function of the pressure of the second reactant in the pick-up cell. The dotted black line in the upper frame shows the signal on mass m = 8 amu measured with the quadrupole mass spectrometer. The dashed red lines represent the number of helium droplets, which do not contain the second reactant. The dotted green curve in the middle frame also gives the number of helium droplets without a second reactant, but assuming a reduced pick-up cross-section for O2 (2.3 times smaller than the geometrical cross-section of the droplet). In the case of oxygen or acetylene as the second reactant, the helium droplets were produced with the nozzle kept at T = 15 K. In the experiment with water, larger helium droplets were produced by adjusting the nozzle temperature to T = 13 K.

of the R2PI signal of Fe on the pressure of the second reactant in the pick-up cell. In the same figure, we have also plotted the number of helium droplets, which did not pick up any molecule of the second species, in addition to iron. These dashed curves were obtained assuming a Poisson distribution for the embedded molecule and a cross-section for the pick-up of the second species equal to the geometrical cross-section of the helium droplet. In contrast to the earlier discussed case of iron, this assumption should be valid for closed-shell molecules at room temperature.29 The aforementioned curve also represents the R2PI signal change if quantitative reaction between the Fe atom and the second reactant took place. For comparison, we also plotted the R2PI signals of Al atoms obtained in different experiments. This comparison is particularly useful for O2 as a reaction partner as it has already been shown that Al atoms react readily with oxygen molecules inside helium droplets.20 If no reaction between the incorporated reactants occurs, we also expect some small attenuation of the R2PI signal. This depletion occurs due to the destruction or scattering of the

Figure 4. Relative energies of the Fe−O2 and Fe−C2H2 complexes as a function of the Fe−O and Fe−C separation, respectively. At each step of the quantum chemical calculation, the molecular structure was fully optimized. 2615

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binding energy in the Fe+OO complex is further reduced to a value of 1225 cm−1. Therefore, the missing FeO2 peak in the mass spectrum of helium droplets doped with iron and oxygen suggests the formation of exactly this linear FeOO structure. In the case if acetylene is the second reactant, a spin change can possibly occur after complex formation. As can be seen from Figure 4 and Table 1, the triplet state of the FeC2H2 complex is predicted to be energetically more favorable than the quintet state. However, the separated reagents have a quintet ground state. Considering that the stabilization of higher spin states was observed in the case of alkali cluster formation,38 it is not clear whether the spin change from quintet to triplet state occurs in the present reaction. However, the amount of energy released in the reaction is not much affected, as the predicted energy difference between the quintet and triplet states of the FeC2H2 complex is only about 1565 cm1 (see Table 1). We also used the calorimetric technique described in our previous publications20,22 to check whether we can determine the energy released in the reactions. We found that the attenuation of the helium droplet beam intensity caused by the pick-up of the second reactant does not depend on whether the helium droplets were doped before by iron atoms or not. Therefore, within the sensitivity of the method, we were unable to detect any heat release in the chemical reaction of iron atoms with oxygen or acetylene. The calorimetric studies could not be performed with water molecules as a reaction partner as it was impossible to change quickly the pressure of the water vapor in the reaction chamber. These results confirm the formation of weakly bound adducts as shown in Table 1 and exclude the formation of structures, where the iron atom was inserted into an existing molecular bond of the reaction partner.

reaction coordinate for the Fe + O2 and Fe + C2H2 reactions. To obtain these curves, the distance between the iron atom and the oxygen or carbon atoms was fixed, while a full geometry optimization (without any other geometry constraints) was performed at each step. At large separations, the iron atoms prefer a linear orientation with respect to the oxygen molecule axis. Only at closer distance, when the Fe−O separation is shorter than 3.3 Å, the iron atom is shifted away from the linear configuration. The molecular structures of the reaction products found in our computations are given in Table 1. Table 1. Relative Energies (RE) and Iron Binding (BE) Energies in cm−1 for the Potential Minimums of the Products of Low-Temperature Chemical Reactions as Determined by B3LYP/6-311G+d,p Computations; the Relative Energies Are Given with Respect to the LowestEnergy State



a

CONCLUSIONS The reactivity of iron atoms with oxygen, acetylene, and water molecules at ultralow temperature (T = 0.37 K) has been studied in liquid helium droplets. It is found that iron atoms prefer the formation of weakly bound adducts and are not inserted into an existing molecular bond of the reaction partner. It follows that these reactions should not proceed in the gas phase at low temperature because there is no possibility for the formed adducts to release the binding energy. These results also suggest only a weak bonding of iron atoms to the surface of carbonaceous or icy interstellar dust grains. Therefore, we can expect that physisorbed iron atoms can be easily desorbed from the surface of such dust grains upon UV irradiation or other excitation. For the Fe + O2 reaction, stabilization of the linear intermediate FeOO reaction product was observed. Finally, it was found that the reaction products could easily be dissociated upon excitation. This is demonstrated by the efficient fragmentation of the FeC2H2 complex and the complete dissociation of the FeO2 complex after charge transfer from He+.

Multiplicity.

Our calculation predicts that the transition from the linear to the triangular orientation of the iron atom occurs before reaching the local energy minimum of the linear configuration. Therefore, the formation of the triangular structure is predicted to be barrierless. Taking into account that the accuracy of the quantum chemical calculations for estimating the interaction energy at large distances is rather low, the small energy barrier can be easily overpassed in this case. Table 1 summarizes the computational results on the possible reaction products of the reactions Fe + O2, Fe + C2H2, and Fe + H2O. Both stable FeO2 structures found in our calculations and in earlier studies37 are characterized by iron binding energies, which are comparable to those found for FeC2H2 and FeH2O complexes. At the same time, contrary to FeC2H2 and FeH2O molecules, the FeO2 complex undergoes complete dissociation upon ionization. This can be explained by the relatively weak binding of the iron cation in the linear configuration of the FeO2 molecule. Our DFT calculations show that the binding energy of the iron cation is about 3 and 4 times weaker in the Fe+OO complex than in Fe+H2O and Fe+C2H2, respectively. With the MP2 technique and an correlation-consistent basis set (aug-cc-pVTZ), the predicted



AUTHOR INFORMATION

Corresponding Author

*(S.A.K.) Tel: +49-3641-947306. E-mail: sergiy. [email protected]. Notes

The authors declare no competing financial interest. 2616

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ACKNOWLEDGMENTS This research was supported by the Max Planck Institute for Astronomy (MPIA) and the Deutsche Forschungsgemeinschaft DFG (Contract No. HU 474/22-3)



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dx.doi.org/10.1021/jp5007704 | J. Phys. Chem. A 2014, 118, 2612−2617

Reactivity of iron atoms at low temperature.

We have studied the reactions of iron atoms and clusters with oxygen, acetylene, and water molecules in superfluid He droplets at T = 0.37 K. For all ...
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