1979
J. Am. Chem. SOC.1989, 111, 1979-1986 a IO-ms pulse. This could perhaps be understood if the bubbles generated by a 2-ms pulse are much smaller than those resulting from a 10-ms pulse. They dissolve more rapidly because of their greater internal pressure 2u/r ( u , surface energy; r, radius). The new phenomenon is discussed now which was observed in the experiments of Figure 7a when the on/off ratio was 1:500. Under these circumstances the pulses after the initial one became rapidly less efficient. The effect can be understood if one postulates that the fresh solution contained "natural" nuclei, X,, which are particularly efficient to promote chemically active cavitation. These nuclei are destroyed to a large extent by the first pulse. The pulse itself produces nuclei, Xu, which are, however, short-lived. Their lifetime is long enough, i.e. >1 s, as pointed out above, so that trains with an interval time 4 ps) to reach the mass spectrometer and be detected because of the large number of internal degrees of freedom in the cluster. Besides the Si25D2+adduct no other products were observed. Owing to the large laboratory energies employed in these studies (because D2 is so light), the lighter products, if any are formed, will be discriminated against. At the highest center-of-mass collision energy studied (3 eV), products lighter than Silof would not be detectable. Furthermore, because of the isotope distribution, it is difficult to detect small amounts of products very close in mass to the reactant ions. So we would not be able to detect a small amount of Si25Df if it were formed. Under the conditions employed to record the data shown in Figure 1, the Si2,D2+ product peak sits on the edge of the much larger Si,,+ peak in the mass spectrum. As a consequence the threshold region (where the Si25D2f signal is small) is quite noisy. As can be seen from Figure 1 the cross sections for Si25D2+ formation show an apparent threshold a t collision energies around 1.5 eV and then rise to around 2.5 A2 a t a collision energy of 3 eV. The shape of the cross-section plot for Si25' is remarkably similar to that for AI25' shown in the lower half of Figure 1, though the cross sections for AI25' are around four times larger. A possible explanation for the smaller cross sections observed for Si,,+ is that steric constraints (the orientation of the D, with respect to the cluster and the site where the D2 strikes the cluster surface) are more severe for Si25' than for Al7.5'. The lines drawn through the data in Figure 1 are the results of a simulation to deduce the true collision energy thresholds. The thresholds and other parameters used to model the experimental data are summarized in Table I and the thresholds for Si25+ and AIz5+are compared in Table 11. The collision energy threshold for adduct formation can be related to the activation barriers for chemisorption. Clearly the activation barriers for chemisorption of D2 on Si25' and Al25' are similar. In their study of the reactions of Si2++ using FT-ICR, Creasy and co-workersI2 found no reaction with D2. It is not clear if this result indicates that there is a significant activation barrier for chemisorption of D2 on Si,-,+. B. Si25++ CH4. Si2*+reacts with C H 4 to give three main products: the Si2&H4+adduct, Si15CH4+,and Sil5+. Owing to the isotope distribution it was not possible to resolve product ion masses separated by 1 amu, so there could be small components of other masses present. For example, the Si15CH4+peak could contain a small component of Si15CH3+. Figure 2 shows a plot of the relative intensities of the products against the gas cell pressure. The data shown in Figure 2 were recorded with a collision energy of 6 eV. All the products show a nonlinear pressure dependence. The intensity of the Si25CH4+product begins to level off a t the higher pressures studied. This probably arises
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Figure 2. Plot of the gas cell pressure dependence of the intensities of the main products from the reaction between Sizs+and CH,. The collision energy was 6 eV. The dashed lines drawn through the data are only guides.
