Near-Threshold Photoionization to Probe Neutral “Met-Car” Clusters

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J. Phys. Chem. 1996, 100, 5654-5659

Near-Threshold Photoionization to Probe Neutral “Met-Car” Clusters L. R. Brock and M. A. Duncan* Department of Chemistry, UniVersity of Georgia, Athens, Georgia 30602 ReceiVed: NoVember 21, 1995; In Final Form: January 25, 1996X

Metal-carbon compound clusters of Ti, Zr, V, and Nb are produced by laser vaporization in a pulsed nozzle cluster source. The distribution of neutral clusters is investigated by time-of-flight mass spectrometry, with photoionization at a variety of laser wavelengths. Laser wavelength and power dependence studies are used to investigate ionization potentials of different metal-carbon clusters and the role of fragmentation in the appearance of mass distributions. The M8C12 met-car clusters of zirconium, vanadium, and niobium have vertical ionization potentials greater than 5.76 eV. Ti8C12 has a vertical ionization potential of 4.9 ( 0.2 eV.

Introduction Especially stable metal-carbon clusters having the stoichiometry M8C12 have recently been reported by Castleman and co-workers for the metals titanium, vanadium, hafnium, niobium, and zirconium,1-14 as well as for several metal mixtures.13 The special character of these clusters was first recognized in mass spectral distributions, but there have now been experiments indicating unique stability with respect to dissociation8,9 and reactions5 for this stoichiometry. Our research group has reported this same special 8/12 stoichiometry in mass distributions of iron, chromium, and molybdenum metal-carbon clusters15 and as a prominent photofragment from larger metalcarbon cluster cations of titanium, vanadium, zirconium, and iron.16-20 Castleman and co-workers have proposed a structure for the M8C12 cluster consisting of a pentagonal dodecahedron with Th symmetry. This structure is consistent with the mechanism we have proposed for the photodissociation of larger clusters, which are believed to have fcc crystalline structures. It is also consistent with the results of saturation chemisorption studies5 and ion chromatography experiments.22 Theoretical calculations, however, indicate that other structures (Td or D2d symmetry) slightly distorted from the Th cage are more likely,23-37 and these structures are also supported by chemisorption experiments.21a Unfortunately, the theoretical calculations on these systems are extremely difficult to perform, and their validity is uncertain. Furthermore, the experiments performed to date have obtained few quantitative data with which to test theory directly. In the present study, we investigate ionization potentials of the various metal-carbon clusters in an attempt to provide a direct comparison to parameters resulting from theoretical calculations. These experiments also yield additional insight into the fascinating electronic structure properties for these clusters. Photoionization studies, which are essentially the only way to study neutral clusters in the molecular beam environment, are interesting because they provide a comparison between corresponding neutral and cationic clusters. Nearly all of the previous experimental work on met-car clusters, which demonstrate the dramatic preference for the M8C12 stoichiometry, have been performed on cluster cations. However, the various theoretical investigations of these clusters have focused primarily on neutral clusters. Geometric effects in cluster stability (i.e., closing of a cage) are not expected to depend strongly on cluster charge, but electronic shell closings do, and either one or both * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, March 1, 1996.

