1566
J. Phys. Chem. 1989, 93, 1566-1569
Growth Patterns and Photoionization Dynamics of In/Sb and In/Bi Intermetallic Clusters M. B. Bishop,+ K. LaiHing, P. Y. Cheng, M. Pesehke, and M. A. Duncan* Department of Chemistry, School of Chemical Sciences, University of Georgia, Athens, Georgia 30602 (Received: June 27, 1988)
Molecular beams of In/Sb or In/Bi mixed-metal clusters are produced by laser vaporization of alloy samples in a pulsed supersonic nozzle source. These clusters are photoionized at ArF (193 nm) and F, (1 57 nm) excimer laser wavelengths and mass analyzed with a time-of-flight spectrometer. The dependence of mass distributions on laser wavelength and power probes photoionization dynamics and growth patterns in these systems. Nonstatistical combinations of component elements are observed in patterns closely related to those reported previously for other main-group alloy clusters. These patterns are predicted by a simple valence-electron-counting model.
Introduction Despite numerous recent theoretical and experimental studies the electronic structure of metal clusters is still not well understood.Id Transition-metal dimer and trimer systems have been examined extensively, revealing the complexity of bonding in d-electron ~ystems.~”As expected, larger transition-metal clusters are even more difficult to study, and there are no detailed measurements of structures or bonding patterns in these systems. In contrast to the transition metals, alkali metals exhibit somewhat more understandable patterns in the electronic structure of larger clusters. Mass spectral distributions are characterized by significant discontinuities at certain cluster sizes corresponding to shell closings for a spherically symmetric potential model.7 Other metals (e.g., copper? silver,9J0aluminum”J2) also produce related patterns in mass distributions. Systematic trends are also emerging for clusters composed of main-group In alloy clusters of the group IVA and group VA metals, for example, highly nonstatistical stoichiometries are preferred following the same pattern for all isovalent metal mixtures (e&, Sn/Bi, Sn/As, Pb/Sb).17 As we have reported recently, this pattern of cluster growth is predicted by a valence-electron-counting model recognized in condensed-phase inorganic ~hemistry.’~ This same model should apply in principle to other main-group cluster systems. In the present report we examine mass distributions in 111-V alloy systems which further test for simple patterns in main-group cluster growth. In some of the first main-group-metal studies, clusters were produced in oven sources by using the inert-gas condensation m e t h ~ d . ~Electron ~ . ’ ~ impact ionization (EI) mass spectroscopy provided a measure of abundances in these experiments, which first illustrated the appearance of “magic numbers” at certain cluster sizes. However, the origin of magic number patterns in these experiments is not clear because of the complex interactions of growth kinetics, ionization efficiencies, and fragmentation. More recently, laser vaporization and UV laser photoionization have been applied to these cluster studies.1s-’8 These methods make it possible to study materials with higher vaporization temperatures and to achieve ionization with lower excess energy to limit fragmentation. Using these methods, Smalley and coworkers have shown that silicon and germanium clusters have virtually identical mass spectra.I5 Experiments in our own laboratory find that tin and lead distributions are very similar to each other and have some features in common with those of silicon and germanium.16 Likewise, we have found a different yet common trend for antimony and bismuth clusters.’* The strong similarities found for isovalent materials suggest that valence s and p electrons play a dominant role in cluster bonding for these elements. The importance of valence electron configurations is more striking for alloy clusters composed of a group IVA-group VA Permanent address: Department of Chemistry, Clemson University, Clemson, SC 2963 1.
