J. Phys. Chem. 1987, 91,6521-6525 than to Cursites. This leads to an electron-hole pair separation and an increase in the lifetime of the active 0- sites. The enhanced activity due to UV excitations of CuMo04 compared with M o o 3 may also be the result of increased hole lifetimes. We have proposed for this case that active 0- sites are produced by band-gap irradiation resulting in a MoV1-02- MoV-O- transition. The MoV is "quenched" by adjacent Cu", resulting in efficient electron-hole separation. The Cu d-0 2p band gap orbitals also influence hydrogen atom and methyl radical adsorption stabilities. Heterolytic adsorption sites are similar to those product stabilities on MoV1and 02calculated in our past M o o 3 study, but because of the low-lying
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empty band-gap orbitals on copper molybdate, homolytic adsorption on @- sites is more stable. This is because the gap orbitals are more easily reduced than empty Mo 4d band orbitals. Furthermore, the filled band-gap orbitals stabilize homolytic adsorption on MoV1sites by charge transfer to the MoH bonding orbitls, making this mode stable whereas it is not stable on Moo3. Finally, homolytic adsorption on CUI*and heterolytic adsorption on CUI' and 02are relatively weak. Registry No. MOO,, 1313-27-5; 02,7782-44-7; CH4, 74-82-8; CH3OH, 67-56-1; CU", 15158-1 1-9; CUMOO~, 13767-34-5; H2, 1333-74-0; 11 1057-65-9; ~ ~ ) ~ , C U ~ M O ~ O110970~~'~-, Cu04&,56509-86-5; C U ~ ~ O 91-7; MopO14'*, 11 1058-56-1.
Ultraviolet Photolysis in a Laser Vaporizatlon Cluster Source: Synthesis of Novel Mixed-Metal Clusters K. LaiHing, P. Y. Cbeng, and M. A. Duncan* Department of Chemistry, School of Chemical Sciences, University of Georgia, Athens, Georgia 30602 (Received: May 8, 1987)
A new variation of the laser vaporization/pulsed nozzle metal cluster source is described for the production of mixed-metal clusters. This source incorporates UV laser photolysis of a gas-phase metal-containing complex with the same laser used for and in the same region of solid metal vaporization. This method is generally useful for the production of mixed-metal, metalsemimetal, and metalsemiconductor clusters and may be especially useful for cluster studies in which alloys of desired componehts are not readily available. Data are presented for clusters of Bi/Cr and Mn/Ag produced by this method.
Introduction Over this same time period, the quantum theory of cluster bonding has achieved increasing sophistication and has both stimulated The past few years have witnessed an almost explosive growth and complemented experimental work! As metal-cluster research in the study of small metal This interest has been stimulated primarily by the development of laser v a p o r i z a t i o r ~ ~ , ~ ~is rapidly coming of age, practical applications of cluster technology are also being considered, such as molecular beam vapor as a general technique producing clusters of metals, semicondeposition to form thin films or supported catalysts." For both ductors, and other materials. Based on this technology, new fundamental and practical motivations, it is desirable to extend experiments have examined the molecular electronic structure of these cluster studies to more complicated materials containing clusters (diatomic and triatomic electronic spectra,4g6ionization mixtures of elements. For example, bimetallic systems form an potentials," electron affinities,12 ion fragmentati~n'~) as well as important class of catalysts exhibiting activity not found with pure their chemical properties,le16 revealing a rich variety of behavior. metals.'