0022-3654/96/20100-5654$12.00/0

of these effects may give rise to the kind of magic number mass distributions which have been observed. The theoretical treatments of the met-car clusters have investigated their valence molecular orbitals for different metal analogues to see which, if any, of these systems are closed-shelled electronically. These studies have led to predictions of stability trends for the different metals. However, the orbital patterns and electronic structures calculated depend on the choice of the geometric structure and its corresponding symmetry (Th, Td, or D2d). In any symmetry, the electronic structure is expected to depend on the charge state. It is therefore important to compare corresponding neutral and ionic clusters for various metals to determine their relative stabilities and reactivities. Experimental determinations of ionization potentials (IP’s) are a key ingredient in the investigation of electronic structure. The IP is a straightforward result of most theoretical calculations, and it can therefore be used to evaluate the quantitative accuracy of various theoretical approaches. IP’s also provide an indicator for the relative stability of corresponding neutral and cation clusters. Closed-shell neutrals are expected to have relatively high ionization potentials, while open-shell neutrals are expected to have low ionization potentials. Comparisons of IP’s for different metal analogues in the met-car family are therefore valuable for establishing trends in electronic configuration stability. Castleman and co-workers have reported previous studies of the photoionization of neutral met-car clusters.4a However, these studies were conducted at the fixed wavelengths of 1064, 532, 355, and 266 nm. Ionization at these wavelengths is expected to be multiphoton in nature, and no information about ionization potentials could be obtained. However, interesting time-delayed thermionic emission processes were observed.4b The Castleman group has also studied the metastable dissociation of titanium met-car clusters.8 The product channels observed in these studies establish that the ionization potential of Ti8C12 is less than or equal to the IP of the titanium atom (6.82 eV). There are no other photoionization studies of these systems. However, various theoretical calculations have examined IP’s for the different metal analogues of the met-car family. The most prominent species, Ti8C12, is predicted to have an IP in the 5-6 eV range.28,30,31,34,36,40 In the present study, we study the photoionization behavior of various metal-carbon cluster stoichiometries for the metals titanium, vanadium, zirconium and niobium. We extend the photoionization studies to shorter wavelengths (near 215 nm) with tunable ultraviolet lasers. At each wavelength studied, we investigate the laser power dependence of the photoionization © 1996 American Chemical Society

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signals observed and extend the laser power used to extremely low levels. The wavelength- and power-dependent studies establish close limits on the ionization potentials of some of the M8C12 species studied and lower limits on others. There is considerable scatter in the ionization potential data predicted by theory, and few treatments reproduce either the values measured here or the trends observed for different metals. Experimental Section The clusters for these experiments are produced by laser vaporization in a pulsed-nozzle source. The source uses a Newport Model BV-100 pulsed beam nozzle with homemade modifications.38 A rotating rod of the metal of interest is vaporized with the focused output of a XeCl excimer laser (Lumonics, 308 nm). The expansion gas for the experiment is helium seeded with 1% acetylene or methane. In the titanium experiments, 13C-substituted methane is employed to avoid the mass coincidence at 48Ti and 12C4. Metal-carbon clusters are formed by condensation of the laser-generated plasma as it flows through a high-pressure growth channel of variable length and are cooled further by a supersonic expansion. Downstream from the source, the molecular beam is sampled through a skimmer into a linear time-of-flight mass spectrometer. The molecular beam machine and detection electronics for these experiments have been described previously.39 Photoionization is accomplished with a Nd:YAG (Spectra Physics DCR-3) pumped dye laser (Spectra Physics PDL-2). The dye laser output is frequency doubled with a BBO crystal to obtain wavelengths in the 215-220 nm range. The power of the ultraviolet laser used is measured with a Laser Precision Model Rm-6600 power meter.

Figure 1. Photoionization mass spectrum of Ti/C clusters at 215 nm with stoichiometries indicated for the most prominent peaks. These stoichiometries were determined with 13C isotopic experiments.

Results and Discussion We began these studies by repeating the photoionization experiments reported previously by Castleman and co-workers4 on the titanium/carbon clusters. At various harmonics of the Nd:YAG laser (532, 355, 266 nm), they observed photoionization of neutral clusters. The mass spectra exhibited enhanced abundance for the M8C12 stoichiometry which had been detected previously in the cation mass distribution. We observe mass spectra similar to those reported by Castleman at these wavelengths. However, the ionization potentials of the various clusters investigated have been calculated to be higher than the single-photon energy at these wavelengths. Castleman did not report studies of the laser power dependence of these signals. We investigate their power dependence and find that ionization is multiphoton at 532 and 355 nm. We did not investigate the power dependence at 266 nm, but we do investigate this wavelength region and others at higher energy with doubled dye laser radiation, as described below. Multiphoton ionization has been documented for many cluster systems to produce significant fragmentation of the neutral cluster distribution. When this occurs, the “magic number” species which are detected may represent those clusters which are abundant as neutrals in the nascent cluster distribution or those which are stable as cations and resistant to further fragmentation. Photoionization can provide information about the neutral cluster distribution if ionization is performed near-threshold to limit the excess energy which causes fragmentation. Therefore, the ionization experiments at 532 and 355 nm do establish that neutral metal/carbon clusters exist in the molecular beam, but they do not necessarily measure the nascent distribution of neutral clusters. Specifically, we are interested in the M8C12 clusters to determine whether they exhibit enhanced stability relative to other clusters as neutrals. Multiphoton ionization

