Ligand Stabilization and Charge Transfer in Dissociative Ionization of

Article pubs.acs.org/JPCC

Ligand Stabilization and Charge Transfer in Dissociative Ionization of Fe(CO)5 Aggregates J. Lengyel,*,† J. Fedor,* and M. Fárník* J. Heyrovský Institute of Physical Chemistry v.v.i., The Czech Academy of Sciences, Dolejškova 3, 18223 Prague, Czech Republic ABSTRACT: We probe dissociative ionization of iron pentacarbonyl, Fe(CO)5, in its elementary stage of aggregation: in clusters. Two types of targets are studied: (i) small pure Fe(CO)5 aggregates and (ii) small Fe(CO)5 aggregates deposited on free argon nanoparticles with the mean size of a few hundred atoms. In both cases, the presence of environment leads to suppressed ligand decay upon dissociative ionization. The appearance energies of fragment ions point to very different ionization mechanisms for the two target systems: while, in pure clusters, direct dissociative ionization dominates, on argon nanosupport, the ionization proceeds via a hole transfer from the support. The observed ligand stabilization and ionization mechanism shed new light on previous surface science studies of precursors used in focused electron beam induced deposition (FEBID).

■

INTRODUCTION Recently, a lot of attention has been paid to the electroninduced fragmentation processes in metal−organic molecules (reviewed in, e.g., ref 1). This interest has been motivated by advances in focused electron beam induced deposition (FEBID), a promising technique for creating well-defined nanostructures. In FEBID, metal-containing precursor molecules are physisorbed on a substrate (generally, in a dynamic equilibrium with the constant gas influx) and the desired structure is directly “drawn” by a high-energy focused electron beam. It is generally accepted that the actual precursor decomposition is to a large degree caused by secondary and backscattered electrons, which are much slower, with the maximum of the energy distribution below 10 eV.2 The need to understand (and optimize) the deposition process has inspired a range of electron-triggered decomposition studies of FEBID precursors both in the gas phase and in the condensed phase. Typically, the studies of the first type are performed in a crossed-beam arrangement with a mass spectrometric analysis of products; in the second type of studies, the molecules are adsorbed on a substrate and irradiated by electrons and products are probed by various surface-science techniques. A picture about relevant elementary processes has been appearing from these combined experimental efforts.1 The gasphase studies have shown that the dissociative ionization (interaction of electrons yielding positive fragments) in these types of compounds is usually strongly fragmentative: in many cases the dominant product is the bare metal cation with all the ligands stripped.3−6 On the other hand, dissociative electron attachment, yielding negative fragments, causes very effective separation of only one ligand, however, only at very low electron energies.3,7−9 The degree of decomposition upon electron-induced neutral dissociation remains questionable.10 In any case, the condensed-phase studies using XPS analysis have shown that, for a number of precursors,11−14 the initial © XXXX American Chemical Society

step in degradation proceeds via electron-triggered removal of one ligand. This has led to the conclusion that the (very fragmentative) dissociative ionization plays a minor role and the electron-triggered precursor decomposition is driven by dissociative electron attachment and/or neutral dissociation, or their combination.1 In this paper, we probe the dissociative ionization for precursor molecules that are in the form of small aggregates (clusters). This allows us to probe the influence of environment (mimicking the condensed phase studies) and relate it to the results known from gas-phase studies (that use experimental approaches similar to ours). The studied precursor system iron pentacarbonyl, Fe(CO)5, is one of the most commonly used precursors in iron nanodeposition,2,15 and its elementary electron-triggered decomposition has been probed previously both in the gas phase5,16,17 and in the condensed phase.18,19 We show that already the presence of few neighboring molecules dramatically reduces the degree of precursor decomposition by stabilizing the ligands. Additionally, we demonstrate that the ionization mechanism itself can be very different if an elementary supportin our case large argon nanoparticleis present. Instead of direct electron-impact ionization, the dominant cause of dissociative ionization in that case is the charge transfer from such support.

