Photofragmentation dynamics of ketene at 308 nm: Initial vibrational and rotational state distributions of CO product by vacuum UV laser-induced fluorescence

Abstract

The vibration-rotation state distribution of CO(v,J) from CH2CO flash photolysis at 308 nm has been determined by pulsed VUV laser-induced fluorescence. The excited ketene molecule appears to internally convert to vibrationally excited ground singlet state and to dissociate on that surface, CH2CO+308 nm→CH2(1A1)+CO+6.7 kcal/mol. There are no energetic CO molecules which would indicate formation of the lower energy CH2 triplet state. The observed rotational distributions of CO(v=0) and CO(v=1) are roughly matched by rotational temperatures of 1300 and 550 K, respectively; the data are accurately matched by phase space theory (PST). PST assumes that all energetically accessible product states have equal probability; it contains no adjustable parameters. PST predicts the ratio of v=1 to v=0 populations to be 0.013 compared to an observed value of 0.09±0.05. A Franck–Condon calculation gives too much vibrational excitation, 0.22±0.07. The separate statistical ensemble (SSE) model of Wittig which decouples overall ketene rotation from the other internal degrees of freedom gives 0.082 in agreement with experiment. This type of statistical distribution is likely to characterize bond fissions for which the barrier is just the dissociation limit itself.