Figure 2. The photoionization mass spectra measured for Ti8C12 at 215 nm and three different laser powers. The lowest power shown approaches our minimum limit for detection of the signal. Many clusters are observed, but the “met-car” stoichiometry is not superabundant. The relative intensities of peaks in this distribution are unchanged with laser power, indicating that fragmentation of the distribution is minimal. The laser power dependence shows that this is the result of one-photon ionization.

does not provide this information, and so we examine higher energy radiation, where near-threshold single-photon ionization may be possible, to investigate the neutral distribution. Figure 1 shows the mass distributions observed for Ti/C clusters at 215 nm (laser power, 365 µJ/cm2). As shown, there are many metal/carbon cluster masses detected in the region from just below 200 amu up to and beyond the mass of the 8/12 stoichiometry (528 amu). Figure 2 shows the laser power dependence of this distribution. The three frames of the figure show the distribution at laser powers (365, 44, and 5 µJ/cm2), extending eventually to a level at the limit of signal detection. While all of the cluster masses observed under these three laser powers are reduced in intensity as the laser power is reduced, the relatiVe intensities of these peaks are approximately constant. This suggests that the distribution of clusters is not fragmented significantly by multiphoton absorption under these conditions.

5656 J. Phys. Chem., Vol. 100, No. 14, 1996 The intensities of the prominent peaks measured here vary linearly with the laser power. Therefore, we conclude that the clusters represented by these mass peaks have ionization potentials lower than the laser photon energy of 5.76 eV. The masses represented in Figure 1 correspond to various Ti/C stoichiometries. The lowest mass peak is Ti+, which shows a nonlinear power dependence and is therefore the result of a small amount of multiphoton ionization. In order to discuss the other mass peaks, it is important to note that the mass of the primary isotope of titanium is 48 amu, which is the same as a C4 increment. We therefore use 13C isotopic substitution to assign the stoichiometry of these Ti/C mass peaks. The first peak detected in all three 12C spectra is mass 168, which corresponds to the stoichiometry Ti3C2. The next peak, which is significantly large in all spectra, is mass 180, which is assigned as 3/3. Beginning with these masses, there is a peak at every increment of 12 amu all the way up to about mass 636, where the distribution gradually dies out in intensity. Two features of the distribution are noticeable. There is a drop in intensity at or about mass 528, which is the mass of the Ti8C12 met-car cage cluster. Second, there is a series of peaks larger than their neighbors at 180, 228, 276, 324, 372, 420, and 492 amu. In the lower masses, these peaks are spaced by 48 amu, suggesting titanium increments. The stoichiometries are respectively 3/3, 4/3, 5/3, 6/3, 7/3, and 6/7, 7/7, and 8/9. The 528 amu peak has the correct isotope shift for the 8/12 met-car stoichiometry. It is tempting to associate these intense mass peaks with clusters having an enhanced abundance in the molecular beam. However, there are two factors which determine these intensities: (1) the density of the respective clusters in the molecular beam and (2) the ionization cross sections of these clusters. Therefore, cluster masses which are ionized in this fashion exist in the molecular beam with reasonable concentration, and they have ionization potentials lower than the laser energy. However, there may also be other clusters with equal or greater density in the molecular beam which are not detected because they have higher ionization potentials. It is therefore not appropriate to try to interpret, for example, a formation mechanism for the met-car cluster from the large mass peaks indicated here, because other abundant masses may not be detected. The prominent masses may be just those species which have the lowest ionization potentials. It is therefore not surprising that the stoichiometries for the prominent clusters listed at low mass above are those rich in metal; carbon-rich molecules would have higher ionization potentials, on average. Likewise, the clusters with masses less than 168 amu, which are not detected, are either not present in high abundance or have high ionization potentials. Higher ionization potentials are expected on average for clusters with fewer atoms. The relationship between peak intensity and cluster abundance is especially important for consideration of the 8/12 met-car cluster. In the present distribution, there is always a noticeable truncation in intensity after this mass, but this peak does not appear with enhanced intensity relative to other clusters. This may be because it is not abundant in the neutral distribution or it may be that it has a high ionization potential, so that it is not ionized efficiently here. On the basis of the mass distribution alone, it is not possible to tell which is true. Therefore, we attempt the measurement of ionization potentials, as described below, to further investigate this issue. Figure 3 shows other mass spectra measured for vanadium/ carbon and niobium/carbon cluster distributions at 215 nm. These mass spectra appear sharper in resolution because both of these metals have only one naturally occurring isotope. Again, several M/C masses are observed in the region below

