Laser Desorption Time-of-Flight Mass Spectrometry of Inorganic

Laboratory Experiment pubs.acs.org/jchemeduc

Laser Desorption Time-of-Flight Mass Spectrometry of Inorganic Nanoclusters: An Experiment for Physical Chemistry or Advanced Instrumentation Laboratories Timothy M. Ayers,§ Scott T. Akin, Collin J. Dibble, and Michael A. Duncan* Department of Chemistry, University of Georgia, Athens, Georgia 30602-2556 United States S Supporting Information *

ABSTRACT: New experiments for the undergraduate laboratory are described using laser desorption time-of-flight (TOF) mass spectrometry to produce and analyze a variety of inorganic nanoclusters. Laser vaporization of solid powder samples of sulfur, phosphorus, and bismuth all produce gas-phase cluster ions with unusual mass spectral patterns. This experiment provides an introduction to TOF mass spectrometry and ultrasmall nanoclusters, as well as hands-on experience in vacuum systems, lasers, oscilloscopes, and pulsed electronics. New concepts in chemical bonding for inorganic materials are explored.

KEYWORDS: Upper-Division Undergraduate, Inorganic Chemistry, Physical Chemistry, Analytical Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Gases, Lasers, Mass Spectrometry

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typically take this class in their third or fourth year after lecture courses in analytical instrumentation and with the physical chemistry course as either a pre- or corequisite. Because the experiment combines lasers and mass spectrometers, it is quite popular.

ass spectrometry is one of the most widely used techniques in chemistry. Time-of-flight (TOF) instruments like those employed for this laboratory have advantages and disadvantages compared to other mass spectrometers.1−3 They are simple in design, inexpensive to construct, and the principles of operation are understandable to undergraduates. The mass resolution is sometimes not as good, but the mass range extends far beyond that of other instruments. As demonstrated here, a TOF instrument can be combined with a laser vaporization ion source to produce unusual mass spectra for many inorganic nanoclusters. Time-of-flight mass spectrometry is used for many applications in chemistry.1−3 When coupled with matrixassisted laser desorption ionization (MALDI), it is popular for the detection of biomolecules,4,5 as illustrated in previous undergraduate experiments.6−13 In contrast to this, the laser desorption experiments here focus on the vaporization of inorganic materials and the growth of nanoclusters from their gas-phase atoms. Like biopolymers, such nanoclusters may have high molecular weights, and a TOF instrument is well suited for their detection. We present here three examples of inorganic nanoclusters produced through vaporization of powder samples with a pulsed Nd:YAG laser and detected with a time-of-flight mass spectrometer. Sulfur, phosphorus, and bismuth oxide all produce unusual mass spectra and cluster growth patterns, illustrating new chemical bonding concepts. This experiment has been used for over 20 years in the undergraduate physical chemistry or advanced instrumentation laboratories at the University of Georgia. Chemistry majors © 2013 American Chemical Society and Division of Chemical Education, Inc.

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EXPERIMENTAL PROCEDURE

Overview of the Instrument

A schematic of our time-of-flight mass spectrometer is shown in Figure 1. This instrument is homemade,14 but comparable commercial instruments suitable for undergraduate laboratories are available.15,16 The samples are solid powders pressed into a 1/8 in. diameter hole on the end of a stainless steel rod probe tip. These are loaded into the probe tip, inserted into the mass spectrometer through a vacuum interlock, and mounted flush with the rear acceleration plate in the ion source. The desorption−ionization laser is a Nd:YAG (New Wave Research, Polaris II) operating at 532 or 355 nm. The beam is aligned visually onto the sample, which is then vaporized and ionized using repeated pulses (10 Hz) from its loosely focused output (20 cm lens). The fluence of the laser is adjusted with a variable attenuator or iris diaphragm to optimize production of the desired clusters; energies of about 1 mJ/pulse are employed for most experiments. The laser is moved periodically to different positions on the sample to access fresh material. High acceleration voltages (typically 10 kV) and delayed-pulsed ion Published: November 8, 2013 291