* The optical, electronic, and magnetic properties of materials also depend critically on well-chosen stoichiometric mixtures of elements. To meet some of these increasing demands (1) Davis, S. C.; Klabunde, K. J. Chem. Reu. 1982, 82, 153. in cluster technology, we have developed a variation of the pulsed (2) Gole, J. L.; Stwalley, W. C., Eds. Metal Bonding and Interactions in laser vaporization technique which makes it possible to produce High Temperature Systems; ACS Symposium Series 179; American Chemical Society: Washington, DC, 1982. a variety of mixed-element clusters. This method incorporates (3) Ozin, G. A.; Mitchell, S. A. Angew. Chem., Int. Ed. Engl. 1983, 22, laser photolysis of a volatile metal-containing complex, such as 674. a metal carbonyl, within the laser vaporization source region. This (4) Weltner, W., Jr.; Van Zee, R. J. Annu. Rev. Phys. Chem. 1984,35, technique provides a surprisingly clean and efficient source of 291. ( 5 ) Moskovits, M. Metal Clusters; Wiley: New York, 1986. mixed-element clusters that could not be produced easily by other (6) Morse, M. D. Chem. Rev. 1986, 86, 1049. methods. (7) Phillips, J. C. Chem. Rev. 1986, 86, 619. A variety of heteronuclear diatomic species have been studied (8) Koutecky, J.; Fantucci, P. Chem. Rev. 1986, 86, 539. previously in cryogenic rare gas matrices.'~~In the gas phase, alloy (9) Dietz, T. G.; Duncan, M. A.; Powers, D. E.; Smalley, R. E. J . Chem. Phys. 1981, 74, 651 1. (10) Bondybey, V. E.; English, J. H. J. Chem. Phys. 1981, 74, 6978. (1 1) (a) Rohlfing, E. A.; Cox, D. M.; Kaldor, A.; Johnson, K. H. J . Chem. Phys. 1984,81, 3846. (b) Whetten, R. L.; Zakin, M. R.; Cox, D. M.; Trevor, D. J.; Kaldor, A. J . Chem. Phys. 1986, 85, 1697. (12) Zheng, L. S.; Karner, C. M.; Brucat, P. J.; Yang, S.H.; Pettiette, C. L.; Craycraft, M. J.; Smalley, R. E. J. Chem. Phys. 1986, 85, 1697. (13) Brucat, P. J.; Zheng, L. S.; Pettiette, C. L.; Yang, S.;Smalley, R. E. J . Chem. Phys. 1986, 84, 3078. (14) (a) Geusic, M. E.; Morse, M. D.; OBrien, S. C.; Smalley, R. E. J . Chem. Phys. 1985,82,590. (b) Morae, M. D.; Geusic, M. E.; Heath, J. R.; Smalley, R. E. J . Chem. Phys. 1985,83, 2293.
(15) (a) Whetten, R. L.; Cox, D. M.; Trevor, D. J.; Kaldor, A. Phys. Reo. Lett. 1985,54, 1494. (b) Trevor, D. J.; Whetten, R. L.; Cox, D. M.; Kaldor, A. J . Am. Chem. SOC.1985, 107, 518. (16) Richtsmeier, S. C.; Parks, E. K.; Liu, K.; Pobo, L. G.; Riley, S. J. J . Chem. Phys. 1985, 82, 3659. (17) (a) Yamada, I.; Tagaki, K. Thin Solid Films 1981, 80, 105. (b) Yamada, I.; Inakawa, H.; Tagaki, K. J . Appl. Phys. 1984,56, 2746. (18) (a) Sinfelt, J. H.; J . Caral. 1973, 29, 308. (b) Sinfelt, J. H. Acc. Chem. Res. 1977, 10, 15. (c) Sinfelt, J. H. Bimetallic Catalysts: Discoueries, Concepts, and Applications; Wiley: New York, 1983.
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uv Voporizotion Loser
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Pulsed V o l v e
/
Metol Rod
Bi X
w
Figure 1. Laser vaporization cluster source in the photolysis configuration. The metal-containing complex, ML,, is blended at its ambient vapor pressure into the helium gas, which is then pulsed over the solid sample rod. The UV vaporization laser (XeCI; 308 nm) both photolyzes the complex and vaporizes the solid rod.
