J. Phys. Chem. 1993,97, 145-7440
7435
Photodissociation of Pb/Sb Bimetallic Cluster Cations K. F. Willey, K. LaiHing, T. G. Taylor, and M. A. Duncan’ Department of Chemistry, University of Georgia, Athens, Georgia 30602 Received: January 21, 1993; In Final Form: May 12, 1993
Lead-antimony metal cluster cations are studied with laser photodissociation in a reflectron time-of-flight mass spectrometer system. Dissociation products resulting from excitation at 308 nm are compared to those found for pure lead or pure antimony cluster cations. All clusters studied exhibit extensive fragmentation, consistent with relatively weak intracluster metal-metal bonding. Prominent dissociation channels include the elimination of neutral or charged lead atoms and dimers and the formation of stable X S b 3 + cation species. There is little or no preference for the formation of cations which exist as magic numbers in the original cluster distribution.
Introduction There have been numerous recent experimental and theoretical studies of the electronic structure of metal clusters.ld These studies are motivated by the need to understand metal bonding, which has many practical implications. Transition metal dimer and trimer systems have been extensively studied,1-9 revealing the complexity in even small systems having d electron participation in the bonding. New photoelectron spectroscopy studies of metal cluster anions have extended these studies to clusters containing several hundred atoms.’ However, characterization of the structure and bonding in large transition metal clusters remains an elusivegoal. In contrast to the situation for transition metal systems, alkali metals exhibit more understandable patterns in cluster bonding.l&ll Mass spectral distributions’O and electronic spectroscopy of plasmon resonances11 provide a consistent picture of bonding through delocalizedvalence electrons residing in a spherically symmetric potential. Mass photoelectron spectra? and plasmon spectra12 of noble metal clusters find evidence for similar bonding trends in these systems. Systematic trends are also emerging for the bonding in metal clusters composed of the main group p-block elements.13-36 Both pure metal systems and various bimetallic alloy clusters have been investigated with mass spectroscopyexperimentsto elucidate bonding patterns in these systems. The alloy systems are particularly interesting, where preferential formation of nonstatistical stoichiometries has been noted.3G32 These stoichiometries are predictable when clusters are assumed to form cage structures with stable bonding configurationsdescribed by a simple electron counting In the present report, we apply massselected photodissociation experiments for the first time to these main group alloy clusters to further explore their bonding energeticsand stability patterns. We focus on the lead-antimony system, which has been studied in previous work in our laboratory.30 In some of the first main group metal studies, clusters were produced in oven sources using the inert gas condensation method.13-14 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.15J6.21-23 Experiments on a variety of main group clusters under a variety of conditions have shown that magic number patterns in mass distributions are similar for those cluster elements in the same periodic table group. Author to whom correspondence should be addressed.
This suggests that valence s and p electrons play a dominant role in cluster bonding for these elements. The importance of valence electron configurations is striking for alloy clusters composed of a group IVA-group VA metal combination.3b32 We have previously reported laser vaporization/ photoionization studies of Sn/Bi, Pb/Sb, and Sn/As systems.30 Castleman and co-workers have reported similar studies on the Pb/Sb system.32 Recknagel and co-workers have used oven inert gas condensation/electron impact ionization to study the Pb/Sb and Pb/As systems.” All of these experiments find that the metals combine nonstatistically in these mixtures in stoichiometries predicted by a valence electron counting model. Analogous five- and nine-atom clusters of all these mixtures are especially abundant. For example, under conditions designed to probe neutral clusters, the five-atom species Sn3Bi2, Pb3Sb2, Sn3As2, and PbsAs2 and the nine-atom species SnsBi4, PbsSb4, SnsAs4, and PbsAs4 dominate their respectivie mass distributions. Under high laser power multiphoton ionization conditions, the corresponding isoelectroniccations are prominent (SnzBi3+,PbzSb3+, Sn2As~+,PbzAs3+ and SndBis+, Pb4Sbs+, S&ss+, Pb&ss+, 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.33-36 The five-atom species are analogues of Pbs2-and Sns2-, which have 12 valence p electrons, while the nine-atom species are analogues of Sn&, Pb&, and Ge&, which have 22 valence p electrons. In the condensed-phase systems, multiply charged clusters are formed to achieve the optimum number of electrons for stable electron configurations. Gas-phase experiments usually probe only the singly charged systems. Therefore, alloys of the electron-deficientgroup IV elementsand the electronrich group V elements can be used to balance the required number of p electrons and preserve the low-charge states. The stable electron configurations, however, occur only at certain prescribed stoichiometries. The experiments so far indicate that these expected stoichiometries are formed preferentially in the cluster growth processes. Photodissociation experiments can be used to see if there are preferential decomposition paths for these clusters. Photodissociation experiments have become a valuable tool in cluster mass spectrometry to identify stable cluster sizes and to determine the energetics of cluster bonding. However, except for experiments on dimers and trimers,3’ there have been few photodissociation experiments on alloy or bimetallic clusters. The experiments described here investigate dissociation in the leadantimony bimetallic system. Photodissociation of the pure lead and pure antimony systems has been explored previously by our research group.21-38 The decomposition dynamics and energetics in the alloy system can therefore be compared to those of the corresponding pure component clusters.