Brock and Duncan

Figure 3. Photoionization mass spectra for V8C12 and Nb8C12 clusters at 215 nm. The laser power dependence studies show that these spectra are the result of multiphoton absorption of at least two photons.

the 8/12 met-car stoichiometry. In the V/C data, the smaller masses are quite weak in intensity until those masses just prior to the 8/12 mass peak, which are larger. For Nb/C, several smaller masses are prominent, and there is a sharp truncation after the 8/12 peak. Of the lower masses, the 4/4 peak was also prominent in the distribution of cation clusters measured earlier by several groups.20 This cluster has been suggested to have the 2 × 2 × 2 cubic structure. However, other Nb/C clusters which were prominent in the nascent cation distribution,20 such as 6/7, 7/9, and 9/9, are not especially noticeable here. We have also measured photoionization spectra for Zr/C at this same wavelength. This distribution appears similar to the V/C distribution, except that the mass resolution is much worse due to the Zr isotope distribution. In all three of these systems, photoionization at 215 nm produces mass spectra in which the 8/12 met-car stoichiometry is prominent. However, variation of the laser power shows that these distributions in all three cases are quite sensitive to the laser power. Figure 4 shows the power dependence of the photoionization at 215 nm for the Zr8C12 and V8C12 clusters. In both systems, the photoionization was studied down into the µJ/pulse‚cm2 energy domain, where the signal is at or near our detection limit. At these low powers, the limiting slope of the log-log plots are greater than two, indicating that these ionization processes are the result of at least two-photon absorption. Similar power dependence plots are obtained for the clusters Nb4C4 and Nb8C12, and the exponents observed from these plots are shown in Table 1. If these measurements are extended to higher laser powers (in the range of mJ/pulse‚m2), the plots bend over and become flatter (slopes less than 2) due to saturation effects. It is therefore important to extend these measurements to extremely low laser powers in order to observe the limiting behavior. These laser power studies indicate that the clusters Nb4C4, V8C12, Zr8C12, and Nb8C12 have ionization potentials in excess of the 5.76 eV laser energy at 215 nm. We are not able to follow the ionization behavior of these clusters to higher laser energies because of the low efficiencies of both the laser dyes and the frequency doubling in BBO.

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Figure 4. Study of laser power dependence for the M8C12 clusters of zirconium and vanadium at 215 nm. Both data sets indicate that ionization requires at least two photons.

TABLE 1: Results of Laser Power Dependence Studies at 215 nma cluster

N

cluster

N

Nb4C4 Ti8C12 V8C12

2.39 1.15 2.26

Zr8C12 Nb8C12

2.47 2.62

a

The measured ion signal, I, for the cluster MxCy is fit to the linear expression log(I) ) K log(N), and the exponent N indicates the apparent photon order of the ionization.