0022-3654/89/2093- 1566$01.50/0
metal ~ombination.’~ We have recently reported laser vaporization/photoionization studies of Sn/Bi, Pb/Sb, and Sn/As systems.17 Recknagel and co-workers have used oven inert-gas condensation/electron impact ionization to study the Pb/Sb and Pb/As systems.lg Both kinds of experiments find that the metals combine nonstatistically in these mixtures in stoichiometries predicted by a valence-electron-counting model. Analogous fiveand nine-atom clusters of all these mixtures are especially abundant. For example, under conditions designed to probe neutral clusters, the five-atom species Sn3Bi,, Pb3Sbz,Sn3Asz,and Pb3Aszand the nine-atom species Sn5Bi4,Pb5Sb4, Sn5As4,and Pb5As4dominate their respective mass distributions. Under high laser power multiphoton ionization conditions the corresponding isoelectronic cations are prominent (Sn2Bi3+,Pb2Sb3+,Sn2As3+, Pb2As3+,and Sn4Bis+,Pb4Sb5+,Sn4As5+,Pb4As5+,respectively). Significantly, these five- and nine-atom mixed clusters are isovalent to the especially stable Zintl polyanion clusters studied extensively in condensed-phase inorganic chemistry?O The five-atom species are analogues of Pb52-and Sn52- (12 valence p electrons), while ~~
(1) (2) (3) (4)
Metal Clusters; Moskovits, M., Ed.; Wiley: New York, 1986. Phillips, J. C. Chem. Reu. 1986, 86, 619. Morse, M. D. Chem. Reu. 1986, 86, 1049. Weltner, W.; Van Zee, R. J. Ann. Reu. Phys. Chem. 1984, 35, 291. (5) Koutecky, J.; Fantucci, P. Chem. Reu. 1986, 86, 539. (6) (a) Smalley, R. E. Laser Chem. 1983, 2, 167. (b) Morse, M. D.; Hopkins, J. B.; Langridge-Smith, P. R. R.; Smalley, R. E. J . Chem. Phys. 1983, 79, 5316. (7) Knight, W. D.; Clemenger, K.; de Heer, W. A,; Saunders, W. A,; Chou, M. Y.; Cohen, M. L. Phys. Rev. Left. 1984, 52, 2141. (8) Pettiette, C. L.; Yang, S. H.; Craycraft, M. J.; Conceicao, J.; Laaksonen, R. J.; Cheshnovsky, 0.;Smalley, R. E. J . Chem. Phys., in press. (9) Katakuse, I.; Ichihara, T.; Fujita, Y.; Matsuo, T.; Sakurai, T.; Matsuda, H. Int. J. Mass. Spectrom. Ion Processes 1985, 67, 229. (10) LaiHing, K.; Cheng, P. Y.; Duncan, M. A,, to be published. (11) (a) Jarrold, M. F.; Boper, J. E.; Kraus, J. S. J. Chem. Phys. 1987, 86, 3876. (b) Jarrold, M. F.; Bower, J. E. J . Am. Chem. SOC.1988, 110, 70. (12) Chou, M. Y.; Cohen, M. L. Phys. Left. 1986, 113A, 420. (13) (a) Martin, T. P. J . Chem. Phys. 1984,80, 170. (b) Martin, T. P. Ibid. 1984, 81, 4426. (c) Martin, T. P. Ibid. 1985, 83, 7 8 . (14) (a) Sattler, K.; Muhlbach, J.; Recknagel, E. Phys. Reo. Lert. 1980, 45, 821. (b) Muhlbach, J.; Recknagel, E.; Sattler, K. Surf. Sci. 1981, 106, 188. (c) Sattler, K. Surf.Sci. 1985, 156, 292. (d) Sattler, K.; Miihlbach, J.; Pfau, P.; Recknagel, E. Phys. Lett. 1982, A87, 418. (15) Heath, J. R.; Lui, Y.; OBrien, S. C. Zhang, Q.L.; Curl, R. F.; Tittel, F. K.; Smalley, R. E. J . Chem. Phys. 1985, 83, 5520. (16) LaiHing, K.; Wheeler, R. G.; Wilson, W. L.; Duncan, M. A. J. Chem. Phys. 1987,87, 3401. (17) (a) Wheeler, R. G.; LaiHing, K.; Wilson, W. L.; Allen, J. D.; King, R. B.; Duncan, M. A. J . Am. Chem. SOC.1986,108,8101. (b) Wheeler, R. G.; LaiHing, K.; Wilson, W. L.; Duncan, M. A. J . Chem. Phys. 1988, 88, 2831. (18) Wheeler, R. G.; LaiHing, K.; Wilson, W. L.; Duncan, M. A. Chem. Phys. Lett. 1986, 131, 8. ( 1 9) (a) Schild, D.; Pflaum, R.; Sattler, K.; Recknagel, E. J . Phys. Chem. 1987, 91, 2649. (b) Schild, D.; Pflaum, R.; Recknagel, E., to be published. (20) (a) Corbett, J. D. Chem. Reo. 1985,85, 393. (b) Corbett, J. D. Prog. Inorg. Chem. 1976, 21, 129.