0022-3654/93/2091-1435~04.00/00 1993 American Chemical Society
Willey et al.
7436 The Journal of Physical Chemistry, Vol. 97, No. 29, 1993
1 Pb4Sb5
i
500
1000
1500
+
2000
1 .o
MASS Figure 1. Photoionization mass spectrum of the alloy clusters produced by laser vaporization of a lead-antimony alloy. The photoionization
laser power was high enough so that multiphoton absorption is efficient, and theclustersdetected represent relatively stablecations. The 2/3 and 4/5 ions are isoelectronic to Pbs*- and PbgC, respectively, which are well-characterized Zintl ions.
Experimental Section Alloy samples for these experiments were prepared by mixing and melting solid powders of lead and antimony in a test tube mold. Laser vaporization was accomplished using techniques and cluster sources described previously.16~22JO A Nd:YAG laser (532 nm) or a XeCl excimer laser (308 nm) was used for vaporization. The molecular beam apparatus and reflectron timeof-flight mass spectrometer system used for photodissociation experiments have also been described previously.38 Neutral clusters were photoionized at 193 nm with an ArF excimer laser and photodissociated at 308 nm with a XeCl excimer laser. As in previous work, the desired cluster is mass-selected prior to photoexcitation by its time-of-flight, and fragment masses are analyzed by their time-of-flight through a second drift tube section.38
I
Pb3Sbn
200
400
600
800
MASS
Figure 2. Photodissociation mass spectra for the two five-atom species PbzSbp+ and Pb3Sbz+. As shown, both are significantly fragmented. 3,l 4'1
ReSUltS
Figure 1shows the distributionof Pb/Sb cluster cations detected at the mass spectrometer following laser vaporization of the alloy sample and photoionization with an ArF excimer laser (193 nm). The ionization laser is operated unfocused but at a moderately 3 high power level (10 mJ/cm2). We have described extensive 2 1 4 5 7 studies of the photoionization behavior of metal cluster systems such as these in prior work from this l a b o r a t ~ r y . ~ J ~ .The ~l.~~.~~ ionization conditions used here are in general believed to cause multiphoton absorption, which results in extensive dissociation of the cluster distribution. The distribution measured, therefore, is not representative of the nascent distribution of neutral clusters formed in the source. Instead, it represents a distribution modified by multiphoton ionization and fragmentation, which is believed to be dominated by stable cluster cations. In a number of experiments,we have verified this by comparing the mass spectrum obtained by multiphoton ionization of neutrals with that resulting 500 1000 1500 2000 from the direct sampling of the cation clusters which condensed from the laser plasma. The intensity patterns in these two kinds MASS of spectra are nearly always the same. In the present system, Figure 3. Photodissociationmass spectra for two nine-atom clusters, the Pb&b+ and Pbg+ species, for excitation at 308 nm. Both are highly both methods produce nearly the same relative intensity patterns fragmented at this energy, and both have a pattern suggesting sequential in the mass spectrum, but multiphoton ionization of neutrals loss of P b neutrals. produces much larger parent ion signals which are needed for the photodissociation experiments. interested in comparing the dissociationbehavior of these "special" The mass distribution shown in Figure 1 is dominated by two ions to that of other clusters in the distribution. mass peaks corresponding to the cluster ions PbzSbs+ and PbdFigures 2-5 show selected examples of photodissociation mass spectra obtained for some of these bimetallic cluster ions with a Sbs+. As discussed in our previous work, these particular species XeCl excimer laser (308 nm, 4.02 eV). In each spectrum, the are formed preferentially from a variety of targets with different data are collected by a difference method, so the parent ion relative metal concentrations. They are isoelectronicto the Zintl ion species PbsL and Pb&, respectively. Therefore, we are intensity without the dissociation laser is subtracted from the
I
The Journal of Physical Chemistry, Vol. 97,No. 29, 1993 7437
Photodissociation of Pb/Sb Cations 1,5
4,2
R
4 .r(
0