At 215 nm, the Ti8C12 cluster is the only one with the 8/12 met-car stoichiometry which has a laser power dependence near unity. We have therefore investigated the ionization of this cluster at lower energies to attempt to measure its ionization potential. As we scan the ionization laser to lower energy, we do not ever under any conditions see a sharp ionization threshold where the ionization efficiency undergoes a noticeable change. Instead, the 8/12 cluster is ionized with about the same efficiency throughout the region of 4.5-5.7 eV. Presumably, there is a high density of excited electronic states so that the absorption probability is strong throughout this region. Thus, we cannot obtain a “sharp” measurement of the ionization energy. We then measure the laser power dependence throughout this region. The power dependence is approximately linear at 215 nm (5.76 eV), which is consistent with a one-photon process, as noted above. As we proceed to lower energies, the power dependence gradually changes over to quadratic, which suggests a twophoton process. Figure 5 shows the laser power dependence studies in the region near 4.9 eV, where the log-log plots show a gradual change from linear to quadratic. Since this appears to be a critical wavelength region, we have measured the laser power dependence here several times on different days with different cluster source conditions. We have worried in particular about the internal temperature of the clusters, because hot clusters are often characterized by ill-defined ionization thresholds. We have therefore used extended growth channels on the cluster source to attempt further cooling of the clusters. Experiments at 1, 2, and 3 cm channel lengths likewise never produce any sharp threshold. Instead, all these experiments are consistent in measuring a “transition” region, where the ionization efficiency changes from linear to quadratic, at 4.9 ( 0.2 eV. We therefore conclude that this transition point best

Figure 5. Power dependence studies of Ti8C12 photoionization in the limit of low laser power. The photon order changes over gradually from a two-photon process at lower energy to a one-photon process at higher energy.

represents the threshold for one-photon ionization, and therefore this is our assignment for the ionization potential for Ti8C12. The ionization potential determined this way is subject to qualifications. First of all, this is a Vertical IP and not necessarily the adiabatic IP. As always, experimental determinations of IP’s are subject to Franck-Condon considerations, and the vertical value measured is greater than or equal to the adiabatic value. The adiabatic value is that which is usually predicted by theory. Additionally, the IP may be affected by the internal temperature of the clusters. There is good reason to suspect that clusters such as these, with extremely strong covalent bonding, will be heated significantly in the growth process. The internal energy generated may or may not be cooled effectively by the entrainment gas collisions and/or supersonic expansion in the cluster source. If clusters are internally hot, the IP measured may be lower than the actual value. However, as noted above, we have investigated different source configurations (longer growth channels) which would be expected to produce additional cooling of the clusters, and these adjustments had no effect on the measured IP. We conclude either that the clusters are not internally hot or that the internal energy does not affect the ionization threshold. Table 2 shows the comparison of this ionization potential measurement to the values calculated by theory. As shown in the table, there have been a variety of calculations and a corresponding variation in IP predictions. In some of the early theory work, the Ti8C12 cluster was assumed to have Th symmetry, and this structure was used in the IP calculation. The resulting numbers (6.0-6.9 eV) tend to be a good deal higher than our measured value. In later theory investigations, other symmetries have been explored, and different groups have arrived at different conclusions about the structure. The D2d and Td structures are now preferred by most theory groups. Of these preferred structures, the Td is predicted to have a lower ionization potential. In fact, the calculated values of 4.534b and 4.37 eV33e are both lower than our measured value. If there is a geometry change between the ion and the neutral cluster, an

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Brock and Duncan

TABLE 2: MxCy Ionization Potentials Determined Here and the Comparison to Theorya cluster Nb4C4 Ti8C12

V8C12 a

exp (eV)

theory (eV)

>5.76 4.9 ( 0.2 6.0 (Th)26 6.82 (Td)28 5.33 (Th)30 5.8 (D2d)34b 5.4 (Th)34b 4.5 (Td)34b 4.37 (Td)33d >5.76 6.74 (Td)28

cluster

exp (eV) theory (eV)

Zr8C12

>5.76

Nb8C12 Ti14C13

>5.76

Nb14C13

5.92 (Th)30 5.53 (Td)33e 4.89 (Th)30 3.99 (Td)33e 5.840 4.4133d 3.9333e

The calculated cluster symmetry is shown in parentheses.

