UV absorption spectrum and rate constant for self-reaction of silyl radicals

The rate constant of self-reaction of silyl radicals, SiH₃ + SiH₃ → products (1), was measured at 300 K over an extended buffer gas pressure range (1-100 bar, He) using excimer laser pulsed photolysis combined with the transient UV spectroscopy. Silyl radicals were produced in fast reaction of chlorine atoms with silane, Cl + SiH₄ → SiH₃ + HCl. Oxalyl chloride, (COCl)₂, and phosgene, COCl₂, were used as "clean" photodissociation sources of Cl atoms at 193 nm (ArF laser). Silyl radicals were monitored using UV absorption. The absorption cross-sections of SiH₃ radicals are determined using the measured photodepletion of the precursor, (COCl)₂, via its transient absorption at 210 nm. The UV absorption of silyl radical has maximum at 218 nm with the absorption cross-section, σ_SiH3(218 nm) = (2.01 ± 0.11) × 10^-17 cm² molecule^-1. No pressure dependence of the overall rate constant of reaction 1 was found over the range 1 - 100 bar (He). The measured rate constant is: k₁ = (8.25 ± 1.05) × 10^-11 cm³ molecule^-1 s^-1 at 300 K. Observed residual UV absorption, tentatively assigned to the dissociation products of the vibrationally excited Si₂H₆* molecules formed in reaction 1, is quenched by the buffer gas at 100 bar pressure due to the collisional stabilization of excited disilane molecules.