from dissociation of the adduct caused by collisional activation in secondary collisions. The intensities of the other two products show a faster than linear increase with pressure. The Sil5+product shows a close to quadratic dependence on pressure. Sil5+is the dominant product from the photodissociation'6 of Si25' and from the collision-induced d i s s ~ i a t i o nof~Si,,+ ~ by argon, and probably arises mainly from collision-induced dissociation here as well. The close to quadratic pressure dependence observed for this product suggests that multiple collisions are required to cause dissociation of Si25' even a t a collision energy of 6 eV. This result does not imply that the dissociation energy of Si25' is more than 6 eV because a substantial amount of excess energy is required to cause such a large cluster to dissociate (in -7 ps) before reaching the mass spectrometer. The SilSCH4+product probably arises from dissociation of the Si25CH4+adduct. Apparently the dominant dissociation channel of both Si25' and Si2,CH4+ is loss of Silo. Though multiple collision processes are clearly important in the 0.2-1 -mTorr pressure regime discussed above, it should be noted that, when the gas cell length (2.5 cm) is taken into account, these pressures correspond to target gas densities of only 1.7-8.8 X lOI3 molecules/cm2. Clearly the cross sections for collisional activation are large; we estimate 10-50 A2, which is much larger than the reaction cross sections. In our previous work4 on we came to the same conclusion. The upper half of Figure 3 shows a plot of the cross sections for the products from the reaction between Si25' and CH4against collision energy, recorded with a gas cell pressure of 0.5 mTorr. The Siz5CH4+adduct appears to have the lowest energy threshold. The threshold for SiI5CH4+is around 1 eV higher in energy. The lines drawn through the experimental data are the results of simulations to deduce the true collision energy thresholds. The measured thresholds are not significantly influenced by collisional activation in secondary collisions. It is apparent from Figure 3 and Table I1 that the thresholds for formation of SizSCH4+and A125CH4+are quite similar. However, Si25CH4ffragments by loss of Sil, to give Sil5CH;, but A12,CH4+ fragments by loss of A1H or A1CH3 to give AlZ4CH3+and A124H+,respectively. C. Si25+ + 02.In the reaction between Si25' and 0, three main products were found: Si23+, Sil6+,and Sil3+. No Si2502+adduct was observed. Figure 4 shows a plot of the cross sections for the products against collision energy. Note that this is an efficient reaction and the cross sections are quite large. At a collision energy
-
(16) Zhang, Q.-L.; Liu, Y . ;Curl, R. F.; Tittel, F. K.; Smalley, R. E. J . Chem. Phys. 1988, 88, 1670.
1982 J . Am. Chem. SOC.,Vol. I l l , No. 6,1989
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Figure 3. Plots of the cross sections against collision energy for the products observed in the reactions between Si25' and CH, (upper), and
between Al25' and CHI (lower). The gas cell pressure was 0.5 mTorr for the Si25' data and 0.25 mTorr for A125'. The points are the experimental data and the lines are the results of simulations to deduce the true collision energy thresholds. The data for were taken from ref 4.
of 3 eV all three products showed a close-to-linear variation with gas cell pressure, up to a pressure of 1.O mTorr. The reactions between silicon cluster ions with up to six atoms and 0, have been studied by Creasy, O'Keefe, and McDonald using FT-ICR.I2 The main product they found was loss of two silicon atoms to give Siw2'. The neutral products are presumably either two S i 0 molecules or an SiZO2species. The main product found for Si25' is also Siw2+.Sil6+and Sil3+are the main products from the collision-induced dissociation of Si23' by argon.15 So the Sil6+ and Sil3+products probably arise from further dissociation of the Si23' product. However, since the products all show a close-tolinear gas cell pressure dependence, the Sil6+and Sil3' products, in this case, probably arise mainly from further fragmentation of Si23' excited in the reaction with oxygen, rather than from collisional activation. There are obvious similarities between the Si25' + O2and AI,,+ O2 reactions. With AI,,+ the main product is AI2]+,arising from loss of two AI20 molecules. At higher collision energies some AI2,+ arises from further fragmentation of the product. As can be seen from Figure 4 the AI25' + O2 reaction occurs with greater probability than the Si25+ + O2reaction. But the collision energy thresholds for the reactions are quite similar. The thresholds deduced from the simulations of the experimental data (see Table I) are shown in Table 11. It appears that the Si25+ reaction may have a slightly lower threshold. In the simulation of the Si25' data only the main product (Si23') was modeled. We were unable to obtain a satisfactory fit to the total reaction cross sections; this might be considered evidence that the other products (Sil6+and Si,,+) arise from a mechanism different from simple fragmentation of the Si23+ product as suggested above. In their FT-ICR study of the reactions of Si2+,+with O2Creasy and co-workers12 found that the reaction rates were slow and decreased with cluster size. Noting that the reactions were very
0
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Figure 4. Plots of the cross sections against collision energy for the products observed in the reactions between Si25' and O2 (upper), and between and O2 (lower). The gas cell pressure was 0.5 mTorr for the Si25' data and 0.25 mTorr for The points are the experimental data and the lines are the result of simulations to deduce the true collision energy thresholds. The data for were taken from ref 4.
exothermic (by -2-3 eV if the products are two Si0 molecules and 1.93 eV" more exothermic if the product is Si202),they suggested that there must be an activation barrier associated with 0-0 bond breaking and Si-0 bond formation. There is no information available for the binding energy of a Si atom to Si25'. however, even using the bulk cohesive energy,I8J9the reaction of Si25' with oxygen is exothermic by at least 2 eV. So it is reasonable to associate the small activation barrier we found for the reaction of Si25' with O2with chemisorption of Oz onto the Si25+ cluster. Once the O2 is dissociatively chemisorbed, the Si2SO2' species dissociates to give the observed products. The overall reaction is presumably sufficiently exothermic that the Si2SO2' adduct dissociates before it reaches the mass spectrometer (indicating a lifetime
