Asymmetric Partitioning of Metals among Cluster Anions and Cations

Article pubs.acs.org/JPCA

Asymmetric Partitioning of Metals among Cluster Anions and Cations Generated via Laser Ablation of Mixed Aluminum/Group 6 Transition Metal Targets Sarah E. Waller, Jennifer E. Mann, and Caroline Chick Jarrold* Department of Chemistry, Indiana University, 800 East Kirkwood Avenue, Bloomington, Indiana 47405, United States S Supporting Information *

ABSTRACT: While high-power laser ablation of metal alloys indiscriminately produces gas-phase atomic ions in proportion to the abundance of the various metals in the alloy, gas-phase ions produced by moderate-power laser ablation sources coupled with molecular beams are formed by more complicated mechanisms. A mass spectrometric study that directly compares the mass distributions of cluster anions and cations generated from laser ablation of pure aluminum, an aluminum/molybdenum mixed target, and an aluminum/ tungsten mixed target is detailed. Mass spectra of anionic species generated from the mixed targets showed that both tungsten and molybdenum were in higher abundance in the negatively charged species than in the target material. Mass spectra of the cationic species showed primarily Al+ and aluminum oxide and hydroxide cluster cations. No molybdenum- or tungsten-containing cluster cations were definitively assigned. The asymmetric distribution of aluminum and Group 6 transition metals in cation and anion cluster composition is attributed to the low ionization energy of atomic aluminum and aluminum suboxide clusters. In addition, the propensity of both molybdenum and tungsten to form metal oxide cluster anions under the same conditions that favor metallic aluminum cluster anions is attributed to differences in the optical properties of the surface oxide that is present in the metal powders used to prepare the ablation targets. Mechanisms of mixed metal oxide clusters are considered.

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INTRODUCTION Laser ablation (LA) is a technique commonly used for the production of gas-phase atoms, molecules, or clusters1 for fundamental scientific exploration (e.g., measuring the melting point of elemental clusters as a function of size)2 or for simple analytical chemical applications (e.g., identification of unknown alloy composition),3 the latter of which tends to rely on the nondiscriminating effect of very high LA power.4 While atomization of a surface via LA can be taken as a straightforward result of extremely high energy content of the surface, the mechanism of subsequent cluster formation is complex and multivariate. LA (which is not necessarily distinct from laser desorption processes) is generally believed to produce cations, neutrals, and photoelectrons which can subsequently undergo complex condensation to form larger cations, neutrals, and anions.5 While slow electron attachment to more weakly bound clusters systems such as neutral alkali metal clusters can result in evaporative cooling,6 in the case of materials such as refractory metal oxides, high bonding energy is sufficient to prevent this mechanism of internal energy release, and subsequent collisional cooling is an important process. While LA is a fundamentally nonequilibrium phenomenon, a very simple difference in metal or metal oxide cluster cation and anion formation lies in the thermodynamics: Cations are © 2013 American Chemical Society

typically on the order of 6−9 eV higher in energy than their corresponding neutrals, whereas the anions are typically on the order of 1−5 eV lower in energy than the neutral clusters. Depending on the cluster source conditions following a pulsed LA event, including carrier gas pressure and presence of reactant molecules, thermodynamics may come into play in the mass distributions of cluster cations versus anions. We recently reported the results of anion photoelectron spectroscopic measurements on LA-produced heteronuclear MoAlOy− (y = 1 − 4)7 and WAlOy− (y = 2 − 4)8 clusters for which production of sufficiently Al-rich species required a LA target with an Al:M (M = Mo or W) ratio of over 10. In contrast, the results presented below on cations produced from LA of the same target under comparable conditions produce abundant Al+. The well-documented preponderance of atomic Al+ in the cluster cation distributions generated from pure Al sources9 is the result of the low ionization energy (IE) of atomic Al, which evidently facilitates the ionic bonding in the mixed metal complex anions. From a survey of the literature, there are relatively few reports that directly compare anion and cation mass Received: December 5, 2012 Revised: January 24, 2013 Published: February 18, 2013 1765