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1567
In/Sb and In/Bi Intermetallic Clusters the nine-atom species are analogues of Snge, Pbge, and G e t - (22 valence p electrons). Unlike any gas-phase systems, the structures and bonding patterns in the condensed-phase polyanion clusters are well characterized. Polyhedral structures have been measured through X-ray diffraction of the crystallized solids and N M R spectroscopy in solution.20 Molecular orbital treatments (extended Hiickel) have also examined the bonding in these systems.20 Through this combined approach structural patterns have been identified for these metal clusters. These patterns are outlined in the form of “Wade’s rules”,2’ which describe the optimum number of skeletal electrons for stable polyhedral structures. In this picture, cluster bonding involves exclusively the p orbitals, and an N-atom system has stable bonding for configurations of 2 N 2, 2N + 4, or 2 N 6 skeletal electrons. As N varies these electron configurations define the “closo”, “nido”, and “arachno” cluster series, respectively. In this scheme the bonding includes both conventional covalent interactions between adjacent atoms and multicentered delocalized bonding. Our studies of IV/V alloy systems represent the first observation of gas-phase growth patterns that could be related to these well-established concepts from condensed-phase chemistry. In the condensed-phase literature, many isoelectronic analogues to the Zintl ions have been studied by substitution of other maingroup metals.20 If these same concepts apply, other gas-phase cluster systems should follow the growth patterns observed for the IV/V alloys. As a further test of these ideas, we have chosen the III/V mixtures In/Sb and In/Bi. III/V systems in general have important electronic materials applications, and the most common example of these species, GaAs, has been studied previously with cluster beam mass spectroscopy.22
+
+
Experimental Section Clusters for these experiments are prepared by laser vaporization of solid metal rods in a pulsed nozzle source described previously.16 Samples are formed by melting the component metals in a test tube under vacuum, producing a 1/2-in.0.d. rod upon cooling. Vaporization is accomplished by using either an excimer laser (308 nm) or the second harmonic of a Nd:YAG laser (532 nm). Cluster photoionization is accomplished with an excimer laser at either 193 or 157 nm in a supersonic beam appratus and time-of-flight mass spectrometer system described High mass resolution experiments use a newly constructed reflectron timeof-flight ~ p e c t r o m e t e r . ~ ~
193 nrr
a1 a2
I
13
157 .im Y
23
t-
34
1om
(21) ?de, K. Ado. Inorg. Chem. Radiochem. 1916, 18, 1. (22) 0 Brien, S. C.; Liu, Y.; Zhang, Q.; Heath, J. R.; Tittel, F. K.; Curl, R. F.; Smalley, R. E. J . Chem. Phys. 1986, 84, 4074. (23) Boesl, U.; Newer, H. J.; Weinkauf, R.; Schlag, E. W. J . Phys. Chem. 1982, 86, 4857. (24) (a) Walstedt, R. E.; Bell, R. F. Phys. Rev. A 1986, 33, 2830. (b) Walstedt, R. E.: Bell, R. F. J . Chem. Phys. 1987, 87, 1423.
