Ultraviolet absorption spectrum and cross-sections of vinyl (C2H3) radical in the 225-238 nm region

The room-temperature gas-phase ultraviolet absorption spectrum and cross-sections of vinyl (C2H3) radicals have been determined in the spectral range 225-238 nm, employing cavity ring-down absorption spectroscopy. Vinyl radicals in these experiments were produced from the 193 nm excimer laser photolysis of methyl vinyl ketone (CH3COC2H3) and vinyl bromide (C2H3Br). The spectra obtained from the two systems were nearly identical. The observed spectrum exhibits a relatively broad and featureless absorption with a cross-section of 5.3 x 10(-18) cm(2) molecule(-1) at 230 nm. A combined uncertainty of similar to 25% for cross-section values has been assessed. The electronic transitions in the vinyl radical have been calculated by ab initio quantum chemical methods at the CIS, EOM-CCSD, CASSCF and CASPT2 levels of theories which assign the new observed band to the highly allowed and 'in plane' pi*(2a ") <-- pi(1a ") transition. (C) 1998 Elsevier Science B.V. All rights reserved.