dx.doi.org/10.1021/jp3119692 | J. Phys. Chem. A 2013, 117, 1765−1772

The Journal of Physical Chemistry A

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that protrudes approximately 0.2 mm into the channel and the 1.6-mm laser entrance hole situated opposite of the surface. The solid metal disk targets are 5.6 mm in diameter and ≤0.7 mm thick. They are prepared by compressing pure metal powder or a mixture of metal powders, using a hydraulic press (4000 pounds per square inch) and a custom-built sample holder. The three samples used in this study were (1) pure Al powder (Sigma-Aldrich, 99.95+%, 1) clusters are less prevalent. The Al/98 Mo mass spectrum (Figure 3b) is dominated by the MoO5− hyperoxide mass peak, and the Mo2Oy− series is skewed to a higher oxidation distribution relative to the mass spectrum shown in Figure 1b. The Al/186W sample mass spectrum (Figure 3c) is more dominated by the “magic” aluminum oxide cluster anions and WO3−. In both cases, an oxide progression of the form MAlOy− (y = 3−6 for M = Mo; y = 2−6 for M = W) is observed, and the most intense member of the oxide progression is MAlO4−. With 1.25% O2/bal He carrier gas, the mass spectrum of the clusters generated with the pure Al target, shown in Figure 4a,

The mole percent of 98Mo and 186W in both Al/98Mo and Al/186W mixed targets is 8% (Al atoms outnumber Mo and W by 11.5 to 1), yet the 98Mo- or 186W-containing cluster anions contribute to a more significant portion of the ion signal. The peak areas, weighted by the number of Al, Mo, or W atoms in the particular ion, were calculated and used to determine the relative abundances of the various metals in the anion mass spectra. For anions generated using the Al/98Mo sample, Mo represented 48(1) % of the metal atoms, while for anions generated using the Al/186W sample, W represented 36(6)% of the metal atoms. Both Mo and W are significantly more abundant in the cluster anion mass spectra than in the mixed metal target (by a factor of 6 in the case of Mo and a factor of 4.5 in the case of W). The relatively high abundance of Mo and W atoms in the cluster anion mass spectra raises the question on the fate of Al atoms in the LA source; in part, the question is answered by analyzing cation mass spectra. Comparison of Cluster Cation Distributions. Figures 5a−5c show the mass spectra of cations produced under similar

Figure 4. Mass spectra of cluster anions generated via LA in 1.25% O2/balance He carrier gas of (a) a pure Al target, (b) a 92% Al/8% 98 Mo target, and (c) a 92% Al/8% 186W target. Main mass peaks are labeled.

exhibits several hyperoxide masses (e.g., AlO2−, Al3O5−, Al4O7−). The mass spectra of clusters generated from the mixed metal targets, shown in Figures 4b and 4c, are both dominated by the MO5− anions, and the most intense member of the MAlOy− series (y = 4−6 for M = Mo; y = 3−6 for M = W) shifts to y = 5. To facilitate a more direct comparison of the Mo- and WMAlOy− series, the Supporting Information includes several mass spectra obtained with different O2 concentrations, in which the mass scales are shifted so the MAlOy− series are aligned for the Al/98Mo and Al/186W targets (Figures S4−S6). Also in the Supporting Information is a series of mass spectra of cluster anions generated using a pure 98Mo sample target with 0%, 0.3%, and 1.25% O2 concentrations, to show the impact of O2 content on the pure transition metal oxide distributions (Figure S7). Mass distributions obtained using a pure 186W target show the same trend. That is, both cluster systems skew toward stoichiometric clusters significantly with 0.3% O2/bal He carrier gas, and hyperoxide MxO(3x+2)− clusters dominate the mass spectra when clusters are produced using 1.25% O2/ bal He carrier gas.

Figure 5. Mass spectra of cations generated via LA in He carrier gas of (a) a pure Al target, (b) a 92% Al/8% 98Mo target, and (c) a 92% Al/ 8% 186W target.

conditions as those used in obtaining the anion mass spectra shown in Figures 1a−1c in that He carrier gas was used in the ion source. However, as mentioned in the Experimental description, higher LA power was used to observe any signal beyond Al+ for all three targets. In Figure 5a, the most intense ion signal is Al+, with several higher masses observed with less than 0.01 the intensity of Al+. In Figure 5b, no signal is observed at any mass corresponding to a Mo-containing ion. Rather, several trialuminum oxides were observed. Note that parts of the traces in Figures 5a and 5c have been scaled by 50 and 10, respectively. The mass spectrum obtained with the Al/186W target (Figure 5c) also shows primarily Al-containing complexes, several of which are labeled to guide the eye. A number of the unlabeled peaks can only be assigned assuming 1768