or mixed-metal clusters have been produced by covaporization of component elements in oven/beam source^^^*^ or by direct laser vaporization of alloy^.^^-^^ For example, Martin has studied "magic" number patterns in compound clusters such as As/S, Cs/S, or Pb/In using the covaporization method.Ig Schild and co-workers have used similar oven sources with inert gas condensation to study intermetallic clusters of Pb, Bi, and Sb." In some of the first laser vaporization studies, Bondybey and coworkers have produced heteronuclear dimers (ASP, SbP, CuGa, CuIn) for laser-induced fluorescence spectroscopy.2' Kaldor and co-workers have examined the mass spectroscopic distributions of Ni/Cr and Ni/Al transition-metal mixtures produced with laser vaporization and have found statistical combinations of the component elements.22 Recent laser vaporization studies in our own laboratory, however, have found preferential mass combinations of elements in group IV (14)/group V ( 1 5 p main-group mixtures (Sn/Bi, Pb/Sb, Sn/As), consistent with patterns predicted by ~ alloy and mixed-metal models based on electron c o ~ n t i n g ? While studies have provided interesting new insights into cluster properties, there are limitations on the existing experimental techniques. For example, covaporization methods fail for components with widely disparate vapor pressures. Additionally, alloys for laser vaporization may not always be available at desired compositions. For systematic studies of various binary systems as a function of stoichiometry, new cluster synthetic techniques may be required. The laser photolysis/laser vaporization source described here overcomes some of the limitations in existing methods for cluster synthesis. In this paper we describe the general details of this source and its operation, as well as some preliminary investigations of the gas-phase chemistry on which it is based. We also present laser photoionization mass spectra for new cluster systems prepared with this source that illustrate some of the fascinating properties of mixed-metal systems. Experimental Section The pulsed molecular beam apparatus and laser vaporization cluster source used for these experiments has been described previo~sly.~'The cluster source is shown in Figure 1. We use a double solenoid pulsed valve (Newport Corp. BV- 100) typically operating with a 6-atm backing pressure of helium, a l-mm-diameter orifice,a 200-ps pulse duration, and a 10-20-Hz repetition rate. Vaporization is accomplished by a focused excimer laser (19) (a) Martin, T. P. J. Chem. Phys. 1984,80, 1970. (b) Martin, T. P. J . Chem. Phys. 1984,81, 4426. (c) Martin, T. P. J . Chem. Phys. 1985,83, 78. (20) Schild, D.; Pflaum, R.; Sattler, K.; Recknagel, E. J . Phys. Chem. 1987, 91, 2649. (21) (a) Bondybey, V. E.; Schwartz, G . P.; English, J. H. J . Chem. Phys. 1983,78, 11. (b) Rasaneu, M.; Heimbrook, L. A.; Schwartz, G. P.; Bondybey, V. E. J. Chem. Phys. 1986, 85, 86. (22) Rohlfing, E. A.; Cox, D. M.; Petkovic-Luton, R.; Kaldor, A. J . Phys. Chem. 1984,88, 6227. (23) Wheeler, R. G.; LaiHing, K.; Wilson, W. L.; Allen, J. D.; King, R. B.; Duncan, M. A. J . Am. Chem. SOC.1986, 108, 8101.
Figure 2. Mass spectra obtained for pure bismuth vaporization and vaporization plus photolysis of Cr(C0)6 for photoionization at 157 nm (fluorine excimer laser; power 0.25 mJ/cm2). The bismuth atom and dimer are off scale in both spectra by a factor of 2. Mixed-metal features appear just above pure bismuth peaks. The more intense peaks correspond to Bi,Cr masses. Carbonyl-containingfeatures are also observed for Bi34. Pure bismuth clusters are not observed larger than five atoms.
(Lumonics) operating at 308 nm. As shown in Figure 1, the sample rod is mounted within a housing attached to the nozzle where the helium flow is confined to a narrow channel (2-mm diameter) over the exposed rod surface. Pulsed vaporization produces a hot plasma above the sample rod containing both neutral and ionic metal species, electrons, and both ionic and metastable helium. Collisional energy transfer with excess helium cools the metal vapor, and clusters grow in the channel extending beyond the vaporization point. The length of the growth region (section d l ) can be varied from 5 to 50 mm by addition of modular channel segments to obtain crude adjustment of cluster size. An additional segment (4-mm diameter X 10 mm long) is added at the end of the growth region to spread the cluster packet (via turbulence) and lessen the timing requirements on downstream detection. The metal-cluster/helium mixture expands freely into a vacuum system described elsewhere,24where it is collimated into a beam by a skimmer before entering the differentially pumped detection chamber. Photoionization is accomplished by a second excimer laser crossing the molecular beam in the ion source of a homemade time-of-flight mass spectrometer. Mass spectra are processed with a Camac-based 100-MHz transient digitizer (DSP Technology) under computer control. The same pulsed valve cluster source described above is used for photolysis experiments producing mixed-metal clusters. For these experiments, one metal compqnent of the mixture is introduced as a solid rod mounted in the usual laser vaporization configuration. The second metal is introduced via a volatile metal-containing species in the vapor which is blended with the helium buffer gas flowing over the rod. The products and mechanism of laser multiphoton dissociation of transition-metal complexes or organometallic complexes have been extensively s t ~ d i e d . ~In~many ? ~ ~ cases, the dominant photophysical process is ligand stripping (either sequential or concerted), which produces neutral metal atoms. Therefore, when this kind of complex is mixed into the helium buffer gas, the same laser pulse that vaporizes the metal rod also photolyzes the metal-containing vapor species, reciulting in two different atomic metals in the vaporization plasma. Mixed clusters then form in the growth channel in (24) Wheeler, R. G.; Duncan, M. A. J. Phys. Chem. 1986, 90, 1610. (25) Gedanken, A.; Robin, M. B.;Keubler, N. A. J . Phys. Chem. 1982, 86, 4906. (26) Hollingsworth, W. E.; Vaida, V. J . Phys. Chem. 1986, 90, 1235.