dx.doi.org/10.1021/ed4003942 | J. Chem. Educ. 2014, 91, 291−296

Journal of Chemical Education

Laboratory Experiment

a commercial instrument, mass analysis is hidden in the software, but in a homemade instrument like ours, the mass assignment requires active engagement from the student. Details of this procedure are given in the Supporting Information. The experiments described here were reproduced using a commercial time-of-flight instrument (Bruker “Autoflex”), and identical cluster distributions were obtained for all samples. Therefore, if a homemade LDI-TOF instrument is not available, instruments in typical departmental mass spectrometry laboratories can be employed for these experiments. Commercial instruments likely have an advantage in mass resolution over homemade devices. However, MALDI instruments usually use a nitrogen laser (337 nm) with lower output power (10−20 μJ/pulse) that is not as efficient as a pulsed Nd:YAG in desorbing metal-containing samples.

Figure 1. Schematic of a laser desorption time-of-flight mass spectrometer. A Nd:YAG laser is used as the vaporization and ionization source.

Laboratory Instruction

extraction are implemented for improved mass resolution.17 Deflection plates are used to deflect the ion beam to position it on the detector, if needed, and an einzel lens is employed to focus the ion beam within the acceptance aperture of the detector. Laser vaporization has been used for many years to make atomic clusters in the gas phase.18−22 The mechanism of vaporization and cluster growth is not well understood, but relevant issues are discussed in a recent review.22 The pulsed (3−5 ns duration) laser focused onto the sample produces a hot plasma containing atoms, ions, and electrons. Condensation of atoms via three-body collisions or direct vaporization of molecular species are both possible. In some experiments, a buffer gas such as argon or helium is employed to promote collisional cooling and atom condensation.18,22 In the neat desorption experiments here, the atomic vapor must condense without collisions with a buffer gas, but this occurs with reasonable efficiency for elements with high vapor densities.19 The mechanism of ionization is also not completely clear. Clusters may grow as charged species beginning with a vaporized cation, or as neutrals, which become ionized later in the expanding plume of material. If electrons are produced in the plume, they can be accelerated by the field in the source region (in the direction opposite to cations), and then collisions with fast electrons can cause electron impact ionization. Photoionization is unlikely because the photon energies (2.3 eV at 532 nm; 3.5 eV at 355 nm) are low compared to the ionization energies of these atoms and their clusters (typically 6−10 eV). Multiphoton ionization is conceivable, but the pulsed laser is only present for a few nanoseconds, and cluster growth is much slower than this. Therefore, the laser light is gone before clusters have grown from the atoms produced by vaporization and could only affect clusters desorbed in molecular form from the solid powder. The time-of-flight instrument is constructed following the design of Wiley and McLaren.17 The time-of-flight measurement begins with the laser pulse, which forms ions almost instantaneously, and the synchronization pulse from the laser triggers the oscilloscope. Ions are detected at the end of the flight tube using a standard electron multiplier tube (EMT). Ion signal (y axis) versus time (x axis) waveforms are accumulated with a digital oscilloscope (LeCroy LT 341) by summing the results of several laser shots and then transferring the result to a PC via an IEEE-488 interface. Excel, Origin, or other database software is employed to manipulate the data. In

This experiment is one of eight typically taught in a onesemester lab in advanced instrumentation. The class has an enrollment of 60 chemistry majors distributed over four sections. Students rotate through four experiments each in the first and second halves of the semester. In this way, a single instrument can serve all the students in the class. Each experiment takes two three-hour lab periods in the same week. Prior to each experiment, a one-hour prelab lecture is given by the faculty member in charge of the lab covering the instrumentation and chemical concepts. Additional lectures at the beginning of the term cover laser safety (one-hour lecture) and other aspects of report formats and preparation. Teaching assistants (TAs) supervise the experiments in the lab and grade the journal-article-style lab reports. Because sample loading requires operation of a vacuum interlock, this part is usually done by the TAs, but other aspects of the experiment are done by the students. Mass spectra stored as Excel files are analyzed in the lab or taken home on a flash drive for subsequent analysis and figure construction. The LDI-TOF instrument for these studies is located in a faculty research lab rather than in the normal undergraduate laboratory room for better laser safety, more convenient maintenance, and limited access to the expensive instrumentation.