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some hydrogen content, which is not surprising considering that the surface of aluminum is typically covered by oxides and hydroxides. Two low-intensity peaks are at masses consistent with W-containing complexes, although the assignments should be considered tentative. Upon addition of O2 to the carrier gas, the mass spectra obtained using the three different samples look similar, with superficial differences in peak widths and small differences in relative intensity of some of the aluminum oxide cation peaks. Figures 6a, 6b, and 6c show the cation mass distributions

Figure 7. Mass spectra of cations generated via LA in 1.25% O2/ balance He carrier gas of (a) a pure Al target, (b) a 92% Al/8% 98Mo target, and (c) a 92% Al/8% 186W target.

oxide clusters. Mass spectra of a mixed 98Mo/186W target (metals were combined to conserve on the isotopically pure material used) generated with 0.3% O2/bal He are included in the Supporting Information along with the cluster anion mass spectrum produced in He (Figure S8). We therefore conclude that the absence of 98Mo and 186W from the mass spectra of cations generated by LA of Al/98Mo and Al/186W targets, respectively, is likely due to lower IE’s of AlnOy and AlnOyHz species.

Figure 6. Mass spectra of cations generated via LA in 0.3% O2/balance He carrier gas of (a) a pure Al target, (b) a 92% Al/8% 98Mo target, and (c) a 92% Al/8% 186W target.

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DISCUSSION Disparities in Composition of Species Populating Anion and Cation Mass Spectra. As mentioned in the Introduction, high LA powers can generate nondiscriminating distributions of atomic species.4 However, the most significant result of this study is the following: In moderate-energy LA/ pulsed molecular beam production of atomic and cluster ions from a mixed metal target, one of the metals can dominate the anionic species while the other can dominate the cationic species. There are several zero-order features of Al, Mo, and W that can be considered when attempting to explain this phenomenon. Table 1 summarizes the atomic electron affinities (EA’s), IE’s, and bulk melting and boiling temperatures for Al, Mo, and W. The EA’s of both Mo and W atoms are higher than the EA of Al,29 although the oxides of all three metals have very comparable EA’s, as summarized for a few species in Table 1. Furthermore, Aln clusters in the n = 3−10 range have EA’s that increase from approximately 1.5 to 2.5 eV.30 Based on this, the lack of atomic aluminum cluster anions observed in mass spectra of cluster anions generated from an aluminum-rich mixed target using pure He carrier gas (e.g., Figures 1b and 1c) cannot be rationalized based on a disparity in cluster EA’s. On the other hand, atomic Al+ tends to dominate mass spectra of cations generated from LA of Al targets.31 We therefore attribute this imbalanced distribution of Al and M (M

measured when the He carrier gas was seeded with 0.3% O2, and Figures 7a, 7b, and 7c show the same, but with 1.25% O2 seeded in the carrier gas. Figures 6a, 6b, and 6c in particular are very similar in that the mass spectra are dominated by Al+ and exhibit similar AlnOy+ cluster distributions, as indicated by the peak labels. Nearly all clusters are suboxide relative to the bulk. Figures 7a, 7b, and 7c show more differences from one another: The mass distribution of clusters generated with the pure Al target, while still dominated by the Al+ ion peak, shows a broader distribution of cluster sizes and oxidation. This is likely to be due to more effective collisional cooling of newly formed clusters by O2 than by He. The mass spectrum of clusters produced from the Al/98Mo target appears somewhat suppressed, showing clusters with roughly the same intensity as shown in Figure 6b, but with many fewer species evident. The mass distribution of cluster cations generated with the Al/186W target exhibits increased prevalence of hydroxides and the loss of Al3O2+. Again, essentially no signal in any of the cation mass spectra can be attributed to 98Mo- or 186W-containing species under the ion source conditions used in these studies. Indeed, we were unable to produce any cationic MxOy− (M = Mo, W) species in UHP He carrier gas. However, by seeding small concentrations of O2 into the He carrier gas, we were able to produce metal 1769



