Pressure dependent photolysis quantum yields for CH3C(O)CH3 at 300 and 308 nm and at 298 and 228 K

Abstract

The quantum yield of formation of CH₃ and CH₃CO in the pulsed laser photo-excitation of acetone at 300 and 308 nm was investigated at several pressures (60 to 740 Torr) and at either 298 or 228 K. The organic radicals generated were monitored indirectly following conversion (by reaction with Br₂) to Br atoms, which were detected by resonance fluorescence. The photolysis of Cl₂ in back-to-back experiments at the same wavelength and under identical experimental conditions served as chemical actinometer. The pressure and temperature dependent quantum yields obtained with this method are in good agreement with previous literature values and are reproduced using the parameterisation developed by Blitz et al. The Br formation kinetics deviated from that expected from reactions of CH₃ and CH₃CO alone and Br atoms were still observed at high yield even when the quantum yield of formation of CH₃ and CH₃CO was low. This is explained by the reactive quenching of thermalized triplet acetone (T₁) by Br₂. High yields of T₁ (>80%) at the highest pressure in this study indicate that any dissociation from the first excited singlet state (S₁) occurs in competition with intersystem crossing, and that physical quenching of S₁ to the electronic ground (S₀) is not a major process at these wavelengths. The rate coefficient for reaction of T₁ with Br₂ was found to be ∼3 × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹, independent of pressure or temperature.