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Issue 31, 2007
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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

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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.

Graphical abstract: 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

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Supplementary files

Article information


Submitted
29 Jan 2007
Accepted
06 Mar 2007
First published
02 Apr 2007

Phys. Chem. Chem. Phys., 2007,9, 4098-4113
Article type
Paper

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

V. Khamaganov, R. Karunanandan, A. Rodriguez and J. N. Crowley, Phys. Chem. Chem. Phys., 2007, 9, 4098
DOI: 10.1039/B701382E

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