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As mentioned in the Introduction, LA and laser desorption can both occur when a laser pulse impinges on a target surface. An obvious difference between the oxides of Mo, W, and Al is the band gaps of their bulk oxides, which is near 10 eV for the various phases of Al2O3 (ref 47) and much lower in energy for WO3 (2.6 eV)48 and MoO3 (4.1 eV),49 with defects (e.g., oxygen vacancies) in both known to introduce lower energy absorptions. It therefore seems possible that the oxide layer on molybdenum and tungsten powders may act as an effective matrix that facilitates desorption of oxide fragments from the surface, while the oxide layer on the aluminum is transparent to the LA laser, resulting in more energy absorbed by the metallic portion. Differences in Cluster Distributions of the Mixed Al/ Mo and Al/W Targets. Minor differences were seen in the mass spectra of cluster anions generated from LA of the two mixed metal targets, which were prepared to have the same mole fraction of Al. The primary difference was that the abundance of 186W in the anion mass spectra was not as enhanced (relative to the abundance of 186W in the target sample) as 98Mo. Being in the same group on the periodic table, molybdenum and tungsten have similar properties. However, both the temperatures of fusion and vaporization of molybdenum are lower than those of tungsten, which, since surface heating accompanies LA, may contribute to the higher relative abundance of 98Mo. However, tungsten powder is also much harder than molybdenum powder.50 We cannot eliminate the possibility that the roughly spherical tungsten particles are simply more deeply embedded into the aluminum matrix than the roughly spherical molybdenum particles, which are more readily deformed upon compression with the aluminum particles (and therefore more abundant on the surface). A less significant difference in cluster anion distributions generated from LA of the mixed targets was that the WAlnOy− species tended to be in slightly lower oxidation states than the MoAlnOy− species. This is surprising, since molybdenum, a second row metal, is more easily reduced than tungsten, a thirdrow metal. There are no obvious explanations for this observation, and we leave it as a topic for further investigation.

Table 1. Atomic IE’s and EA’s and Bulk Melting and Boiling Temperatures of Al, Mo, and W, along with Several Known EA’s of the Respective Metal Oxide Clusters bulk bulk melting boiling temp temp (Tfus, °C)c (Tvap, °C)c

atomic IEa (eV)

atomic EAb (eV)

Al

5.99

0.44

660

2519

Mo

7.09

0.75

2623

4639

W

7.98

0.81

3422

5555

approximate EA’s of pure metal oxide clusters observed in mass spectrad Al3O3 2.8 eV26 Al4O3 0.7 eV; Al4O5 2.3 eV37 Al5O4 3.5 eV27,37 Al6O5 2.8 eV; Al7O5 3.4 eV37 MoO2 2.0 eV38; MoO3 3.2 eV39 Mo2O2 2.4 eV; Mo2O3 2.3 eV Mo2O4 2.1 eV; Mo2O5 2.7 eV WO2 ∼ 2 eV40 W2O2 1.7 eV; W2O3 2.2 eV W2O4 3.2 eV; W2O5 3.6 eV41,42

a

Reference 32. bReference 29. cReferenec 43. dEA’s for M3Ox, x = 3− 6 and M = Mo, W were measured to be between 1.5 and 2.8 eV, while for x > 6, EA > 3.5 eV.44

= Mo and W) over the different charge states to the large “sink” of atomic Al+ produced via LA, either directly or via the energetically favorable Aln+ → Aln−1 + Al+ dissociation process.9 The IE of Al is 5.99 eV, which is only nominally lower than Mo (7.09 eV) and W (7.86 eV).32 Assuming the abundance of Al+ reflects large quantities of neutral Al, the question of why atomic Mo+ and W+ (or Mo− and W−, and all elemental clusters that might form) are not observed under the source conditions may due to the very high melting and boiling temperatures of these refractory metals (Table 1). That is, under the source conditions employed in these experiments, the surface temperature, which is expected to be less than 5000 K,24 is sufficient to result in significant evaporation of Al atoms but insufficient to evaporate the refractory metal atoms. The low IE of Al relative to Aln explains the high abundance of Al+ relative to Aln+ in mass spectra,9 and differences in IE extend to the metal oxides as well. While existing data is limited, aluminum suboxide clusters have lower IE’s (ca. 6.8 eV)33 than MoOy suboxide species (MoO, 7.45 eV;34,35 MoO2, 9.2 eV36), so if MoxOy+ or WxOy+ clusters are produced directly via LA, they could quickly gain an electron via collisions with aluminum-containing species, resulting in the aluminum-rich cation mass spectra we observed (Figures 5−7). Laser Ablation versus Laser Desorption? Mass distributions of clusters generated from LA of Mo and W targets in He carrier gas present a range of oxide and suboxide clusters, in contrast to the more metallic species measured from the similarly ablated Al target (e.g., Aln−, n = 2−6, AlnC−, n = 2− 10). All three metals have very strong metal−oxygen bonds. For neutral diatomic species, D0(Al−O) = 5.335 eV,45 D0(Mo− O) = 5.210 eV,46 and D0(W−O) = 7.46 eV.46 Therefore, we conclude that oxygen affinity is not the cause of this difference in aluminum and Mo (or W) oxide cluster distributions. Further, given that powder forms of all metals were used in preparing samples, and the propensity of all three metals to have oxide (and hydroxide, in the case of Al) surface coats, we conclude that the targets all have comparable oxide content.

