Photolysis of CH3C(O)CH3 (248 nm, 266 nm), CH3C(O)C2H5 (248 nm) and CH3C(O)Br (248 nm): pressure dependent quantum yields of CH3 formation

Abstract

The formation of CH₃ in the 248 or 266 nm photolysis of acetone (CH₃C(O)CH₃), 2-butanone (methylethylketone, MEK, CH₃C(O)C₂H₅) and acetyl bromide (CH₃C(O)Br) was examined using the pulsed photolytic generation of the radical and its detection by transient absorption spectroscopy at 216.4 nm. Experiments were carried out at room temperature (298 ± 3 K) and at pressures between ≈5 and 1500 Torr N₂. Quantum yields for CH₃ formation were derived relative to CH₃I photolysis at the same wavelength in back-to-back experiments. For acetone at 248 nm, the yield of CH₃ was greater than unity at low pressures (1.42 ± 0.15 extrapolated to zero pressure) confirming that a substantial fraction of the CH₃CO co-product can dissociate to CH₃ + CO under these conditions. At pressures close to atmospheric the quantum yield approached unity, indicative of almost complete collisional relaxation of the CH₃CO radical. Measurements of increasing CH₃CO yield with pressure confirmed this. Contrasting results were obtained at 266 nm, where the yields of CH₃ (and CH₃CO) were close to unity (0.93 ± 0.1) and independent of pressure, strongly suggesting that nascent CH₃CO is insufficiently activated to decompose on the time scales of these experiments at 298 K. In the 248 nm photolysis of CH₃C(O)Br, CH₃ was observed with a pressure independent quantum yield of 0.92 ± 0.1 and CH₃CO remained below the detection limit, suggesting that CH₃CO generated from CH₃COBr photolysis at 248 nm is too highly activated to be quenched by collision. Similar to CH₃C(O)CH₃, the photolysis of CH₃C(O)C₂H₅ at 248 nm revealed pressure dependent yields of CH₃, decreasing from 0.45 at zero pressure to 0.19 at pressures greater than 1000 Torr with a concomitant increase in the CH₃CO yield. As part of this study, the absorption cross section of CH₃ at 216.4 nm (instrumental resolution of 0.5 nm) was measured to be (4.27 ± 0.2) × 10⁻¹⁷ cm² molecule⁻¹ and that of C₂H₅ at 222 nm was (2.5 ± 0.6) × 10⁻¹⁸ cm² molecule⁻¹. An absorption spectrum of gas-phase CH₃C(O)Br (210–305 nm) is also reported for the first time.