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Mass (amu) Figure 4. Photodissociation of the Pbabs+ cation, which is a magic number in the cluster growth proccss. 2,o
Y 0
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Pb,Sb,
1200
1600
Mass (amu) 283
0
600
4.3
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‘0
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Figure 5. Photodissociationof Pb,Sb6+ and PbSb4+clusters at 308 nm. Both species exhibit fragmentation to odd-numbered antimony cations. These and the 4/5 cation in Figure 4 are the only species for which production of the 2/3 fragment is important. spectrum of the depleted parent ion plus its daughter ions. The result is a spectrum with a negative peak indicating the parent ion depletion and positive peaks indicating the fragment ions. In each spectrum, the laser power dependence is studied, and the data presented are accumulated in the limit of low laser power to limit multiphoton dissociation processes. The typical laser power used is 1 mJ/cm2. At higher laser powers in nearly every system, more extensive fragmentation is observed, which is attributed to multiphoton absorption and sequential multistep dissociation processes. As discussed previously, the intensities in these fragment spectra are sensitive to the spectrometer focusing conditions used (deflection plates, etc.), and to the exact firing time of the dissociation laser.3* Therefore, we have also studied these spectra under a variety of instrument conditions to guarantee that no fragment ions are missed. Table I provides a complete listing of all the bimetallic ions studied and their dissociation fragments. The choice of parent ions to be dissociated is limited
TABLE I: Photodissociation Products of Pb/Sb Cluster Cations from Excitation at 308 nm parent ion fragment ions Pb+ Pb+(s),PbSb+(m),Sbz+(w) Pb+(s),PbSb+(m),Pbz+(m) Pb+(s),PbSb+(m),PbSbz+(m) PbZSb+(m),Pbz+(m) Pb+(s) ,PbSb+(m) ,PbZ+(m) ,PbZSb+(m),Pb3+(w) Pb+(s),PbSb+(m),PbSbZ+(w),Sbl+(s),PbZ+(w), PbzSb+(m),PbSb3+(m),PbzSbz+(w) Pb+(s),PbSb+(s),Pbz+(s),PbSbz+(s),PbzSb+(m), PbZSbz+(W) Pb+(s),PbZ+(m),PbZSb+(m) Pb+(s),PbzSbZ+(m),Pbz+(m),PbzSb+(s) Pb+(S),Pbz+(w), PbpSb+(s) Pb+(S) ,PbsSbz+(S) PbZSb+(s),Pbl+(w),PblSb+(s),Pb&b+(s),PbsSb+(m), Pb&b+(s) PbzSbZ+(m) ,PbSb4+(w ),Pb,Sbz+(m) , PbSbs(s) , PbzSbs+(m),PbZSbs+(w),Pb&bZ+(s) PbZ(s), Sbs+(m),PbzSb3+(w),Pb3Sb3+(s),PbZSbs+(w) Pb&bl+(m),PbsSba+(m), Pb&b~+(s),Pb$b3+(w), Pb~Sbz+(w), PbzSbo+(~) TABLE 11: Energetics for Lead and Antimony Atoms, Dimers, and Their Ions1~E0,40,41 species IP (eV)40 (eV) mf,298(g) (kcal/mol)@ Pb 7.416 46.6 Sb 8.641 62.7 Pb+ 218 Sb+ 262 Pbz 6.3 0.8241 74.3 8.4 3.0941 56.3 Sbz 1.94’ 309.2 Pbz+ Sbz+ 3.33“ 247.9 Derived from the literature ionization potential and AHf data. to those species having significant parent ion density in the cluster distribution which can be cleanly mass selected. Relative intensities (strong, medium, weak) are indicated rather than quantitative branching ratios because of the uncertain effects of focusing on these intensities. However, as discussed previously, there is no reason to believe that focusing effects could cause spurious peaks where there is no fragment or cause fragment ions which are formed to be missed entirely. Table I1 provides a list of the known thermochemical data for the atomic and diatomic species of lead and antimony.*”20+40.41 The cation enthalpies are derived from the corresponding neutral values and the respective ionization potentials. Likewise, dimer cation dissociation energies are derived from the neutral dissociation energies and the atomic and diatomic ionization potentials. In the small cluster dissociation events, where only atomic or diatomic fragments are formed, these data can be used to derive additional information about the previously unstudied bimetallic parent ions. The first parent ion listed in Table I is the PbSb+ diatomic. The only fragment ion detected at low laser power is Pb+. We only detect charged fragments in these photodissociation systems, but the corresponding neutral fragments can be inferred by simple mass conservation. Thus, detection of Pb+ implies a dissociation process of PbSb+ Pb+ + Sb. This is expected to be the lower energy dissociation channel because the ionization potential of the Pb atom (7.416eV) is lower than that of the Sb atom (8.641 eV).a The only thermochemistry resulting from this measurement is that the molecular dissociation energy, DO,must be less than or equal to the photon energy (308nm, 4.02eV, 92.8kcall mol). For comparison, the respective lead and antimony homonuclear dimer cations have dissociation energies of 1.94and 3.33 eV.40 Although it is not possible to conclude this from the data here, it seems likely that the dissociation energy for the mixed
