NASA - National Aeronautics and Space Administration

The visible absorption spectrum of the acetyl radical, CH3CO, was measured between 490 and 660 nm at 298 K using cavity ring-down spectroscopy. Gas-phase CH3CO radicals were produced using several methods including: (1) 248 nm pulsed laser photolysis of acetone (CH3C(O)CH3), methyl ethyl ketone (MEK, CH3C(O)CH2CH3), and biacetyl (CH3C(O)C(O)CH3), (2) Cl + CH3C(O)H f CH3C(O) + HCl with Cl atoms produced via pulsed laser photolysis or in a discharge flow tube, and (3) OH + CH3C(O)H f CH3CO + H2O with two different pulsed laser photolysis sources of OH radicals. The CH3CO absorption spectrum was assigned on the basis of the consistency of the spectra obtained from the different CH3CO sources and agreement of the measured rate coefficients for the reaction of the absorbing species with O2 and O3 with literature values for the CH3CO + O2 + M and CH3CO + O3 reactions. The CH3CO absorption spectrum between 490 and 660 nm has a broad peak centered near 535 nm and shows no discernible structure. The absorption cross section of CH3CO at 532 nm was measured to be (1.1 ( 0.2) × 10-19 cm2 molecule-1 (base e).