Shock wave and modelling study of the unimolecular dissociation of Si(CH)F: an access to spectroscopic and kinetic properties of SiF.

The thermal dissociation of Si(CH)F was studied in shock waves between 1400 and 1900 K. UV absorption-time profiles of its dissociation products SiF and CH were monitored. The reaction proceeds as a unimolecular process not far from the high-pressure limit. Comparing modelled and experimental results, an asymmetric representation of the falloff curves was shown to be most realistic. Modelled limiting high-pressure rate constants agreed well with the experimental data. The UV absorption spectrum of SiF was shown to be quasi-continuous, with a maximum near 222 nm and a wavelength-integrated absorption cross section of 4.3 (±1) × 10 cm (between 195 and 255 nm, base e), the latter being consistent with radiative lifetimes from the literature. Experiments over the range 1900-3200 K showed that SiF was not consumed by a simple bond fission SiF →SiF + F, but by a bimolecular reaction SiF + SiF → SiF + SiF (rate constant in the range 10-10 cm mol s), followed by the unimolecular dissociation SiF → SiF + F such that the reaction becomes catalyzed by the reactant SiF. The analogy to a pathway CF + CF → CF + CF, followed by CF → CF + F, in high-temperature fluorocarbon chemistry is stressed. Besides the high-temperature absorption cross sections of SiF, analogous data for SiF are also reported.