adiabatic value of 4.4-4.5 eV could conceivably give rise to a vertical IP near 4.9 eV. The D2d structure is predicted to have an IP of 5.8 eV.34b For this to be consistent with our measured value, there would have to be a significant red shift in the measured IP, such as would occur if the clusters were extremely hot internally. At the higher levels of theory pursued more recently, the Th structure is also calculated to have a low IP of 5.4 eV.34b However, it may be misleading to compare these IP’s from experiment and theory at this level of detail. We have estimated the errors involved in the experiment, but the error bars on the theory numbers are difficult to quantify. It is especially discouraging to note that theory does not predict the trends in IP’s for different metals correctly. While our IP threshold is not as sharp as we would like, it is clear from our powerdependent studies that the IP's of the vanadium, zirconium, and niobium met-car clusters are significantly greater than that of the titanium met-car. There are a limited number of investigations which have examined different met-car systems at the same level of theory, but there is no study which predicts this IP trend properly. Reference 28 predicts that the IP of the vanadium met-car is less than that of titanium, rather than greater. References 30 and 33e correctly predict that the vanadium metcar IP is greater than that of titanium, but both of these predict that the zirconium met-car IP is lowest of all. There is thus considerable cause for concern when the theoretical IP values are considered. It would be useful if the various theoretical methods employed in different groups could be analyzed to quantify their systematic errors. These ionization potential data provide some important insights into the nature of the cluster distributions shown in Figures 1-3. First of all, and perhaps most significant, the Ti8C12 met-car cluster is not present with enhanced abundance in Figure 1 or 2. We mentioned earlier that this could be because of either a low abundance or a high IP. Now that the IP is measured, the simplest remaining explanation for the low abundance of this peak is that this cluster is not present as a superabundant neutral. This suggestion is extremely surprising because of the superabundance of this cluster as a cation in previous experiments. The V/C, Zr/C, and Nb/C photoionization spectra have prominent 8/12 masses, but these spectra are the result of multiphoton ionization. The prominence of the 8/12 mass in all cases is greater as the laser power is increased, consistent with the production of this peak by fragmentation of larger clusters. Another interesting point about these ionization experiments is the absence of the M14C13 cluster mass in any of our spectra. This cluster stoichiometry is extremely prominent in the distribution of cation clusters, and its photodissociation has been discussed previously by our group.17-19 However, it is missing completely in the neutral photoionization data. To check that our cluster source conditions were as before, we have switched

back and forth in the same experiment between cation and neutral detection. Under the exact same source conditions which produce extremely large 14/13 cation masses, there is no 14/13 signal detected by neutral photoionization. Again, it is curious that this should occur, but it is not possible to say exactly why. It could be that this cluster has an IP greater than the photon energy at 215 nm (5.76 eV). Reference 40 calculates an IP of 5.8 eV for the Ti14C13 cluster, but ref 33e calculates an IP of 4.41 eV. On the other hand, it may also be that this cluster is not formed from the cluster source as an abundant neutral. Even if it had a high IP, we might expect that an abundant cluster could be detected by multiphoton ionization, but it is apparently not. The absence of large neutral concentrations of Ti8C12 and Ti14C13 (as well as other M14C13) is the most surprising observations in this study. Both of these clusters are extremely abundant in the corresponding nascent cation distributions, and both are also produced as abundant photofragment cations from larger clusters.17-19 Therefore, the enhanced stability of these species as cations is well established. The cluster source and the gas concentrations which produce the neutral distribution here are identical to those which produce the cation distribution. Indeed, we are able to switch back and forth between the two instrument configurations within a few minutes. It is conceivable that the growth kinetics for the neutral clusters is somehow different from that for the cations, leading to these different distributions of magic numbers. Unfortunately, this effect cannot be measured. It is also conceivable that both of these clusters have enhanced stability as cations, but not as neutrals. The only experimental fact directly supporting enhanced neutral stability is the truncation which occurs after the Ti8C12 mass peak in the low-power near-threshold photoionization data. On the other hand, the low Ti8C12 ionization potential and the relatively low abundance in the total distribution argue against a Ti8C12 neutral with exceptional stability. Every other experimental fact measured previously by our group and others for the Ti8C12 cluster is also understandable if it is the cation rather than the neutral which has enhanced stability. The present data here do not prove definitively either the cation or neutral stability, since thermochemistry is not directly measured. However, it does raise this issue for the titanium system, where the magic numbers observed in previous studies are perhaps the most prominent. The data for other M8C12 (i.e., ionization potentials too high to measure) are inconclusive on this point. However, no M14C13 clusters are observed by photoionization for any of these metals. It is difficult to understand how there could be a significant difference in the stability of clusters in this size range which differ only in charge state. The symmetric geometric structures, of course, are available to both charge states. To explain a difference in relative stability, there would have to be a significant electronic effect in the addition or subtraction of one electron. Such an effect is not evident, for example, in C60 clusters, where neutral, positive, and negative ions are all superabundant. However, in other ionic systems, dramatic electronic structure effects have been noted, and ionic bonding is believed to be important in all these metal-carbon systems. Other facts are also consistent with an electronic structure effect. For example, attempts in other labs to produce anions of the Ti8C12 clusters for photoelectron spectroscopy have failed; the 8/12 anion is also not superabundant.41 In the nanocrystal systems, it is always the M14C13 cluster which is detected, rather than M13C14 which would have the same geometric stability. On the other hand, one of the only theoretical calculations to compare both neutral and ionic clusters has examined the Ti/C