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HAZARDS

The main hazards associated with this lab are exposure to the laser light and to high voltages associated with the mass spectrometer. All personnel should complete appropriate laser safety training before operating a Nd:YAG laser. Such courses are provided by university safety personnel or are available online. A Nd:YAG is a Class IV laser system, and severe eye damage is possible from exposure to the main beam or its partial reflections from shiny surfaces or optics. If laser safety training is not feasible, exposure can be minimized if the laser is isolated within an appropriate light-tight enclosure. High voltages are typically enclosed in commercial instruments but may not be in homemade systems. Students are warned against touching any cables or connector associated with the power supplies. Finally, students should access MSDS forms related to this laboratory and identify hazards and proper handling and disposal of chemicals used. 292



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cluster ions of the form Sn+, for n = 2−8, are observed. The signal for S+ is extremely weak, but the S2+ and S5+ mass peaks are prominent. The spectrum extends out to S8+, but the mass peaks corresponding to larger cluster ions, S9+ and S10+, are much less intense. The anion spectrum is noticeably different from that for the cations. Only the mass peaks for n = 1−4 are observed, with S3− by far the most intense. These observations agree with previous results reported by Johnson and coworkers.23 The interpretation of these mass spectra should stimulate speculation about which exact atomic or molecular species are desorbed, whether there is subsequent growth of larger species, and whether there is fragmentation in the ionization process. It is well known that solid sulfur primarily contains the S8 allotrope.24,25 The vapor above heated sulfur samples has also been shown to contain neutral Sn molecules.26,27 It is therefore possible that laser vaporization desorbs molecular S8 directly into the gas phase and that the ions observed represent fragmentation products from this molecular species. Another possibility is that desorbed sulfur atoms recombine to form these clusters and that the growth ends near the S8 species. Unfortunately, there is no way of determining from mass spectra alone which is the predominant mechanism of cluster production. However, either mechanism suggests the likely importance of eight-membered rings in the gas-phase cations. These results for sulfur cations vary noticeably from corresponding experiments reported by Martin in a review article,27 in which clusters formed by heating the powder in an oven and ionizing it with electron impact produced multiples of the S8 species. It is understandable that different vapor production methods and ionization processes lead to different mass spectra. Interested readers are referred to ref 22 for a more detailed discussion of cluster growth under different conditions and the role of ionization conditions on the appearance of mass spectra. An example of fragmentation that occurs in higher power laser desorption is described in a recent study of fullerene LDI with infrared lasers.28 It is also interesting to note the differences between anions and cations. This suggests that the number of electrons present in these species plays an important role in their stability. The details of ion formation and cluster growth in laser vaporization processes are not well understood, and therefore speculation about these processes is entirely acceptable. Students should be encouraged to propose structures and bonding configurations for both the prominent cation and anion species in an attempt to rationalize why certain clusters are produced more than others. In particular, they should question why the abundant clusters here (S3−, S2+, and S5+) all have an odd number of electrons. It is particularly useful to discuss what information can and cannot be derived with certainty from mass spectra. Ab initio theory on these systems is possible to elucidate cluster ion structures, and this kind of work is also recommended for more advanced students.