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six-atom bismuth species must either be difficult to form by singleatom growth or be thermodynamically unstable with respect > to fragmentation once it is formed. t In the photolysis experiment, clusters containing more than five Ln z atoms of bismuth are produced. Careful inspection of the mass w spectra, however, shows that these larger species always contain c Z at least one atom of chromium. There are still no pure bismuth m species larger than five atoms. The slight exception to this observation is the Bi7+mass channel in Figure 3. Bi7+ is unusually prominent in our previous studies of bismuth cations,28 and moderate-to-higher laser powers such as that used here are likely to cause fragmentation. Therefore, we believe this mass peak, > i I I I which is not observed in the F2 laser data, results from fragL i B i Cr mentation of larger mixed clusters, rather than the direct ionization of Bi7 neutrals. The observation of these clusters, therefore, suggests that chromium atoms must alter either the growth dynamics or the stability problems in the six-atom region. In this sense, then, chromium acts as a nucleating agent for the growth of larger bismuth-containing clusters. 1 5 10 It is especially surprising that mixed-metal clusters are formed B I CLUSTER SIZE in these experiments without the addition of carbonyl ligands. Figure 3. Mass spectra for the bismuth-chromium system at a phoEven if carbon monoxide is initially liberated by the photolysis toionization wavelength of 193 nm (ArF excimer laser; 1.0 mJ/cmz). As step, it should be both thermodynamically and kinetically favorable in Figure 2, bare metal species are dominant, but features corresponding for these ligands to readsorb on the surface of the newly formed to two or three chromium atoms per bismuth unit are much more promixed-metal clusters before the mixture leaves the growth region. nounced. There is very little evidence for carbonyl-containingspecies. One simple explanation for the absence of metal-cluster carbonyls in these spectra is that they are not formed efficiently. However, processes similar to those for pure metals. While this photolysis Smalley and co-workers have shown that carbon monoxide remethod was intended when we designed our mixed-metal exactions with other metal clusters (e.g., Nb) under similar conditions periments, other gas-phase chemistry may also be possible. Details are reasonbly e f f i ~ i e n t . ' Alternatively, ~ it could be that cluster about other possible mechanisms are presented below. In excarbonyls are formed initially but carbonyl ligands are preferperiments to date we have used iron pentacarbonyl (Fe(CO)5), entially fragmented in photoionization processes. It could also chromium hexacarbonyl ( c r ( c o ) , ) , and methylcyclobe that cluster carbonyls are not ionized efficiently because of pentadienylmanganese tricarbonyl [(CH3C5H4)Mn(CO),] comhigh ionization thresholds or low ionization cross sections. In fact, plexes to produce mixtures containing iron, chromium, and extensive fragmentation and ligand stripping is the usual mechmanganese, respectively. These complexes were used at their room anism for atomic metal carbonyls at the ArF laser wavelength temperature vapor pressures (approximately 20,0.1, and 1.0 Torr, for which ionization necessarily occurs by at least two-photon respectively). absorption, and similar behavior might be expected for clusters.26 Results and Discussion At the F2 laser wavelength, however, atomic metal carbonyl complexes (Cr(CO),, Fe(CO)5) are single-photon ionized and the The Bismuth-Chromium System. Mass spectra at two phocorresponding parent ion is the dominant mass spectral feature. toionization wavelengths (157 and 193 nm) are shown in Figures Cluster carbonyls should have lower ionization potentials than 2 and 3 for the bismuth-chromium mixed-metal system. In each the corresponding saturated metal atom complexes and should figure the laser-vaporized bismuth distribution is presented both therefore be ionized with good efficiency a t the F2 laser wavebefore and after the addition of chromium hexacarbonyl as a length. Inspection of Figures 2 and 3 does shown that there is photolysis precursor. The UV photochemistry of both iron and more evidence for metal carbonyl species under fluorine laser chromium carbonyls has been extensively studied.