■

EXPERIMENTAL METHODS

The present experiments were performed on the cluster beam apparatus (CLUB) described elsewhere.20−22 The setup allows for versatile experiments;23−26 in the present work we irradiate Received: June 10, 2016 Revised: July 13, 2016

A

DOI: 10.1021/acs.jpcc.6b05852 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

spread of fitted values as dependent on initial parameters, is ±0.4 eV.

the cluster beam with electrons of controlled energy, and the positive product ions are analyzed mass spectrometrically. Two types of target systems were probed: (i) pure Fe(CO)5 clusters and (ii) multiple Fe(CO)5 molecules that were picked up on large argon clusters and aggregated on their surface. In the first case, the clusters were produced by the supersonic expansion of a mixture of Fe(CO)5 and helium under a stagnation pressure of 0.1 bar through a 2 mm long conical nozzle of 55 μm diameter and 30° opening angle (nozzle temperature was 43 °C). The nature of the beam produced at these expansion conditions is discussed herein (see Discussion). In the second case, the argon clusters were produced by the expansion of Ar at the stagnation pressure of P0 = 6 bar through the same nozzle (temperature T0 = −50 °C). Under these conditions the generated ArN clusters follow a log-normal size distribution with a mean size N = 230 determined by a modified Hagena’s formula.22,27,28 The argon clusters passed a skimmer and entered a differentially pumped pickup chamber filled with Fe(CO)5 vapor. The efficiency of the pickup process was judged by the cluster beam attenuation monitored by a quadrupole mass spectrometer at the end of the apparatus. The low temperature of the argon clusters (approximately 40 K) and effective momentum transfer generally cause a sticking probability of guest molecules to be close to one.29 In principle, it is possible to exactly determine the number of Fe(CO)5 molecules that are picked up on the argon cluster;24,29,30 however, the absolute pressure in the chamber has to be known. In the present experiment, the pressure is monitored by the ionization gauge, however, its correction factor for Fe(CO)5 is not known. It has been shown in numerous cases31,32 that guest molecules on the surface of argon clusters are mobile and coagulate efficiently. The sequential pickup process thus leads to creation of small Fe(CO)5 aggregates attached to argon nanoparticles, as judged from the recorded mass spectra. In both cases, the clusters passed through another differentially pumped chamber, and entered the vacuum chamber hosting a reflectron time-of-flight mass spectrometer (RTOF). The beam was ionized by electrons with variable energy Ee up to 90 eV at 10 kHz repetition frequency. The mass spectra were recorded with the resolution of M/ΔM ≈ 104. Recording the mass spectra while scanning the electron energy, 3D data were obtained from which the ion yields of each fragment were obtained. The appearance energy (AE) for a particular ionic fragment was determined by fitting the Ee dependency of the ion yield. The ion yield was obtained by integrating the signal intensity over the entire selected mass peak. The fitting procedure was based on the generalized Wannier law33 using the function S(Ee) = b , Ee ≤ AE; S(Ee) = b + a(Ee − AE)d , Ee ≥ AE

■

RESULTS Fragmentation Patterns. Figure 1 shows the mass spectra for various target systems: gas-phase Fe(CO)5, small pure

Figure 1. Mass spectra of (a) gas-phase Fe(CO)5, taken from ref 5; (b) pure Fe(CO)5 clusters generated in coexpansion with He; (c) ArN, N = 230, nanoparticles doped with multiple Fe(CO)5 molecules. All three spectra were measured at 70 eV electron energy. Spectrum d is sum of the mass spectra in the energy range 8−20 eV.

clusters, and clusters deposited on argon nanoparticles. In the present setup, it turned out to be basically impossible to produce a molecular beam of isolated Fe(CO)5 molecules. The RTOF is located 150 cm from the nozzle, and even the most gentle expansion conditions that produce a stable beam that far from the source (i.e., low stagnation pressures in coexpansion with helium) resulted in clustering of iron pentacarbonyl in the beam. As the reference gas-phase data in Figure 1 we thus plot the most recent data of Lacko et al.5 All spectra were recorded at 70 eV electron energy with the exception of the spectrum in Figure 1d, which shows the sum of all the spectra between 8 and 20 eV (energy step 0.2 eV). The RTOF mass analyzer does not fully resolve the main isotope of iron (55.935 amu) from the two CO ligands (55.990 amu). To overcome the ambiguity of spectral assignment we exploit the isotopic ratio of 54Fe:56Fe (6.3%) (CO does not have any isotopic contribution at two masses below the main isotope). We plot the intensity ratios of the peak at a given mass (k × 28) and the corresponding isotope mass (k × 28 −

(1)

where the fitting parameters were the background b, multiplication factor a, Wannier exponent d, and the appearance energy AE. In principle, in a multiparameter fitting procedure, one should use a global-minimum optimizing routine and statistical weighting of the signal. Such an approach, when used with high-resolution measurements, can yield very precise appearance energies.34 In view of the energy resolution of our electron gun (≈600 meV) we have decided to use a commonsense approach: the parameter b has been determined in the energy range below AE, and the local minima fits with various starting values AE, b, and d were performed (using the Levenberg−Marquardt algorithm as implemented in Gnuplot). The uncertainty in appearance energies, estimated from a B