Neutral “Met-Car” Clusters stoichiometries of 8/12 and 14/13.33d In both systems, the neutral cluster is calculated to have a slightly greater binding energy than the corresponding cation, in apparent contradiction to our observations. However, it is the stability relatiVe to other larger and smaller clusters with the same charge state which makes a particular system stand out in a cluster distribution, and calculations have not been done, in general, on other clusters with similar sizes. If sharp electronic structure effects are in operation in the bonding of these metal-carbon clusters, their outcomes will of course vary across the periodic table of metals. The comments above apply specifically to titanium, because this is the only M8C12 system for which we are able to measure the ionization potential. To determine the extent of such effects in other metal systems will require photoionization at higher photon energies into the 6-7 eV range. Such wavelengths are available with more advanced optical technology (such as the Berkeley Advanced Light Source), but they are not presently available in our laboratory. On the theory side, these results can serve to focus efforts on more reliable calculations of measurable quantites like the ionization potential and to investigate the role of cluster charge state on stability. In theoretical investigations to date, there have been only limited reports of comparative stabilities for clusters with different charge states. Finally, a study of ionization potentials for several M8C12 and M14C13 systems could identify or rule out unusual kinetic effects in cluster growth. If there is some kinetic growth effect in the titanium system, for example, which reduces the apparent abundance of the neutral 8/12 cluster, chances are that such an effect would not always occur for other M8C12 systems. A final possibility for the “missing” magic number neutrals is an anomalous ionization mechanism. Castleman and coworkers4b have recently shown that Ti8C12 exhibits time-delayed “thermionic emission” under multiphoton ionization conditions. For the near-threshold ionization here, the low excess energy might lead to a pronounced time delay effect, which could consequently result in a low intensity in the corresponding mass channel. The ionization behavior of these clusters is quite unusual, and further study is clearly warranted on the relative stability of cations versus neutrals. Acknowledgment. We gratefully acknowledge support for this work from the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences (Contract DEFG05-93ER14402) and the Air Force Office of Scientific Research (Grant F49620-94-1-0063). References and Notes (1) Guo, B. C.; Kearns, K. P.; Castleman, A. W., Jr. Science 1992, 255, 1411. (2) Guo, B. C.; Wei, S.; Purnell, J.; Buzza, S.; Castleman, A. W., Jr. Science 1992, 256, 515. (3) (a) Wei, S.; Guo, B. C.; Purnell, J.; Buzza, S.; Castleman, A. W., Jr. Science 1992, 256, 818. (b) Wei, S.; Castleman, A. W., Jr. Chem. Phys. Lett. 1994, 227, 305. (4) (a) Wei, S.; Guo, B. C.; Purnell, J.; Buzza, S.; Castleman, A. W., Jr. J. Phys. Chem. 1992, 96, 4166. (b) May, B. D.; Cartier, S. F.; Castleman, A. W., Jr. Chem. Phys. Lett. 1995, 242, 265. (5) Guo, B. C.; Kerns, K. P.; Castleman, A. W., Jr. J. Am. Chem. Soc. 1993, 115, 7415.

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