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7438 The Journal of Physical Chemistry, Vol. 97, No. 29, I993
dimer should be between the homonuclear values. It is clear that it is not significantly higher than the Sb2+ value. A lower limit of AHr(PbSb+) 2 187.9 kcal/mol may be derived from this dissociation energy and from the known ionization potentials for the respectiveatoms. The Pb+ channel observed here is prominent throughout the smaller mixed clusters studied here. This is likely caused by higher ionization potentials in the mixed cluster fragments which contain one or more antimony atoms. Likewise, pure lead clusters16ahave ionization potentials significantlylower than those of pure antimony clusters.16b In small polyatomic parent ions, such as the trimers Pb2Sb+ and PbSb2+, the observation of an atomic ion fragment, such as Pb+,implies that there may be either a neutral diatomic fragment or two neutral atoms. However, the channel with a neutral diatomic will always lie lower in energy. Dissociation seems to be similar in the two triatomics studied here. Both yield the lead atomic ion, the mixed dimer PbSb+, and the respective homonuclear diatomic cation. Neither system fragments to the antimony atomic cation. Presumably, these respective channels lie higher in energy because of the high ionization potential of Sb. As described above, the products observed here can be used to infer some simple thermochemistry about the parent ion. For example, the reaction PbSb2+ Pb+ + Sb2, which is indicated by the observation of the Pb+ ion, must have an endothermicity no greater than the photon energy, 92.8 kcal/mol. Using the standard enthalpies of formation for Pb+ and Sb2,40we derive the lower limit that AjYr(PbSbz+) 2 181.5 kcal/mol. By a similar line of reasoning, AHr(Pb2Sb+) 1 279.1 kcal/mol. More interesting fragmentation patterns are possible for larger clusters with more atoms of each element. These studies make it possible to look for preferential elimination of one element or the other, or to observe special fragment ions which are formed from many cluster decomposition processes. In particular, it is interesting to compare these results to the dissociation behavior of pure antimony or lead cluster ions. Pb,+ cluster decomposition is dominated by the sequential loss of neutral Pbz units.39 Sb,+ cluster decomposition is dominated by neutral dimer loss and the formation of the stable Sb3+ cluster.21 As shown, both of these patterns influence the mixed clusters. Dissociation patterns in the four-atom speciesseem to be driven by the low ionization potential (IP) of lead, the higher IP of antimony, and the stability of the antimony neutral dimer. Pb+ is a prominent fragment from PbSb3+and PbsSb+,which is favored by the low IP of Pb relative to Sb. It appears that the presence of Sb in mixed clusters gives them higher IPS as well. Sb, Sb2, or Sb3 are the neutral fragments implied by mass conservation to correspond to several of the fragment channels here. The process Pb,Sb+ PbSb+ implies the loss of Pb2. This is the smallest cluster illustrating this behavior, but this process becomes more important for larger clusters which are rich in lead (see below). Three five-atom species have been studied: PbzSb3+, PbsSbz+, and Pb4Sb+. As a convenient shorthand notation, we indicate thesespeciesas2/3,3/2,and4/l,respectively.The213 and312 parent ions, as shown in Figure 2, exhibit a wider variety of fragments than any other clusters studied here. Pb+ is a strong channel for both, as it is also for the 4/1 parent ion. The process PbzSb3+ Sb3+ is favored by the stability of Sb3+, which is the most prominent fragment ion for pure antimony clusters. This process also indicates the loss of Pb2, which is the most prominent process for pure lead clusters. The 3/2 2/0 process is likely favored by the low IP of Pbz (6.3 eV). The process of 312 112 also corresponds to the loss of Pbz neutral. Apparently, in these five-atom species there are many fragmentation processes with about the same energetics. There is no obvious differencebetween the behavior for the Pb2Sb3+species, which is the magic number in the initial distribution, and that for the other two species with