% The chromium photoionization, thus suggesting either some fragmentation loss carbonyl was chosen for this initial experiment rather than iron of C O under A r F ionization or inefficient ionization of carbonto avoid the inconvenient mass coincidence of Fe, versus (CO)2x yl-containing species at this wavelength. Even under fluorine laser at multiples of 56 amu. Bismuth was chosen as the laser-vaporized though, the ratio of peak intensities all through the metal because it has been extensively studied in our l a b o r a t ~ r y ~ ~ ~ conditions, *~ spectrum favors bare metal mass channels over carbonyl-conand because the large atomic mass makes mixed-metal mass taining channels by a factor of 5-1O:l. Additional investigations spectra easier to interpret. Two very interesting observations are of the role of ligands in these mixed-metal systems are presented readily apparent in these mass spectra. First of all, the cluster below. mass spectra resulting from photolysis extend out to beyond 10-15 Another interesting phenomenon can be found by comparison total atoms, while the pure bismuth spectra end abruptly at 5 of the fluorine and ArF mass spectra in Figures 2 and 3. The atoms. Second, the mass peaks observed correspond primarily A r F data in Figure 3 is significantly enhanced with respect to to Bi/Cr mixed-metal species without carbonyl ligands. Figure 2 at all mass channels corresponding to clusters with 2 or The truncated bismuth spectrum, which is also observed for 3 chromium atoms. All bismuth,clusters in this size range27have antimony clusters, is itself an interesting phenomenon and has ionization potentials greater than 6.4eV. Power-dependent studies ~ ~ have shown that been studied previously in our l a b o r a t ~ r y .We at the ArF wavelength indicate that this is probably also true for the five-atom limit is not caused by fragmentation of larger clusters these mixed clusters. At the lowest ArF powers used, when in the photoionization process but instead results because bismuth two-photon ionization first occurs, the mass spectrum resembles clusters larger than five atoms are not formed in the laser vathe Fz data in that mixture peaks representing one chromium atom porization source. This effect is only observed for neutral bismuth added to a bismuth cluster are dominant. As the laser power is clusters; larger charged clusters of both polarities have been obincreased, two and three chromium atom peaks become more served out of this same source.28 This latter result also rules out prominent, particularly for the smaller bismuth units. This insufficient vaporized atoms as the source of the growth limitation. suggests that, on the average, photofragmentation in the ionization Clusters in these laser vaporization sources grow primarily by process results in preferential loss of bismuth rather than chromium sequential addition of atoms. Apparently, then, the next larger
!
I l l
1
I l l
c (
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(27) Wheeler, R. G.; LaiHing, K.; Wilson, W. L.; Duncan, M. A. Chem. Phys. Leu. 1986, 131, 8.
(28) Geusic, M. E.; Freeman, R. R.; Duncan, M. A. J . Chem. Phys., in press.
6524 The Journal of Physical Chemistry, Vol. 91, No. 26, 1987
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tlE l
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t-
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-L
t
.
.