DOI: 10.1021/acs.jpcc.6b05852 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C 2). Figure 2a shows these ratios for the clusters deposited on Ar nanoparticles. For each peak in the mass spectrum, the

dominant ion at the given mass can be unambiguously assigned: mass spectra in Figure 1 are marked according to these dominant ions. However, the signals at many peaks seem to have a minor contribution from ions containing more iron atoms. We have verified the assignment by measuring several mass spectra in a high-mass-resolution mode; Figures 2b and 2c shows the details of such peaks for masses 112 and 168 amu. The weak contributions from ions containing more than one iron atom are clearly resolved in these spectra, and their relative intensity is in good agreement with the isotope ratios obtained via the integration of the unresolved mass peaks in standardresolution mode (Figure 2a). Appearance Energies. The ion-yield curves for several fragment ions are shown in Figure 3, together with the Wannier law fits, eq 1. We show these curves for Fe(CO)5 aggregates on argon nanoparticles: in this case the “molecular” fragments Fe(CO)n+, n = 1−5, show two-threshold behavior. They were thus fitted by a superposition of two Wannier curves with thresholds AE 1 and AE 2 . The masses with dominant contribution from ions with two iron atoms have only one discernible threshold; the same is true for all masses in the case of pure Fe(CO)5 clusters. In order to prevent too many free parameters when fitting the two-threshold curves, AE1 and the corresponding Wannier exponent were fixed at the value obtained for the same mass in pure Fe(CO)5 clusters. Figure 4 and Table 1 summarize the evaluated appearance energies. For molecular fragments Fe(CO)n+, n = 1−5, originating from aggregates on argon nanoparticles, the symbols in Figure 4 correspond to the higher threshold, AE2. In pure Fe(CO)5 clusters, the appearance energies of ions Fe(CO)n+ and Fe(CO)5Fe(CO)n+ are very similar to the thresholds for the corresponding ions from the gas-phase Fe(CO)5. On the contrary, the appearance energies of Fe(CO)n+ fragments from the ionization on ArN nanoparticles are significantly higher, around 15 eV, and they are independent of n within the experimental error bars. In addition, the cluster fragments Fe2(CO)5+n+, n = 1−5, and the

Figure 2. (a) Intensity ratio of the peak with the mass k × 28 − 2 to the mass k × 28 reflecting the number of Fe atoms in the ion according to the ratio 54Fe:56Fe = 0.063. (b, c) Detailed shape of the peaks at 112 and 168 amu. The dashed line is a measurement with very low concentration of Fe(CO)5 in the pickup cell (no aggregates created on ArN surface).

Figure 3. Ion yield curves for several dissociative ionization fragment ions for Fe(CO)5 aggregates on argon nanoparticles. Solid lines are the Wannier law fits. C

DOI: 10.1021/acs.jpcc.6b05852 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