200G
MASS
Figure 1. InBi cluster photoionization at different laser wavelengths and power conditions. In the lower spectrum, photoionization occurs at 157 nm in the limit of low laser power (0.1 mJ/cm2). Reduction in the laser power causes no further changes in mass spectral intensities. The features observed under these conditions are believed to represent single-photon ionization of neutral clusters in the molecular beam. In the upper spectrum, ionization occurs at 193 nm at moderately high laser fluence (10 mJ/cm2). Multiphoton ionization and fragmentation are efficient under these conditions, producing a mass spectrum characterized by surviving stable cluster cations.
133 i r r
2cc3
23C I I
>
i5’
t
Results and Discussion Mass spectral distributions for InBi and InSb clusters produced by laser vaporization with a XeCl eximer laser (308 nm) are shown in Figures 1 and 2. The samples for these experiments are 1:l molar mixtures. Photoionization is accomplished at ArF (193 nm, 6.4 eV) and fluorine (1 57 nm, 7.9 eV) excimer laser wavelengths. Clusters for both systems are produced into the 1520-atom size range. There is no evidence for truncated mass distributions like those observed for pure antimony or bismuth under these same conditions.I8 As has been observed for all metal cluster distributions, the peak intensity patterns in these spectra are sensitive to both the laser wavelength and the laser power chosen for photoionization. Unfortunately, ionization potentials (IPS) are not known for these mixed-metal systems. However, electron impact appearance potentials have been measured for pure bismuth clusters up to 30 atoms in size, produced by oven vaporization and inert-gas c ~ n d e n s a t i o n . ~Appearance ~ potentials have also been measured for smaller indium, antimony, bismuth, and InSb molecules in the equilibrium vapors of these species (e.g., InSb 8.4 eV, h S b 2
55
r T
i
3
.lCCC
___ 2CDC
MASS
Figure 2. InSb cluster photoionization under the same laser wavelength and power conditions described for InBi in Figure I .
6.6 eV).25-27 Pure indium clusters up to 30 atoms and pure antimony or bismuth clusters up to 5 atoms have been studied in our laboratory with fixed-frequency excimer laser photoionization and power dependences.’8,28 All indium clusters appear to be single-photon ionized at the ArF laser wavelength and therefore should have ionization potentials less than 6.4 eV. No antimony or bismuth clusters are ionized at 6.4 eV, but all bismuth species and Sb4 and Sb5 are ionized at the fluorine laser wavelength. These species therefore have IPS greater than 6.4 eV but less than 7.9 eV. Sb2 and Sb3 are not ionized at 7.9 eV. Where comparisons are possible, the conclusions drawn from fixed-fre(25) DeMaria, G.; Drowart, J.; Inghram, M. G . J . Chem. Phys. 1959,31, 1076. (26) Kohl, F. J.; Carlson, K. D. J. Am. Chem. SOC.1968,90, 4814. (27) Kordis, J.; Gingerich, K. A. J . Chem. Phys. 1978, 58, 5141. (28) Geusic, M. E.; Freeman, R. R.;Duncan, M. A. J . Chem. Phys. 1988, 89, 223.