T
I
II)
Z
w
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10 15 5 NUMBER OF A S ATOMS
20
Figure 4. Mass spectra obtained for silver vaporization and (MeCp)Mn(C0)3photolysis with photoionization at 157 nm (0.25 mJ/cm2). As described in the text, Ag,Mn, Ag,Mn(MeCp), and AgJMeCp) mass features appear throughout the spectrum. The parent ion of (MeCp)Mn(CO)3at 218 amu overlaps the Ag, mass channel in the lower trace. species from these mixed-metal clusters. Perhaps the most fascinating aspect of cluster Eormation in the bismuth-chromium system is that these two metals are insoluble in bulk phases and do not exist as alloys.29 Cluster formation, on the other hand, is quite efficient. In the laser vaporization source, cluster growth occurs at or near room temperature under collision-dominated conditions. Previous estimates have shown that species which survive these growth conditions must have bond energies greater than about 10 kcal/mol.” Therefore, mixed-metal bond energies must at least be this strong in the bismuth-chromium system. We have now studied several other mixed-metal systems that also do not form bulk phase alloys (bismuth-iron, antimony-iron, silver-chromium, antimony-chromium). In every case, clusters in the size range up to about 20 atoms are formed quite readily. In this sense, then, the structure and bonding in these systems must be significantly different from those in bulk solids. The Silver-Manganese System. To further investigate both the mechanism and the applicability of this photolysis method, we have applied it to other metal mixtures. Figures 4 and 5 illustrate the mass spectra obtained for the silver-manganese system. The precursor for manganese in this experiment is methylcyclopentadienylmanganese tricarbonyl [(MeCp)Mn(CO),]. This complex was chosen to introduce a more strongly bound ligand into the photolysis step which should have a larger cross section for recombination with newly formed mixed clusters than carbon monoxide. Additionally, based on our multiphoton ionization studies of ( M ~ C ~ ) M I I ( C O )we , , ~expected ~ that mixed-metal clusters containing adsorbed MeCp would be less likely to lose this ligand by fragmentation in photoionization. This system, then, would constitute a “worst-case” scenario for the observation of mixed-metal clusters without ligands. Silver was chosen for the laser-vaporized metal in this experiment because of our interest in cluster ionization potentials. We have recently studied the pattern of even-odd I P alternation in silver clusters which causes smaller even-numbered species to be missing in mass spectra at 193 nm. The presence or absence of this simple pattern in mixed clusters should provide a convenient indicator for the effect of added atoms on ionization potentials. Figures 4 and 5 show both the pure silver cluster distribution and the mixture produced by photolysis plus vaporization at the two photoionization wavelengths, 157 and 193 nm. To our (29) Hansen, M. Consrirurion of Binary Alloys; McGraw-Hill: New York, 1958; p 307. (30) LaiHing, K.; Taylor, T.; Duncan, M. A,, unpublished results.
1
5 NUMBER
10
15
OF Ag A T O M S
20
Figure 5. Mass spectra obtained for the silver-manganese system with ArF photoionization. The top trace shows pure silver clusters in the limit of low laser power, where the missing peaks reflect the even-odd IP alternation. Under the same low-power single photoionization conditions in the photolysis experiment, no mixed clusters are observed, suggesting that their ionization potentials are greater than 6.4 eV. The lower trace shows ArF photoionization at moderate power (1.0 mJ/cm2) where mixed-cluster peaks are observed under multiphoton ionization conditions. knowledge, the pure silver photoionization mass spectra at these two wavelengths have not been reported previously. At 157 nm all clusters appear in the mass spectrum down to the lowest laser power where the signal could be detected (approximately 0.01 mJ/cm2). In the low-power limit, cluster peak intensities vary linearly with the laser power. These combined observations indicate that silver clusters in the range of 2-30 atoms all have ionization potentials below 7.9 eV. At 193 nm, however, a very different behavior is observed. In the limit of low-laser power at this wavelength, the atom and the even-numbered cluster peaks N = 2,4,6, and 8 either are missing completely from the spectrum or appear with very low intensity relative to the odd-numbered N = 3, 5, 7, and 9 species. These observations suggest that silver cluster ionization potentials oscillate above and below the energy of the 6.4-eV ArF photon. A similar even-odd alternation has been observed for copper clusters” and alkali metal clusters32and has been attributed to spin-pairing effects in the valence electrons. Just as in the Bi/Cr experiment, the photolysis of (MeCp)Mn(CO), in conjunction with laser vaporization of silver produces the corresponding mixed-metal clusters. Mass spectra at both photoionization wavelengths contain peaks of the form Ag,Mn for all values of x , which appear evenly spaced between Ag, mass peaks. Ag,Mn, features fall within three mass units of the corresponding Ag,+, mass peak and are therefore not resolved cleanly in our experiment. In contrast to the Bi/Cr data, however, there is substantially more evidence for the incorporation of photofragment ligands into the product mixed clusters. Mass features corresponding to both Ag,(MeCp) and Ag,Mn(MeCp) species occur throughout the spectrum. The Ag,(MeCp) peaks fall between Ag,Mn and Ag,+, features while the Ag,Mn(MeCp) peaks appear just above the corresponding Ag,+l features. Carbonyl-containing cluster peaks such as Ag,MnCO are not clearly evident in these data, but they would be partially overlapped by the near mass coincidence with Ag,,(MeCp) peaks. In spite of the observed presence of ligand-containing species in these data, however, bare metal mass channels corresponding to Ag,Mn are still the most intense mass features resulting from this photolysis process throughout the spectrum. (31) Powers, D. E.; Hansen, S. G.; Michalopoulos, D. L.; Smalley, R. E. J . Chem. Phys. 1983, 78, 2866.