part of the signal at the mass peaks below 196 amu originates from ions with more than one iron atom. However, as can be seen in Figure 2, this contribution is small and at least 70% of the signal originates from ions with one iron atom. The degree of fragmentation when Fe(CO)5 is in an elementary aggregation stage is thus much lower than when it is isolated. Several effects can contribute to this stabilization. First is the mechanistic caging, when the Fe−C bond is not broken due to presence of neighboring cluster constituent and the available energy rather leads to cleavage of the intermolecular bonds. The second contributing effect could be a transfer of CO ligands: the first ionization step results in a stripped monomer ion which then “grabs” the CO ligands from the neighboring molecules. The Fe−C bond dissociation energies in neutral and cationic iron pentacarbonyl are very similar, approximately 1.25 eV;5 however, this can be changing with the number of ligands removed. Finally, the origin of the stabilization can be thermodynamic: the fragmentation in the gas phase results from a vibrationally hot cation ground state wheareas in a cluster the energy can dissipate, which effectively cools the cation. All of these effects have been observed previously in various molecular clusters.31,37,38 In the context of fragmentation, it is interesting to compare the present results to the recent communication39 on the dissociative ionization of W(CO)6 dimers, where the authors report production of a bare metal ion W2+. A similar channel, production of Fe2+, is observed here, Figure 2b, and we even see clear evidence of the pure iron trimer ion Fe3+, Figure 2c. However, similarly to the W(CO)6 case,39 the abundance of these completely stripped ions is very low when compared to other fragments. Charge Transfer Ionization on Argon Nanosupport. The appearance energies from pure Fe(CO)5 clusters are very similar to those from the gas phase (Figure 4). The neutral beam (created in the coexpansion of iron pentacarbonyl with helium) contains a distribution of neutral cluster sizes, including monomers. The ions can originate from the neutrals with the same number of iron atoms or from larger clusters. For ions with one iron atom, the thresholds (the appearance of the signal at the lowest energies) will thus probably originate from the ionization of monomers. The appearance energies for Fe(CO)5Fe(CO)n+ clusters are within experimental error the same as the corresponding Fe(CO)n+ ions. The situation is dramatically different for the Fe(CO)5 aggregates deposited on argon nanoparticles, where all ions show the threshold around 15 eV. This suggests that the generation of all the observed fragments starts with the same ionization process: namely, that an Ar atom in the nanoparticle is ionized by the incoming electron and the iron pentacarbonyl molecules are ionized by the subsequent charge transfer. This process is schematically depicted in Figure 5. The ionization energy of an isolated Ar atom in the gas phase is 15.7 eV, however, it is shifted toward lower energies in clusters.40,41 A similar shift of appearance energies has been observed (and attributed to the same charge-transfer mechanism) for Arcoated water and acetylene clusters.22,38 In principle, this is a transfer of an electron hole between the nanosupport and the guest molecules/aggregates. As can be seen in Figure 3, ions containing one iron atom have non-negligible intensity also below 15 eV, with the threshold at the lower appearance energy AE1. It can originate either from the ionization of gas-phase Fe(CO)5 molecules diffused from the pickup chamber or from a direct ionization of

Figure 4. Appearance energies AEs for dissociative ionization fragments for three different target systems: gas-phase Fe(CO)5 (ref 5), pure Fe(CO)5 clusters, and Fe(CO)5 clusters on argon nanoparticles. All the appearance energies from ArN fall within the shaded area.

Table 1. Measured Appearance Energiesa pure Fe(CO)5 clusters ion

mass (u)

FeCO+ Fe(CO)2+ Fe(CO)3+ Fe(CO)4+ Fe(CO)5+ Fe2(CO)6+ Fe2(CO)7+ Fe2(CO)8+ Fe2(CO)9+ Fe2(CO)10+ Fe(CO)5·Ar+ Fe(CO)5·Ar2+ Fe(CO)5·Ar3+

84 112 140 168 196 280 308 336 364 392 236 276 316

a

AE (eV) 12.8 11.7 9.7 9.0 8.3 11.6 10.9 9.9 9.5 8.3

Fe(CO)5 on ArN AE1 (eV), fixed

AE2 (eV)

12.8 11.7 9.7 9.0 8.3

14.8 14.5 15.0 14.5 13.6 15.6 15.4 15.6 15.1 15.3 14.8 15.1 14.9

The standard deviation is ±0.4 eV.

Ar-containing fragments Fe(CO)5Arn+, n = 1−3, also exhibit similarly high appearance energies. All the values fall within 14.6 ± 1.0 eV.

■

DISCUSSION Stabilization of Ligands with Respect to Dissociative Ionization. Comparison of the molecular region (below 200 amu) of the mass spectra in Figure 1 for the gas phase and two different types of clusters immediately reveals that the relative abundance of ions with more than one ligand, Fe(CO)n+, n = 2−5, is dramatically increased in both types of clusters. It should be noted, that among the various gas-phase mass spectra in the literature5,35,36 the relative abundance of these peaks somewhat varies (as is common when various types of mass spectrometers are used for the analysis); nonetheless in all of the previous gas-phase measurements the spectra are strongly dominated by the Fe+ and Fe(CO)+ fragments. When comparing individual panels in Figure 1, it should be taken into account that, in the present cluster measurements, a certain D

DOI: 10.1021/acs.jpcc.6b05852 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

