Ion beam studies of the reaction of Si sup + ( sup 2 P) with methane reaction mechanisms and thermochemistry of SiCH sub x sup + (x = 1-3)

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{sup +} and SiH{sub 3}C{sup +} as the major ionic products. There is evidence that the latter species has two forms, Si{sup +}-CH{sub 3} formed at low energies and a higher energy form that could be a triplet state of SiCH{sub 3}{sup +} or HSiCH{sub 2}{sup +}. Minor ionic products include SiCH{sub 2}{sup +}, CH{sub 3}{sup +}, and SiCH{sup +}. 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 HSiCH{sub 3}{sup +} 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: {Delta}{sub f}H{degree}(SiH) = 91.4 {plus minus} 1.8, {Delta}{sub f}H{degree} (SiCH{sup +}) = 339 {plus minus} 7, {Delta}{sub f}H{degree}(SiCH{sub 2}{sup +}) = 285 {plus minus} 3, and {Delta}{sub f}H{degree} (SiCH{sub 3}{sup +}) = 235 {plus minus} 5.