RESULTS AND DISCUSSION

General Comments on Atomic Clusters in the Gas Phase

The experiments here demonstrate an area of research that is fundamental, with wide-ranging application, but that is unfamiliar to many students and faculty. It has been demonstrated now in many laboratories that gas-phase atoms of any material condense as they cool, first to form diatomics, then triatomics, and then so on to make larger clusters.18−22 These species need not be stable in terms of “normal” bonding patterns because the experimental conditions (only one species present) give the atoms no choice in what they can do. Even when there is no simple way to describe the bonding (i.e., when the component atoms have unused valence electrons), bonds will still always form because it is better (i.e., downhill energetically) to have molecules than to have separated atoms. Stability is then a relative concept for these systems. If there is a closed-shell-bonding configuration for one particular size cluster, that size will be more stable than others and may be produced more effectively as condensation occurs. Abundant species may also be formed if the ionization process (involving ionization potential) is easier than that for others. Therefore, the intensities in mass spectra of clusters are actually quite difficult to interpret. This has been an area of much confusion in the literature; therefore, it is also one that can stimulate much student discussion. This is why this experiment is one of our favorites. Sulfur

The mass spectra measured for cations (bottom) and anions (top) when sulfur powder is vaporized with the laser at 355 nm is shown in Figure 2. These and other mass spectra presented below are typical of student data obtained in this lab. Sulfur

Phosphorus

A mass spectrum obtained by the vaporization of red phosphorus powder is presented in Figure 3. Previous laser desorption experiments have obtained similar results.29−31 Unlike sulfur, phosphorus forms much larger clusters in these experiments. The most interesting details in this spectrum are the prominent P3+, P4+, P5+, and P7+ ions and the preference for odd-numbered cations that begins after n = 11.

Figure 2. Mass spectra of sulfur cluster cations (bottom) and anions (top) using 355 nm. 293




explaining their stability. Similar cations were observed previously in experiments involving antimony and bismuth.36 Theoretical research has investigated the stability of phosphorus cluster cations,29−31 but no experiments have yet been able to measure their structures. Bismuth Oxide

A mass spectrum of bismuth oxide cluster cations (bottom) and anions (top) produced from partially oxidized bismuth metal powder is shown in Figure 4. Similar mass spectra were

Figure 3. Mass spectrum of phosphorus clusters obtained using 355 nm.

As in the case of sulfur, the mechanism of cluster growth is fascinating in this system. Again, we consider growth from vaporized atoms versus desorption of larger intact molecular species. Because red phosphorus is composed of polymeric chains,32 it is tempting to conclude that clusters are produced by direct desorption of these, perhaps followed by additional fragmentation. However, it is difficult to imagine that chains could be desorbed efficiently, as they must necessarily be entangled in the solid and have significant interactions (van der Waals) with their surroundings. Desorption of atoms or small molecules would require the breaking of fewer bonds, but it is difficult to understand how such large clusters could grow on the time scale of the vaporization without a collision gas present. However, larger phosphorus clusters were reported by Martin using an oven source,33 and they were interpreted to grow from atomic vapor. Additionally, McElvaney and coworkers have shown that large clusters of other elements can grow without an added collisional gas.19 If enough elemental density can be produced in the vaporization process, selfcollisions can cool the atomic vapor and stabilize growing clusters. Another interesting observation is the preference for oddnumbered cations, where P7+ is the most prominent. These odd-numbered clusters all have an overall even number of electrons, perhaps explaining their stability.33 However, an additional feature is the prominence of the P3+, P5+, and P7+ ions compared to other odd-numbered species. Electroncounting rules found in the polyhedral skeletal electron pair theory, also known as “Wade’s rules”, may provide further insight into this.34,35 These electron-counting rules are frequently applied to atomic clusters in the same way that the 18-electron rule is applied to transition metal−ligand complexes. They relate the number of skeletal electrons in clusters to specific polyhedral structures with delocalized electron clouds in their interiors that provide enhanced stability. Clusters that are electron deficient can share electrons more effectively in this way, and some are believed to possess three-dimensional aromaticity.34,35 The bonding in these clusters only involves valence p electrons, and an N-atom cluster achieves stability when there are 2N + 2, 2N + 4, or 2N + 6 skeletal electrons, representing the “closo”, “nido”, and “arachno” polyhedral structures, respectively. P3+, P5+, and P7+, with 8, 14, and 20 valence electrons respectively, meet the criteria for the closo, nido, and arachno structures, thus