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CONCLUSIONS Mass spectra of anions and cations generated via LA of pure aluminum targets and mixed metal targets composed of 92 mol % aluminum and 8 mol % molybdenum or tungsten, both of which are Group 6 transition metals, were reported and analyzed. Under a certain set of LA/pulsed molecular beam conditions, cluster anion distributions showed molybdenum or tungsten at higher abundances than the LA target abundance by a factor of 6 in the case of Mo and by a factor of 4.5 in the case of W. Under the same conditions, mass spectra of cationic species were dominated by atomic Al+. While the intensity of larger cluster cations was enhanced with increased O 2 concentration in the carrier gas, no cationic cluster species could be definitively assigned to a Mo- or W-containing species. Rather, all species appeared to be of the form, AlnOyHz+. The absence of Mo- or W-containing species in the cation mass spectra was attributed to the lower IE’s of the Al atom and aluminum suboxide clusters relative to Mo and W atoms and their oxides. From a survey of the anion mass spectra, it was evident that Mo and W tend to form a range of MxOy− cluster anions when He is used as the carrier gas in the cluster source, while Al tends to form pure metallic clusters (along with contaminated 1770



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metallic clusters, such as AlnC− and AlnC2− under our experimental conditions). This may be due to the optical properties of the oxide (and hydroxide) layers that form on the metal powders used to prepare the samples; Aluminum oxides/ hydroxides have ∼10 eV band gaps, while molybdenum and tungsten oxides can absorb visible light, which may facilitate oxide cluster formation via visible light absorption. Addition of O2 to the carrier gas resulted in the production of hyperoxidized Mo and W clusters, but also the elimination of the aluminum and aluminum carbide cluster anions and the appearance of the typical magic aluminum oxide cluster anions.26,27 The mixed metal oxide cluster anions (MoAlnOy−; WAlnOy−) were observed in a range of oxidation states which skewed to higher oxidation state with increased O 2 concentration in the carrier gas, as expected.

