ION-BEAM STUDIES OF THE REACTION OF SI+(2P) WITH METHANE - REACTION-MECHANISMS AND THERMOCHEMISTRY OF SICH1-3+

Guided ion beam mass spectrometry is used to examine the reaction of ground-state silicon ion with methane. Absolute cross sections of all products are measured from near-thermal to 14-eV relative kinetic energy. Only endothermic processes are observed with SiH+ and SiH3C+ as the major ionic products. There is evidence that the latter species has two forms, Si+-CH3 formed at low energies and a higher energy form that could be a triplet state of SiCH3+ or HSiCH2+. Minor ionic products include SiCH2+, CH3+, and SiCH+. The former product can be formed via the concomitant formation of molecular hydrogen or two hydrogen atoms. The latter process is much more efficient. All observed products are consistent with a reaction that occurs via an HSiCH3+ intermediate. From the measured thresholds of the reactions and other information, the 298 K heats of formation (kcal/mol) for the following silicon species are derived: ΔfH°(SiH) = 91.4 ± 1.8, ΔfH°(SiCH+) = 339 ± 7, ΔfH°(SiCH2+) = 285 ± 3, and ΔfH°(SiCH3+) = 235 ± 5. © 1990, American Chemical Society. All rights reserved.