Figure 4. Mass spectra of bismuth oxide cluster cations (bottom) and anions (top) obtained from an oxidized sample of pure bismuth at 532 nm.

reported previously for these systems.37−40 The fascinating observation here is that the metal oxide stoichiometries are not purely random. Instead, for each number of bismuth atoms there is one main oxide stoichiometry (e.g., Bi3O4+). This preference must arise from either the geometric or electronic stability of these clusters. Additional insight is provided by a comparison of the corresponding cations and anions. If geometry alone is important, clusters containing the same number of bismuth atoms should exhibit the same oxide stoichiometry regardless of their charge. However, if the number of bonding electrons is more important, clusters containing the same number of bismuth atoms would have different stoichiometries, as they do (e.g., Bi3O4+ versus Bi3O5−). Electronic stabilization is therefore apparently the primary consideration. Students should be encouraged to explain these oxide patterns in terms of specific bonding configurations for each of these clusters. As discussed previously,38 reasonable configurations can be obtained in terms of Bi−O−Bi−O network structures having localized twoelectron bonds. There is no need to invoke delocalized electron configurations as we did for phosphorus because there is a better balance in the number of electrons between bismuth and 294




(7) Counterman, A. E.; Thompson, M. S.; Clemmer, D. E. Identifying a Protein by MALDI−TOF Mass Spectrometry. J. Chem. Educ. 2003, 80, 177−180. (8) Moe, O. A.; Patton, W. A.; Kwon, Y. K.; Kedney, M. G. Ladder Sequencing of a Peptide using MALDI-TOF Mass Spectrometry. Chem. Educ. 2004, 9, 272−275. (9) Dopke, N. C.; Lovett, T. N. Illustrating the Concepts of Isotopes and Mass Spectrometry in Introductory Courses: A MALDI-TOF Mass Spectrometry Laboratory Experiment. J. Chem. Educ. 2007, 84, 1968−1970. (10) Arnquist, I. J.; Beussman, D. J. Incorporating Biological Mass Spectrometry into Undergraduate Teaching Labs, Part 2: Peptide Identification via Molecular Mass Determination. J. Chem. Educ. 2009, 86, 382−384. (11) Albright, J. C.; Dassenko, D. J.; Mohammed, E. A.; Beussman, D. J. Identifying Gel-separated Proteins using In-gel Digestion, Mass Spectrometry, and Database Searching. Biochem. Molec. Biol. Educ. 2009, 37, 49−55. (12) Harmon, C. W.; Mang, S. A.; Greaves, J.; Finlayson-Pitts, B. J. Identification of Fatty Acids, Phospholipids and their Oxidation products using Matrix-Assisted Laser Desorption Ionization Mass Spectrometry and Electrospray Ionization Mass Spectrometry. J. Chem. Educ. 2010, 87, 186−189. (13) Eibisch, M.; Fuchs, B.; Schiller, J.; Süβ, R.; Teuber, K. Analysis of Phospholipid Mixtures from Biological Tissues by Matrix-Assisted Laser Desorption and Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF MS): A Laboratory Experiment. J. Chem. Educ. 2011, 88, 503−507. (14) Cornett, D. S.; Amster, I. J.; Duncan, M. A.; Rao, A. M.; Eklund, P. C. Laser Desorption Mass Spectrometry of Photopolymerized C60 Films. J. Phys. Chem. 1993, 97, 5036−5039. (15) Comstock, Inc. http://www.comstockinc.com/ (accessed Oct 2013). (16) R. M. Jordan Company, Inc. http://www.rmjordan.com/ (accessed Oct 2013). (17) Wiley, W. C.; McLaren, I. H. Time-of-Flight Mass Spectrometer with Improved Resolution. Rev. Sci. Instrum. 