when Fe(CO)5 clusters are on argon, in pure clusters the relative abundance of Fe(CO)n+, n = 1−5, is still considerably higher than in the gas phase. Since these are created by the direct electron-impact ionization, one can conclude that the stabilization is rather a consequence of the environment than a consequence of the ionization mechanism. Relevance to the Condensed-Phase Studies and FEBID. As outlined in the Introduction and summarized in the recent review,1 the quest for insight into the primary deposition mechanisms in FEBID has inspired a number of experimental approaches to study precursor fragmentation. One of them is ultrahigh vacuum surface studies on adsorbed precursor molecules, where, following the controlled electron irradiation, various surface-science techniques are used to study the composition of the adsorbed layers or the desorption of species from it. It has been found for several precursors that, in the first electron-induced decomposition step (fragmentation of intact precursor molecules), the degree of fragmentation is very low. For example, one ligand is removed for precursors Pt(PF3)4 and MeCpPtMe3,11,12 between one and two ligands are removed for Co(CO)3NO, and two ligands are removed for W(CO)6.13,14 The comparative gas-phase studies in these compounds have found that the dissociative ionization is very fragmentative, the dominant products being either bare metal ion or metal with one ligand attached.3,4,6 On the other hand, dissociative electron attachment (DEA) leads to removal of only one ligand at electron energies below 1 eV.3,7,8 The comparison between the surface and gas-phase data has thus indicated that the initial electron-induced fragmentation is principally through DEA, and is primarily affected through secondary electrons with incident energies below 1 eV (with the notion that very little is known about the neutral dissociation channel). The present results show that the similar, very fragmentative, behavior of Fe(CO)5 upon dissociative ionization in the gas phase is dramatically changed already in an elementary stage of aggregation. Both in the pure clusters and when the aggregates are attached to argon nanoparticles, the degree of fragmentation significantly drops and abundance of ions with more ligands attached rises by approximately an order of magnitude. The relative intensity of individual fragments of course varies with the electron energy. At FEBID conditions, the secondary electron distribution usually peaks around 5 eV and slowly drops: at 70 eV their abundance is up to a factor of 10 lower (this depends on the substrate and on the energy of the primary electron beam).2 It is interesting to note that at low electron energies the most abundant fragment ion for aggregates with the argon nanosupport (Figure 1d) is ion with one ligand removed, Fe(CO)4+. The stabilization of molecular cluster constituents has been observed previously for a number of molecules;31,37,38 it is thus reasonable to assume that a similar effect will be operative also in FEBID precursors listed in the previous paragraph. Their degree of electron-induced fragmentation observed in condensed phase can thus be well caused by the dissociative ionization, and not exclusively by DEA or neutral dissociation as assumed until now. The hole-transfer ionization that we observe on the surface of argon nanoparticle can in principle also occur at FEBID conditions. The primary electron beam interacting with the substrate will create a large amount of electron−hole pairs via sequential ionization events. The electrons create the spatially spread secondary electron maze and contribute to the deposit formation. The precursor molecules that are physisorbed on the

Figure 5. Schematic picture of the ionization of (Fe(CO)5)n on argon nanoparticle proceeding mostly via hole transfer with the support, leading mainly to Fe(CO)2+ ions or fragment ions with even more ligands attached.

Fe(CO)5 molecules on Ar nanoparticles. The diffusion is not expected to contribute significantly: there are two differentially pumped ultrahigh-vacuum chambers between the pickup and RTOF chamber, and over 1.5 m long flight path. Besides, we subtracted the background measured with the cluster beam blockedthis background would still contain the diffused signal. The second possibility is that some part of Fe(CO)5 monomers on the argon nanoparticles are ionized directly by the electron impact. Nevertheless, this contribution is rather small and the stronger increase of the ion signal occurs at AE2 where the argon ionization starts. The hole-transfer ionization mechanism from argon thus clearly dominates over the direct ionization of the Fe(CO)5 aggregates. We presume that the Fe(CO)5 aggregates are actually on the surface of the nanoparticle and do not submerge into the argon, as has been observed for few organic molecules.31 Strong evidence for this comes from the anion yields from such aggregates, where no influence of argon coverage, e.g., via the inelastic scattering or the energy shifts, has been observed.32 The dominance of the charge-transfer ionization simply reflects a higher probability of the interaction of the incoming electron with Ar atoms than with Fe(CO)5 molecules. The average nanoparticle size is N = 230, while there are only a few Fe(CO)5 molecules adsorbed. Although the degree of fragmentation upon ionization on argon nanoparticle cannot be determined unambiguously, by comparison of the positively and negatively32 charged spectra we can assume that the neutral (Fe(CO)5)m clusters with m ≈ 5 are generated on the nanoparticles. This is still order(s) of magnitude less than the number of Ar atoms, and thus the Ar ionization is more probable. The second factor influencing this is the ionization cross sections, which are, however, to our knowledge not known for Fe(CO)5. With respect to the discussion in the next section a question arises, whether such hole transfer does occur between a substrate and the adsorbate in condensed phase. Unfortunately, experimental identification of such a process is not straightforward. Massey et al.18 have measured electron-stimulated desorption of positive ions from thin Fe(CO)5 films condensed on xenon and on platinum foil. The appearance energies (desorption thresholds) of various ions varied between 15.5 eV for Fe+ and 23.7 eV for Fe(CO)5+. The opposite trend with the number of ligands hints that these energies are more influenced by the desorption probability (which is higher for lighter fragments) than by the energetics or by the ionization mechanism. It should be stressed that the ligand stabilization cannot be attributed exclusively to the charge-transfer mechanism. When comparing the mass spectra in Figures 1a, 1b, and 1c, one can see that even though the abundance of bare iron atoms is lower E

