Reactions with Silane SiH

The difference in the behavior of these four ions is easily rationalized. The ionization energies (lEs) of He and Ne fall in a region where resonant ionization preferentially removes an electron from the 3ai orbital of SiH4. Photoionization studies indicate that ionization from this orbital yields Si+ and SiH products almost exclusively (Cooper et al., 1990). In contrast, the lEs of Kr and Xe are resonant with the lower-energy 2t2 orbital of SiH4. Photoionization from this orbital results in less dissociation, preferentially forming SiHj and SiH. Thus, the dominant products observed are consistent with the photoionization results for removal of an electron from the two distinct valence molecular orbitals of silane. 

The variation of product cross sections with translational energy in the laboratory frame (upper scale) and the center-of-mass frame (lower scale) for the reaction of Ar+( P) (in a statistical distribution of spin-orbit states) with SiH4. The solid line shows the total cross section. The dashed line shows the collision cross section divided by 10, given by the maximum of either the ion-induced dipole (LGS) or the hard sphere cross section. The arrow indicates the energy level (2.2 eV) where the 3a, state of SiH4 becomes accessible. Reprinted with permission from Fisher and Armentrout (1990b). Copyright 1990, American Institute of Physics. 

Other products observed in these systems include SiOH+ (a major product), SiO , SiOH, and SiOH in the reactions of 0+ with silane. Reactions of OJ with silane formed these same products, and again SiOH+ was produced in good yield. The OJ reaction also yields Si02H+ ( = 0 to 3), where both SiOj and Si02H+ have sizable cross sections. Atomic N+ reacts with silane to form SiNH+ (n = 0 to 2), where both SiNH+ and SiNHj are abundant products. In all cases, these SiOH,, Si02H,, and SiNH+ product ions are formed in exothermic reactions, so that only limits on the thermochemistry of these species can be determined. 

At higher energies, several additional reaction pathways open. These are shown in reactions (6) to (11). These reactions are endothermic in all cases, and their cross sections can be analyzed to provide thermodynamic information regarding the products. A particularly interesting aspect of reactions (6) to (8) is that both ionic and radical silicon hydrides are formed such that coupled information about the cations and neutrals can be obtained from these data. This is discussed in more detail in Section III.E. 

Thermochemistry for silicon hydride species is derived from either atomic silicon or silane reactions. However, Grev and Schaefer (1992) have found that the 

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