1955, 26, 1150−1157. (18) 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. (19) McElvaney, S. W.; Nelson, H. H.; Baronavski, A. P.; Watson, C. H.; Eyler, J. R. FTMS Studies of Mass-Selected, Large Cluster Ions Produced by Direct Laser Vaporization. Chem. Phys. Lett. 1987, 134, 214−219. (20) Clusters of Atoms and Molecules I: Theory, Experiment, and Clusters of Atoms; Haberland, H., Ed.; Springer-Verlag: Berlin, 1995. (21) Johnston, R. L. Atomic and Molecular Clusters; Taylor & Francis: New York, 2002. (22) Duncan, M. A. Laser Vaporization Cluster Sources. Rev. Sci. Instrum. 2012, 83, 041101/1−19. (23) Hearley, A. K.; Johnson, B. F. G.; McIndoe, J. S.; Tuck, D. G. Mass Spectrometric Identification of Singly-Charged Anionic and Cationic Sulfur, Selenium, Tellurium and Phosphorus Species Produced by Laser Ablation. Inorg. Chim. Acta 2002, 334, 105−112. (24) Elemental Sulfur; Meyer, B., Ed.; John Wiley & Sons: New York, 1965. (25) Steudel, R.; Eckert, B. Elemental Sulfur and Sulfur-Rich Compounds II. Top. Curr. Chem. 2003, 230, 1−79. (26) Berkowitz, J.; Marquart, J. R. Equilibrium Composition of Sulfur Vapor. J. Chem. Phys. 1963, 39, 275−283. (27) Martin, T. P. Cluster Beam Chemistry - from Atoms to Solids. Angew. Chem., Int. Ed. 1986, 25, 197−211. (28) Cheng, T. C.; Akin, S. T.; Dibble, C. J.; Ard, S.; Duncan, M. A. Tunable infrared laser desorption ionization of fullerene films. Int. J. Mass. Spectrom. 2013, No. 10.1016/j.ijms.2013.05.031. (29) Chen, M. D.; Li, J. T.; Huang, R. B.; Zheng, L. S.; Au, C. T. Structure Prediction of Large Cationic Phosphorous Clusters. Chem. Phys. Lett. 1999, 305, 439−445.

oxygen. Other trivalent metals (yttrium, lanthanum) form some of the same stoichiometries seen for bismuth in their oxide cluster ions.41

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CONCLUSIONS These experiments show how laser vaporization in a time-offlight mass spectrometer can be employed to explore a variety of inorganic nanoclusters. The same methods can be applied to many other elements and compounds, as described in the Supporting Information. This laboratory exposes undergraduates to vacuum systems, lasers, and pulsed electronics. By sampling a variety of materials, students find that the inorganic clusters grown in a laser plasma can vary greatly depending on an element’s chemistry. Sulfur produces only small clusters, whereas phosphorus produces much larger distributions. Bismuth oxide has a balance of electron density and the stoichiometries can be explained via localized twoelectron bonding, whereas phosphorus has a tendency for delocalized bonding and more extensive electron sharing. Electron-counting rules can help to rationalize the clusters seen. However, it is shown throughout these systems that elements prefer to form bonded clusters rather than separated atoms, even when the normal considerations of valence do not work out in any simple way. These experiments provide valuable exposure to modern instrumentation and an introduction to concepts and issues now important in inorganic nanocluster chemistry.

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ASSOCIATED CONTENT

S Supporting Information *

Student materials on the measurement and assignment of mass spectra and instructor materials about atomic cluster growth. This material is available via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*M. A. Duncan. E-mail: [email protected]. Present Address §

T. M. Ayers, Department of Chemistry, University of West Georgia, Carrollton, Georgia, 30118−6310, United States Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Air Force Office of Scientific Research (grant FA9550-12-1-0116) and the National Science Foundation (grant CHE-0956025).