DOI: 10.1021/acs.jpcc.6b05852 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

(5) Lacko, M.; Papp, P.; Wnorowski, K.; Matejčík, Š Electroninduced Ionization and Dissociative Ionization of Iron Pentacarbonyl Molecules. Eur. Phys. J. D 2015, 69, 84. (6) Wnorowski, K.; Stano, M.; Barszczewska, W.; Jówko, A.; Matejčík, Š Electron Ionization of W(CO)6: Appearance Energies. Int. J. Mass Spectrom. 2012, 314, 42−48. (7) Engmann, S.; Stano, M.; Matejčík, Š.; Ingólfsson, O. The Role of Dissociative Electron Attachment in Focused Electron Beam Induced Processing: A Case Study on Cobalt Tricarbonyl Nitrosyl. Angew. Chem., Int. Ed. 2011, 50, 9475−9477. (8) May, O.; Kubala, D.; Allan, M. Dissociative Electron Attachment to Pt(PF3)4 - a Precursor for Focused Electron Beam Induced Processing. Phys. Chem. Chem. Phys. 2012, 14, 2979−2982. (9) Engmann, S.; Omarsson, B.; Lacko, M.; Stano, M.; Matejčík, Š.; Ingólfsson, O. Dissociative Electron Attachment to Hexafluoroacetylacetone and its Bidentate Metal Complexes M(hfac)2; M = Cu, Pd. J. Chem. Phys. 2013, 138, 234309. (10) Zlatar, M.; Allan, M.; Fedor, J. Excited States of Pt(PF3)4 and Their Role in Focused Electron Beam Nanofabrication. J. Phys. Chem. C 2016, 120, 10667−10674. (11) Wnuk, J. D.; Gorham, J. M.; Rosenberg, S. G.; van Dorp, W. F.; Madey, T. E.; Hagen, C. W.; Fairbrother, D. H. Electron Induced Surface Reactions of the Organometallic Precursor Trimethyl(methylcyclopentadienyl)platinum(IV). J. Phys. Chem. C 2009, 113, 2487−2496. (12) Landheer, K.; Rosenberg, S. G.; Bernau, L.; Swiderek, P.; Utke, I.; Hagen, C. W.; Fairbrother, D. H. Low-Energy Electron-Induced Decomposition and Reactio n s of Ad sorbe d Te t r ak i s(trifluorophosphine)platinum [Pt(PF3)4]. J. Phys. Chem. C 2011, 115, 17452−17463. (13) Rosenberg, S. G.; Barclay, M.; Fairbrother, D. H. Electron Beam Induced Reactions of Adsorbed Cobalt Tricarbonyl Nitrosyl (Co(CO)3NO) Molecules. J. Phys. Chem. C 2013, 117, 16053−16064. (14) Rosenberg, S. G.; Barclay, M.; Fairbrother, D. H. Electron Induced Reactions of Surface Adsorbed Tungsten Hexacarbonyl (W(CO)6. Phys. Chem. Chem. Phys. 2013, 15, 4002−4015. (15) Lukasczyk, T.; Schirmer, M.; Steinrück, H.-P.; Marbach, H. Electron-Beam-Induced Deposition in Ultrahigh Vacuum: Lithographic Fabrication of Clean Iron Nanostructures. Small 2008, 4, 841−846. (16) Compton, R. N.; Stockdale, J. A. D. Formation of Gas Phase Negative Ions in Fe(CO)5 and Ni(CO)4. Int. J. Mass Spectrom. Ion Phys. 1976, 22, 47−55. (17) Ribar, A.; Danko, M.; Országh, J.; Ferreira da Silva, F.; Utke, I.; Matejčík, Š Dissociative Excitation Study of Iron Pentacarbonyl Molecule. Eur. Phys. J. D 2015, 69, 117. (18) Massey, S.; Bass, A. D.; Sanche, L. Role of Low-Energy Electrons (