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Willey et al. five atoms, except as determined by the available numbers of each kind of atom. In the six-atom species Pb+Sbz+ and PbsSb+, there are fewer fragmentation channels observed. Loss of lead atom or dimer is again prominent, with this loss occurring for both charged and neutral Pb and Pb2 elimination. The only chahnel in which the antimony is split between charged and (implied) neutrals is the process Pb4Sbz+ Pb2Sb+ (+ PbzSb), which is the strongest channel for the 412 parent other than the Pb+ channel. Only one seven-atom parent ion, Pb~Sb2+,has been dissociated. Only two charged fragments are observed, Pb+ and PblSbz+. These both represent the loss of lead, in the form of the charged atom and then in the form of the neutral dimer. Three nine-atom clusters have been dissociated: PbgSb+,Pb4Sbs+, and PbsSbs+. These vary in concentration from the leadrich 8/ 1 species to the antimony-rich 316 species. The 415 cation is the magic number speciesin the initial cluster distribution. The photodissociation mass spectrum of the 811 species is shown in Figure 3, where it is compared to the dissociation behavior of the pure lead Pb+species. As shown, both nine-atom clusters appear to fragment by the sequential loss of Pbz neutral units. Prominent fragments for Pbs+ are P b + , Pbs+, and Pb3+, while PbgSb+ has the prominent fragments of Pb&b+ and PbdSb+. All thecharged fragments from the 811 parent retain the single antimony atom, suggesting that the Sb-Pb bonding is stronger in.these clusters than thePb-bbondmg. Eachofthesespecieshasanevennumber of electrons, as do all Pb/Sb cations with an odd number of antimony atoms. The fragmentation behavior of the Pb4Sbs+ magic number cation is shown in Figure 4. The prominent charged fragments here are Pb4Sbz+and PbSbs+. The 412 fragment corresponds to the loss of an Sb3 neutral. There is no strong evidence for the importance of Pbz loss like that observed for many of the leadrich clusters. The 1/ 5 species and 213 species observed have odd numbers of antimony atoms. The 115 species is predicted to be stable by Wade’s rules (2N 4 = 16 valence electrons), as is the 213 species discussed earlier. Odd-numbered antimony clusters have an even number of electrons, and this is believed to explain in part their relative stability. However, the 412fragment, which is also quite prominent, has an odd number of electrons. Interestingly, the 213 fragment ion has an even number of overall electrons and it is a magic number ion in the initial cluster distribution. However, it is only observed with weak intensity here. The only general comment that can be made about this fragmentation process is that it is qualitatively quite different from that of the lead-rich 811 species. Figure 5 shows the fragmentation spectra of PbsSbs+, the antimony-richnineatomcluster, and thePbSb4+11-atomcluster. The prominent fragments from 316 are 210 and 313. The Pbz+ fragment was indicated above for several of the smaller parent ions. The 313 channel corresponds to the loss of neutral Sb3, which was also a prominent neutral implied to be lost from the 415 parent ion. The other fragments here all have either three or five antimony atoms. The 0/5,2/3,3/3, and 215 species are all even-electron species predicted to be stable by Wade’s rules. The Pb+b4+ fragmentation spectrum is similar to that of Pb3sb6+in the importanceof odd-antimony species. All the fragments observed have three antimony atoms and an even number of electrons. Of these, the two most prominent are the 2/3and 4/3 species. Interestingly, the 213 species is the five-atom magic number in the initial cluster distribution. However, this is the only dissociation spectrum in which it is a prominent fragment ion. The 413 ion is not prominent in the initial distribution, but it is a species predicted to be stable with Wade’s rules (seven atoms; 16 valence electrons = 2N 2).