1568
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989
quency photoionization for these species and those obtained from electron impact appearance potentials are all completely consistent. On the basis of the trends observed for pure clusters of the component metals and the general rule that ionization energies decrease with cluster size, we expect IPS for InSb and InBi mixed clusters to be in the range of 6-8 eV. Therefore, except perhaps for diatomic and triatomic species, these clusters should be single-photon ionized at the fluorine excimer laser wavelength. The ArF laser will not generally ionize smaller clusters in a singlephoton process, but it may do so for some of the larger ones. These general conditions governing photoionization are commonly found Experience in for metal or semiconductor cluster our own laboratory and others indicates that when these conditions apply, low-power fluorine laser radiation provides the best possible measure of the neutral cluster distribution, via near-threshold photoionization with minimized fragmentation. At higher laser fluences, ArF laser multiphoton ionization (MPI) is efficient, leading to extensive fragmentation of the neutral cluster distribution. These conditions tend to produce greater mass spectral intensities for stable cluster cations that survive the fragmentation conditions. Where comparisons are possible, cluster cation abundances probed by MPI fragmentation are essentially the same as those produced by direct ion sampling from the laser vaporization source without photoionization.28 The limiting conditions described, therefore, provide a first approximation for considering abundant neutrals and cations occurring in cluster distributions. The mass spectra shown in Figures 1 and 2 are generated under the limiting conditions described above so that neutral and cation abundances can be considered. Under these conditions, InSb and InBi can be compared to other main-group cluster metals and to the predictions of the Wade’s rules electron-counting model. For these comparisons, InBi is considered first because larger mass differences in the component elements make assignments more straightforward. InBi. Following the reasoning outlined above, the lower trace of Figure 1 is expected to be dominated by abundant neutral InBi clusters. Of the prominent peaks observed, the bismuth dimer and tetramer (indicated as 0,2 and 0,4, respectively) are wellknown stable molecules observed in the pure cluster distribution.I8 As noted previously, both of these molecules also fit the Wade’s rules stability criteria.28 The remaining features correspond to 5, 6-, 7-, 9-, and 10-atom mixed clusters. The 2/3, 314, and 4/5 species appear at moderate intensity in both spectra of Figure 1. When considered as cations they are even-numbered electron species (2N in each case), but these species have no known analogues in other cluster systems and they are not predicted to be stable by electron counting. The most prominent feature under neutral conditions is the 3/3 species, likewise having 2 N (12) p electrons and not predicted to be stable. However, this species is isovalent with the abundant Ge6 and Ga3As3clusters observed p r e v i o ~ s l y . The ’ ~ ~515 ~ ~cluster ~ ~ ~ most ~ prominent at higher mass is also a 2N species isovalent with the abundant 10-atom clusters observed for all the group IV elements (e.g., Silo, Gel,,, Snlo, Pb10).15.16,22,29-32 The remaining abundant neutral clusters 214 and 416 are both 2 N + 2 species (14 and 22 p electrons, respectively) and are thus predicted to be stable “nido” type clusters by Wade’s rules. The upper spectrum in Figure 1 is expected to be dominated by InBi cations. The trimer cation (0,3) is observed in pure bismuth distributions18.28and is predicted as a stable nido/2N 2 species. The most prominent mixed feature under ArF MPI conditions is the 1,4 species, which is predicted as a stable cation (1 2 electrons, 2 N + 2). This species is isovalent with the 5-atom IV/V alloy clusters observed previously in beam experiments (e.g., Sn,Bi2, Pb3Sb2,Sn2Bi3+,Sn2As3+)17and with the stable polyanions Sns2-and Pb?- observed in condensed phases.m*21The 215 cluster
+
~~~
~
~~
(29) Martin, T.P.; Schaber, H. J . Chem. Phys. 1985, 83, 8 5 5 . (30) Phillips, J. C. J . Chem. Phys. 1985, 83, 3330. (31) Phillips, J. C. J . Chem. Phys. 1986, 85, 5246. (32) Bloomfield, L. A,; Freeman, R. R.; Brown, W. L. Phys. Rev Lett. 1985, 54, 2246. (33) Raghavachari, K.; Logownsky, V Phys. Rev.Lett. 1985, 55, 2853.
Bishop et al.
