Shock wave and modelling study of the unimolecular dissociation of Si(CH 3 ) 2 F 2 : an access to spectroscopic and kinetic properties of SiF 2

Abstract

Falloff curves of the unimolecular dissociation of Si(CH 3 ) 2 F 2 are determined in shock waves and compared to modelling results. High-temperature UV absorption cross sections of SiF 2 are also derived.

The thermal dissociation of Si(CH 3 ) 2 F 2 was studied in shock waves between 1400 and 1900 K. UV absorption-time profiles of its dissociation products SiF 2 and CH 3 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 2 was shown to be quasi-continuous, with a maximum near 222 nm and a wavelength-integrated absorption cross section of 4.3 (±1) × 10 −23 cm 3 (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 2 was not consumed by a simple bond fission SiF 2 →SiF + F, but by a bimolecular reaction SiF 2 + SiF 2 → SiF + SiF 3 (rate constant in the range 10 11 –10 12 cm 3 mol −1 s −1 ), followed by the unimolecular dissociation SiF 3 → SiF 2 + F such that the reaction becomes catalyzed by the reactant SiF 2 . The analogy to a pathway CF 2 + CF 2 → CF + CF 3 , followed by CF 3 → CF 2 + F, in high-temperature fluorocarbon chemistry is stressed. Besides the high-temperature absorption cross sections of SiF 2 , analogous data for SiF are also reported.

Falloff curves of the unimolecular dissociation of Si(CH 3 ) 2 F 2 are determined in shock waves and compared to modelling results. High-temperature UV absorption cross sections of SiF 2 are also derived.

The thermal dissociation of Si(CH 3 ) 2 F 2 was studied in shock waves between 1400 and 1900 K. UV absorption-time profiles of its dissociation products SiF 2 and CH 3 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 2 was shown to be quasi-continuous, with a maximum near 222 nm and a wavelength-integrated absorption cross section of 4.3 (±1) × 10 −23 cm 3 (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 2 was not consumed by a simple bond fission SiF 2 →SiF + F, but by a bimolecular reaction SiF 2 + SiF 2 → SiF + SiF 3 (rate constant in the range 10 11 –10 12 cm 3 mol −1 s −1 ), followed by the unimolecular dissociation SiF 3 → SiF 2 + F such that the reaction becomes catalyzed by the reactant SiF 2 . The analogy to a pathway CF 2 + CF 2 → CF + CF 3 , followed by CF 3 → CF 2 + F, in high-temperature fluorocarbon chemistry is stressed. Besides the high-temperature absorption cross sections of SiF 2 , analogous data for SiF are also reported.