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While only crude estimates can be gained from studies at two photoionization wavelengths, it is interesting to consider relative ionization potentials in these systems. Under limiting low-power radiation at 157 nm (Figure 4), all Ag,Mn, Ag,Mn(MeCp), and Ag,(MeCp) features are observed, suggesting that all these species have ionization potentials lower than 7.9 eV. In the limiting low-power data at 193 nm, however, (not shown) the mixed-system mass spectrum resembles that of the pure silver distribution containing no Ag,Mn, Ag,Mn(MeCp), or Ag,(MeCp) features. This suggests either that these species have ionization potentials greater than 6.4 eV or that ionization cross sections are very low at this wavelength. Under higher laser fluence conditions a t this wavelength, when multiphoton ionization becomes possible, all such features are observed (Figure 5). The Mechanism of Mixed-Cluster Formation. The results of photolysis experiments to data are all consistent with the original mechanism envisioned for these experiments. That is that photolysis of the organo-transition-metal precursor produces added metal atoms in the vaporization zone which are incorporated into the growing clusters resulting from laser vaporization. Consistent with this idea, photolysis in this configuration without the solid sample rod yields small clusters of the complexed metal atoms (e.g., Cr(C0)6 photolysis yields Cr2 and Cr, at the end of the growth region). However, other mechanisms could also explain our results. For example, reactions of vaporized atoms or clusters with unphotolyzed gas-phase complexes could also produce mixed-metal species. Ion-molecule chemistry involving these species is also likely in the laser vaporization plasma, but reneutralization of charged species would be required prior to their detection in our current mass spectrometer configuration. In any scenario, it is difficult to explain why ligand-containing species are not more prominent in the product clusters. As mentioned previously, similar reactions between carbon monoxide and other metals are reasonably effi~ient.’~ The partial pressures of the precursor metal complexes used here are on the order of 1 Torr. Under the high-powered photolysis/vaporization conditions used, photofragmentation of these complexes should take place with near unit efficiency. Therefore, ligand concentrations in the growth zone must also be at least 1 Torr, making these conditions very much like those in cluster flow reactor experim e n t ~ . ~Limited ’ ligand concentration, therefore, does not seem to explain our results. Previous cluster chemistry studies have involved pure transition metals, however. The main-group metals and silver are likely to be less reactive, and this tendency may carry over into the mixed species described here. Photoionization effects, while possible, do not seem adequate to explain low relative intensities for ligand-containing species throughout these mass spectra under ionization at different wavelengths. In data not reported here, we have observed similar cluster production without significant CO incorporation for the mixed-metal systems Fe/Bi, Fe/Sb, Ag/Cr, and Cr/Sn, using photolysis of iron or chromium carbonyl over appropriate metal rods. Thus, this seems to be a general characteristic of mixed-metal systems produced by this photolysis method. While this effect is more pronounced for C O than for MeCp ligands, pure mixed-metal species are dominant in all our data to date. Besides low reactivity for these mixtures, another effect that could explain low ligand incorporation involves the exothermicity of atom or ligand addition during the cluster growth process. On the basis of measured dissociation energies of m e t a l d i a t ~ m i c s ~ , and calculations for larger clusters,8 typical metal-metal bond energies are expected to be on the order of an electronvolt (1 eV = 23.06 kcal/mol). The sequential addition of each metal atom to the growing cluster then results in substantial heating of that cluster. If ligand-metal bond energies are weak compared to the energy released by this heating, then they could be preferentially desorbed during the growth process. Carbon monoxide heats of (32) Kappes, M.; Schar, M.; Radi, P.; Schumacher, E. J . Chem. Phys. 1986,84, 1863.