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REFERENCES

(1) Cotter, R. J. Time-of-Flight Mass Spectrometry: Instrumentation and Applications in Biological Research; American Chemical Society: Washington DC, 1997. (2) Gross, J. H. Mass Spectrometry; Springer-Verlag: Berlin, 2004. (3) Watson, J. T.; Sparkman, O. D. Introduction to Mass Spectrometry; John Wiley & Sons: Chichester, UK, 2007. (4) MALDI MS; Hillenkamp, G.; Peter-Katalinić, J., Eds.; WileyVCH: Weinheim, Germany, 2007. (5) Electrospray and MALDI Mass Spectrometry; Cole, R. B., Ed.; John Wiley: Hoboken, 2010. (6) Muddiman, D. C.; Bakhtiar, R.; Hofstadler, S. A.; Smith, R. D. Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry. J. Chem. Educ. 1997, 74, 1288−1292. 295




(30) Bulgakov, A. V.; Bobrenok, O. F.; Kosyakov, V. I. Laser Ablation Synthesis of Phosphorous Clusters. Chem. Phys. Lett. 2000, 320, 19− 25. (31) Chen, M. D.; Huang, R. B.; Zheng, L. S.; Zhang, Q. E.; Au, C. T. A Theoretical Study for the Isomers of Neutral, Cationic and Anionic Phosphorus Clusters P5, P7, P9. Chem. Phys. Lett. 2000, 325, 22−28. (32) Corbridge, D. E. C. The Structural Chemistry of Phosphorus; Elsevier Scientific Publishing Co.: Amsterdam, 1974. (33) Martin, T. P. Compound Clusters. Z. Phys. D: At., Mol. Clusters 1986, 3, 211−217. (34) Wade, K. Structural and Bonding Patterns in Cluster Chemistry. Adv. Inorg. Chem. Radiochem. 1976, 18, 1−66. (35) Corbett, J. D. Homopolyatomic Ions of the Post-Transition Elements - Synthesis, Structure, and Bonding. Prog. Inorg. Chem. 1976, 21, 129. (36) Geusic, M. E.; Freeman, R. R.; Duncan, M. A. Neutral and Ionic Clusters of Antimony and Bismuth: A Comparison of Magic Numbers. J. Chem. Phys. 1988, 89, 223−229. (37) Struyf, H.; van Vaeck, L.; van Grieken, R. Desorption/Ionization of Inorganic Compounds in Fourier Transform Laser Microprobe Mass Spectrometry with External Ion Source. Rapid Commun. Mass Spectrom. 1996, 10, 551−561. (38) France, M. R.; Buchanan, J. W.; Robinson, J. C.; Pullins, S. H.; Tucker, J. L.; King, R. B.; Duncan, M. A. Antimony and Bismuth Oxide Clusters: A New Family of Magic Number Clusters. J. Phys. Chem. A 1997, 101, 6214−6221. (39) Kinne, M.; Bernhardt, T. M.; Kaiser, B.; Rademann, K. Formation and Stability of Antimony And Bismuth Oxide Clusters: A Mass Spectrometric Investigation. Int. J. Mass Spectrom. Ion Processes 1997, 167/168, 161−172. (40) Van Stipdonk, M. J.; Justes, D. R.; English, R. D.; Schweikert, E. A. Ion-Neutral Correlations from the Dissociation of Metal Oxide Cluster Ions in a Reflectron Time-of-Flight Mass Spectrometer. J. Mass Spectrom. 1999, 34, 677−683. (41) Reed, Z. D.; Duncan, M. A. Photodissociation of Yttrium and Lanthanum Oxide Cluster Cations. J. Phys. Chem. A 2008, 112, 5354− 5362.

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Laser Desorption Time-of-Flight Mass Spectrometry of Inorganic

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