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Photoelectron Spectroscopy and DFT Calculations. J. Chem. Phys. 2012, 137, 024302. (8) Mann, J. E.; Waller, S. E.; Jarrold, C. C. Electronic Structures of WAlOy and WAlOy− (y = 2−4) Determined by Anion Photoelectron Spectroscopy and Density Functional Theory Calculations. J. Chem. Phys. 2012, 137, 044301. (9) Jarrold, M. F.; Bower, J. E.; Kraus, J. S. Collision Induced Dissociation of Metal Cluster Ions: Bare Aluminum Clusters, Aln+ (n = 3−26). J. Chem. Phys. 1987, 86, 3876−3885. (10) Liu, J.-B.; Han, C.-Y.; Zheng, W.-J.; Gao, Z.; Zhu, Q.-H. Formation of Lead/Sulfer Binary Cluster Ions by Laser Ablation. Int. J. Mass Spectrom. 1999, 189, 147−156. (11) Zhang, X.; Li, G. L.; Xing, X. P.; Zhao, X.; Tang, Z. C.; Gao, Z. Formation of Binary Alloy Cluster Ions from Group-14 Elements and Cobalt and Comparison with Solid-State Alloys. Rapid Commun. Mass Spectrom. 2001, 15, 2399−2403. (12) Dance, I. G.; Fisher, K. J.; Willett, G. D. Molecular Manganese Sulfide Clusters Formed by Laser Ablation. J. Chem. Soc., Dalton Trans. 1997, 2557−2561. (13) Han, C.; Zhao, X.; Zhang, X.; Gao, Z.; Zhu, Q. Formation, Photodissociation and Structure of Chromium/Phosphorus Binary Cluster Ions. Rapid Commun. Mass Spectrom. 2000, 14, 1255−1259. (14) Hamrick, Y. M.; Morse, M.D . Compartitive Cluster Reaction Studies of the V, Nb, and Ta Series. J. Phys. Chem. 1989, 93, 6494− 6501. (15) Berg, C.; Schindler, T.; Lammers, A.; Niedner-Schatteburg, G.; Bondybey, V. E. Dehydrogenation of Xylene Isomers on Niobium Cluster Cations Nbn+ (n = 2−26). J. Phys. Chem. 1995, 99, 15497− 15501. (16) Zemski, K. A.; Justes, D. R.; Bell, R. C.; Castleman, A. W., Jr. Reactions of Niobium and Tantalum Oxide Cluster Cations and Anions with n-Butane. J. Phys. Chem. A 2001, 105, 4410−4417. (17) Thomas, O. C.; Zheng, W.; Bowen, K. H., Jr. Magic Numbers in Copper-Doped Aluminum Cluster Anions. J. Chem. Phys. 2001, 114, 5514−5519. (18) Moravec, V. D.; Jarrold, C. C. Study of the Low-Lying States of NiO− and NiO using Anion Photoelectron Spectroscopy. J. Chem. Phys. 1998, 108, 1804−1810. (19) Mayhall, N. J.; Rothgeb, D. W.; Hossain, E.; Raghavachari, K.; Jarrold, C. C. Electronic Structures of MoWOy− and MoWOy Determined by Anion Photoelectron Spectroscopy and DFT Calculations. J. Chem. Phys. 2009, 130, 124313. (20) Dietz, T. G.; Duncan, M. A.; Powers, D. E.; Smalley, R. E. Laser Production of Supersonic Metal Cluster Beams. J. Chem. Phys. 1981, 74, 6511−6512. (21) Posey, L. A.; Deluca, M. J.; Johnson, M. A. Demonstration of a Pulsed Photoelectron Spectrometer on Mass-Selected Negative Ions, O−, O2− and O4−. Chem. Phys. Lett. 1986, 131, 170−174. (22) Bakker, J. M. B. Beam-Modulated Time-of-Flight Mass Spectrometer. 2. Experimental Work. J. Phys. E: Sci. Instrum. 1974, 7, 364−368. Bakker, J. M. B. Beam-Modulated Time-of-Flight Mass Spectrometer. 1. Theoretical Considerations. J. Phys. E: Sci. Instrum. 1973, 6, 785−789. (23) Hossain, E.; Rothgeb, D. W.; Jarrold, C. C. CO2 Reduction by Group 6 Transition Metal Suboxide Cluster Anions. J. Chem. Phys. 2010, 133, 024305. (24) Bogaerts, A.; Chen, Z. Effect of Laser Parameters on Laser Ablation and Laser-Induced Plasma Formation: A Numerical Modelling Investigation. Spectrochim. Acta, Part B 2005, 60, 1280− 1307. (25) See the Supporting Information for the cluster source diagram, mass spectra of the primarily carbon-based source contamination found at low mass, mass spectra of clusters generated from W, Mo and Au rods, Mo/Al and W/Al samples, shifted to align common transition metal compositions, mass spectra of pure 98MoxOy− clusters as a function of O2 concentration in the carrier gas, and mass spectra of anions and cations generated from a 98Mo/186W (Al-free) target.

ASSOCIATED CONTENT

S Supporting Information *

Detailed diagram of the cluster source (Figure S1), mass spectra of low-mass carbon-containing contaminants in the experiment (Figure S2), mass spectra of clusters generated by ablation of cleaned, solid Mo, W, and Au rods in pure He carrier gas (Figure S3), mass spectra of clusters produced with different O2 concentrations in the carrier gas and mixed metal targets, with mass scales shifted to easily compare MAlOy− mass distributions for M = 98 Mo and 186W (Figures S4−S6), mass spectra of 98MoxOy− clusters generated with He, 0.3% O2/bal He, and 1.25% O2/bal He (Figure S7), and a comparison of cluster anions and cations generated with an aluminum-free 98 Mo/186W target (Figure S8). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel. 812-856-1190; e-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge support for this work from the Chemical Catalysis Program of the National Science Foundation (CHE-1012641). C.C.J. thanks the research group of Prof. Bogdan Dragnea for the use of their optical microscope.

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REFERENCES

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Asymmetric Partitioning of Metals among Cluster Anions and Cations

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