(33) Huber, K. P.; Herzberg, G. Constants of Diatomic Molecules; Van Nostrand Reinhold: New York, 1979.
adsorption on various single-crystal metals span a wide dynamic range and can be very low depending on the metal and the crystal orientation considered.” It is therefore conceivable that C O could be weakly bound enough to Cr/Bi or Ag/Mn clusters to be desorbed in the growth process. Earlier cluster reaction studies with C O were conducted further downstream in the cluster source where cluster growth had been arrested and therefore would not have been sensitive to this effect.14 MeCp ligands are expected to adsorb more strongly than C O and to be less susceptible to desorption, consistent with their greater abundance in mixed-metal mass spectra.
Conclusion It should not be surprising that mixed-metal species such as those described here can be formed by addition of metal-containing complexes to the helium buffer gas in laser vaporization cluster experiments. Other reactions, especially metal cluster oxide formation, have been observed previously when the helium contained impurities such as air.36 When oxygen is introduced in the helium, monoxide formation results in contrast to dioxide formation in reactions carried out downstream of the laser vaporization region.36 This result reflects the extreme conditions in the vaporization plasma. What is surprising in these experiments is the fortuitous tendency to formed bare mixed-metal species without ligands. The photolysis method described here appears to be a very general method for forming a variety of mixed-element clusters. In addition to Cr/Bi and Ag/Mn mixtures, we have already produced Sn/Cr, Fe/Bi, Fe/Sb, and Ge/Cr mixed clusters. There are gas-phase complexes of many metals that could be used for the photolysis precursor in these experiments. In fact, there is no reason to limit the photolysis step to metal complexes. Many photochemical processes are known to produce atomic nonmetals (e.g., S, P, Si)35that could also be incorporated into metal clusters with this general method. The disadvantage to this method is that concentrations are determined by the vapor pressure of the photolysis precursor, and it is not possible to obtain all possible stoichiometries in the product clusters. As we have shown, however, this disadvantage is offset by the ability to make mixed clusters when alloys or other mixed-element solids are not available for direct laser vaporization. It is especially intriguing that mixed-metal bonding makes cluster formation possible in systems that do not form alloys. Effects such as this in bimetallic clusters will provide new challenges for quantum-mechanical theories of metal bonding and molecular structure in these systems. As detailed spectroscopic techniques such as resonant two-photon ionization and laser photoelectron spectroscopy are applied to these systems, additional challenges and new insights into metal bonding are expected. Acknowledgment. We thank Prof. M. D. Morse for helpful discussions related to this work. This research was sponsored by the U S . Army Research Office through Contract No. DAAG 29-85-K-0040. We also acknowledge the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. Registry No. Bi, 7440-69-9; Cr, 7440-41-3; Mn, 7439-96-5; Ag, 7440-22-4. (34) Somorjai, G. Chemistry in Two Dimensions: Surfaces; Cornell University Press: Ithaca, NY, 1981. ( 3 5 ) Okabe, H. Photochemistry of Small Molecules; Wiley: New York, 1978. R. L.; Cox, D. M.; Trevor, D. J.; Kaldor, A. J. Phys. Chem. 1985, (36)89, Whetten, 566. (37) Geusic, M. E.; Morse, M. D.; OBrien, S. C.; Smalley, R. E. Reu. Sci. Instrum. 1985,56. 2123. (38) In this.paper the periodic group notation in parentheses is in accord with recent actions bv IUPAC and ACS nomenclature committees. A and B notation is eliminated because of wide confusion. Groups IA and IIA become groups 1 and 2. The d-transition elements comprise groups 3-12, and the p-block elements comprise groups 13-18. (Note that the former Roman number designation is preserved in the last digit of the new numbering: e.g., 111 3 and 13.)
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