C)
MASS
600
620
Figure 3. Modeled and experimental isotope patterns for 5-atom InSb clusters. In 3a, the isotope pattern expected for a binomial distribution of stoichiometries is shown. The experimental distribution under ArF MPI conditions is shown in 3b, where the higher mass resolution is obtained with a reflectron time-of-flight spectrometer. 3c shows the isotope pattern for the InSb4+ stoichiometry, which is the only one matching the main features in the experimental spectrum. Smaller amounts of lower mass species in the experimental spectrum fit reasonably well with a combination of the 4/1 and 2/3 stoichiometries. is also a nido type cation isovalent with IV/V alloy species observed in the gas phase (e.g., Pb4Sb2,Sn4Bi3+). The other less prominent species (413, 314,415) have no previous analogues. Power dependencies show that the 416 cluster has a low ionization potential so that it is assigned as an abundant neutral single-photon ionized at both laser wavelengths. Overall, most of the prominent cluster species shown in Figure 1 are either predicted to be stable by the electron counting model or have isovalent analogues in other III/IV mixed clusters (GaAs) or in pure component group IV clusters. The species detected do not represent the full range of possible combinations of the component elements. For example, at 157 nm only two of the seven possible 6-atom stoichiometries and two of the eleven possible 10-atom stoichiometries are observed. More clusters are detected under MPI conditions at 193 nm, but the distribution is still not statistical. Most noticeably, there are few if any clusters observed containing only indium (e.g., X,O species). ZnSb. Mass spectra for InSb clusters, produced and detected under the same conditions described for InBi, are shown in Figure 2. Indium and antimony are relatively close in mass, and both have two naturally occurring isotopes with significant abundance. Therefore, the individual mass peaks for most of the clusters shown are not clearly resolved and peaks are therefore labeled only as the total number of atoms. In other main-group cluster systems, isovalent species exhibit strong similarities in mass distributions. Therefore, although exact mass assignments are not possible for all these InSb species, it is useful to consider these distributions for similarities or consistencies with the InBi data. Higher resolution data presented below makes exact comparisons possible for the 5-atom clusters. Species smaller than 4-5 atoms are not detected efficiently. At 157 nm the InSb data are exactly consistent with the patterns observed for InBi. Neutral Sb4, like Bid, is a known stable molecule abundant in other antimony-containing systems, and it is also observed here. The prominent 6-atom mass channel is expected if InSb forms the same 313 and 214 stoichiometries observed for InBi. Likewise, the abundant IO-atom channel is consistent with the formation of the 515 and 416 stoichiometries. Corresponding patterns are also observed at 193 nm. The most abundant 5-atom
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1569
In/Sb and In/Bi Intermetallic Clusters TABLE I: Isoelectronic Comparisons of Abundant Species Observed in This Work and in Previous Studies condensedthis work gas-phase analogues phase analogues InSb4+, InBi4+ Sn3Bi2,Sn3As2," Pb3Sb2, Sn?-, Pb?-* (trigonal bipyramid) Sn,Bi2+. Pb,Sbl+. InPI+, -~ Gab,' In2Bi4,(In2Sb4) Pb4Sb2,Sn4As2," Pb3Sb3+, B4C2H6*(octahedron) Sn3Bi3+ Ge6, Ga3As3' In3Bi3, (In3Sb3) In2Bi5+,(In2Sb5+) Pb5Sb2,Sn4Bi3+,0Sn4As3+ B5C2Hlb(pentagonal bipyramid) InSBi5,(In5Sb5) Silo,Gelo? Snlo,Pbl& B9CHl0-* In4Bi6, (In4Sb6) "Reference 17. Reference 16.
*Reference 20.
cReference 35.
dReference 15.
channel is expected if InSb forms the 1 /4 stoichiometry observed for InBi. The 7-atom channel would include the 3/4 and 2/5 analogues to InBi. However, the 1 1-atom channel for InSb has no corresponding feature in the InBi data. Figure 3b shows a higher resolution version of the 5-atom mass region for InSb photoionized at 193 nm. These data were obtained with a reflectron type time-of-flight spectrometer recently constructed in our laboratory. Under these conditions, the isotope patterns for the possible 5-atom stoichiometries are resolved and can be used to obtain a definitive mass assignment. Figure 3a shows the isotope pattern expected in the 5-atom mass region if the elements combine in a purely statistical fashion. For this modeled spectrum, the six possible stoichiometries (0/5, 1/4, 2/3, 3/2, 4/1, 5/0) and their respective isotopes are combined according to binomial distributions. Comparisons of Figure 3, parts a and b, clearly indicate that the observed distribution is not statistical. To determine the composition in the observed spectrum, we also modeled the isotope patterns of individual stoichiometries or combinations of stoichiometries. Figure 3c shows the isotope pattern expected for InSb4+. As shown, this stoichiometry produces the correct total mass and the correct isotope pattern matching the experimental spectrum. No other modeled stoichiometry or combination of stoichiometries fits the observed pattern. The reduced intensity mass features at lower mass can be fit approximately by a combination of the 4/1 and 2/3 species. These isotope data show convincingly that InSb4+ is preferentially produced under these conditions. Significantly, this is the same stoichiometry observed for InBi, and it is predicted as a stable cation by the electron-counting rules. This same 1/4 stoichiometry has recently been observed with enhanced abundance for other III/V cluster cations (Gap, InP) produced by laser vaporization in a Fourier transform mass spectrometry system.34 Under the conditions described here, therefore, InSb and InBi mass distributions have significant similarities. Admittedly, these distributions are measured with fixed-frequency photoionization without a full knowledge of ionization potentials, ionization cross sections, or fragmentation patterns. It is impossible to prove, therefore, that these similarities are not purely the result of coincidences in the photoionization dynamics, rather than in the intrinsic cluster growth patterns. However, it is an unlikely set of coincidences that would produce distributions so similar at different cluster sizes and at the same time produce the consistent (34) Bondybey, V. E.; Reents, W. D.; Mandich, M. L. J . Phys. Chem., in press.