-
+
+
Discussion Overall, these data are somewhat surprising in that there is little or no evidence for the formation of the 213 cation species
Thre Journal of Physical Chemistry, Vol. 97, No. 29, 1993 7439
Photodissociation of Pb/Sb Cations as a preferred fragment in these system. In contrast to this behavior, the magic number species observed previously for pure antimony clusters and pure bismuth clusters are found in both the cation distribution produced by the growth process and as preferred photofragments.2’ The only evidence for formation of the magic numbered species here is found in the larger clusters (e.g., the 316 and 7/4 parent species) which produce a prominent 2 / 3 fragment. Unfortunately, we have not been able to study any parent ion species which are large enough to have the 415 ion as a possible fragment ion. It may be that the 213 and 415 speciesare produced in the source distribution through the growth and evaporation of larger clusters. Unfortunately again, this hypothesis will be difficult to test. Larger species are produced only at low abundance in our source,and there is seriouscongestion at higher mass because of the many possible bimetallic species having nearly the same mass. These conditions make it impossible to mass-select large parent ions which have high enough abundance to study. In the small clusters which we have been able to study, there seem to be two guiding principles which influence photodissociation. The first is the preference for the formation of fragment ions containing an odd number of antimony atoms. The valence electron configuration of lead is 16~26~2. As has been discussed p r e v i o ~ s l ythe , ~ ~6s electrons are regarded as an “inert pair” in the chemistry of the heavy main group elements, and the bonding occurs primarily through the 6p electrons. Therefore, each lead atom contributes two p electrons to the bonding scheme. The configuration of antimony is ] 15~25~3. The s electrons are again inert, and there are three valence electrons contributed to cluster bonding for each Sb atom. Therefore, clusters cations which contain an odd number of antimony atoms, such as the fragments prominent in Figure 5 , have an even number of valence electrons. As indicated above, many of these species also have 2N 2,2N 4, or 2N 6 valence electrons, making them stable Wade’s rule clusters. The second guiding principle in these fragmentation processes seems to be the tendency for these clusters to “get the lead out”. The loss of lead atom or dimer is prevalent for all the clusters studied. In the case of the smaller systems, the loss of lead appears as the charged fragments Pb+ and Pb2+. In the larger clusters, lead is lost as an implied Pbz neutral dimer. This pattern is understandable considering the ionizationpotential trends in these clusters. The small clusters (three to five atoms) are generally expected to have higher IPSthan thelarger clusters. The presence of antimony drives the overall cluster IP higher than the values for Pb or Pbl. Therefore, these lead fragments are lost as charged species. The larger clusters have lower ionization potentials than Pb or Pb2, and so these fragments are then lost as neutrals. In both cases, antimony is usually retained in the larger cluster fragment. This suggests that Sb-Sb and Pb-Sb bonding is generally stronger than Pb-Pb bonding in these cations. This pattern is consistent with the strong bond in the neutral antimony dimer (DO= 3.09 eV)41and the weaker one in neutral lead dimer (Do= 0.82 eV).41 Likewise, the antimony dimer cation is more strongly bound than lead dimer cation (DO= 3.33 and 1.94 eV, respectively). In a sense, these clusters may be regarded as weakly bound lead atoms nucleated around a more strongly bound antimony core. A final general comment is that these clusters are significantly fragmented at the energy of photoexcitation used here, and that there is little preference for special fragments. This is in contrast to pure antimony or bismuth clusters, which almost always fragment to the M3+cation clusters.21 It seems that the relative differences in the energetics of the available fragmentation channels here are quite small. This suggests that the relative stabilities of the various fragment ions are also similar. Thus, the strong preference for the formation of the 213 cation implies that it is relatiuely more stable than other clusters in the same
+
+
+
size region, but the fragmentation data suggest that these energy differences may not be very great. At this point, we can only speculate about just how stable the larger 415 species might be. However, the degree of fragmentation exhibited by this species is also not much different from other clusters of similar size.
Conclusion We have synthesized a variety of mixed metal cluster cations containing antimony and lead. The distribution of clusters produced by the source exhibits a strong preferential formation for the five- and nine-atom species PbzSb3+ and Pb4Sbs+. These species are isoelectronic to stable main group metal clusters which have been isolated in condensed-phaseinorganic chemistry. Their bonding is predicted to be stable by the Wade’s rule electron counting model. Photodissociationchannels for a variety of small mixed cations, however, do not exhibit a strong preference for the formation of the 213 cation. Larger species which could possibly produce the 415 cation could not be studied. General principles which are followed in the species studied here are the preference for odd-antimony cations, which have an even number of electrons, and the preference for the elimination of lead. The extensive fragmentation observed and the numerous fragmentation channels suggest that these clusters are relatively weakly bound in general and that the more stable clusters are only marginally more stable than the others.
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