Energetics and mechanisms in the reaction of silicon(1+) with silicon tetrachloride. Thermochemistry of silicon monochloride, silicon chloride (SiCl1+), and silicon chloride (SiCl21+)

Abstract

The title reaction is studied by guided ion beam mass spectrometry. Absolute reaction cross sections are measured as a function of kinetic energy from thermal to 20 eV. Reaction occurs on virtually every collision at low energies. Production of SiCl{sup +} + SiCl{sub 3} is exothermic and is the dominant process. SiCl{sub 3}{sup +} + SiCl and SiCl{sub 2}{sup +} + SiCl{sub 2} production are both seen to be slightly endothermic. Also observed are the dissociative channels SiCl{sup +} + Cl + SiCl{sub 2} and SiCl{sub 2}{sup +} + Cl + SiCl. No Si{sub 2}Cl{sub x}{sup +} species are observed. Isotopic labeling studies indicate that SiCl{sup +} and SiCl{sub 3}{sup +} are produced by direct, coupled mechanisms, while SiCl{sub 2}{sup +} is formed through an intimate collision involving a symmetric intermediate. These mechanisms are interpreted in terms of the molecular orbital correlations. The cross section behavior and proposed mechanisms are consistent with those of a previous study of the Si{sup +} + SiF{sub 4} reaction. Reaction thresholds are analyzed to derive the following thermochemical values: {Delta}H{sub f}{degree}{sub 298} (SiCl) = 44 {plus minus} 6 kcal/mol, {Delta}H{sub f}{degree}{sub 298} (SiCl{sup +}) = 217 {plus minus} 7 kcal/mol, IP (SiCl) = 7.44more » {plus minus} 0.40 eV, and {Delta}H{sub f}{degree}{sub 298} (SiCl{sub 2}{sup +}) = 188 {plus minus} 3 and 190 {plus minus} 6 kcal/mol. These values and those in the literature are compared and evaluated.« less