analogues to other main-group cluster systems. In the case of cluster cations, growth patterns for other main-group systems have been observed without photoionization by using direct ion sampling methods. For pure antimony or bismuth, as well as for group IV/V alloys, these experiments confirm the cation abundances obtained by the MPI fragmentation method described here. The evidence is therefore compelling that the distributions measured are strongly influenced by the intrinsic cluster growth patterns. Table I summarizes the comparisons of these III/V systems to the isovalent gas-phase and condensed-phase systems described previously. The comparisons in Table I suggest that two apparently unrelated structural tendencies influence growth for these clusters. On one hand, certain cluster stoichiometries are predicted to be stable by the Wade's rule electron-counting model. As a natural consequence of the number of valence p electrons, fewer stoichiometries are predicted to be stable for these III/V systems than for the IV/V systems described previously. However, virtually every stoichiometry predicted by this model is in fact observed as an abundant cluster. As described previously, cluster stability in the context of electron counting is intimately related to cluster structure. Therefore, a good first guess for the gas-phase structures of clusters observed here would be the same as those found for their respective condensed-phase analogues (e.g., InSb4+and InBi4+ should be trigonal bipyramids). The other apparent pattern in electronic structure is that 6- and 10-atom species are produced analogous to those observed for pure group IV semiconductors. Possible structures for these species have been c o n ~ i d e r e d ~ ~ * ~ * previously and have been the subject of a b initio calculation^.^^ However, there is no simple unifying concept such as Wade's rules to predict when these growth patterns are to be expected.
Conclusion The results presented here show that both neutral and cation InSb and InBi cluster distributions have common growth patterns and that these patterns are related to those found previously for other main-group clusters. Some of the abundant species observed are predicted to be stable by a valence-electron-counting model (Wade's rules). This is the most recent in a growing list of systems that appear to follow this understandable electronic structure pattern. Other systems include IV/V alloy^,'^*'^ V/VI alloys,lg and pure group V cations.28 Other abundant clusters observed here are isovalent analogues to group IV and other III/V semiconductors studied previously. There is a growing body of evidence indicating that main-group clusters may have more understandable electronic structure and bonding patterns than their transition-metal counterparts. This is especially true for clusters larger than three atoms, for which there are virtually no spectroscopic structural data. As cluster research efforts continue to grow, main-group systems deserve increased theoretical and experimental attention. Continued studies on these systems, including mass-selected photodissociation and resonant two-photon ionization spectroscopy, are currently underway in our laboratory. Acknowledgment. This research was supported by the U S . Army Research Office. We are also grateful for a Department of Defense University Research Instrumentation contribution toward the Nd:YAG laser system used for this work. Registry No. InSb, 1312-41-0; InBi, 12010-46-7; In, 7440-74-6; Sb, 7440-36-0; Bi, 7440-69-9.
