Photofragmentation of ketene to CH sub 2 ( tilde X sup 3 B sub 1 ) + CO. 1. Barrier height and dissociation rate constant

Ketene cooled in a supersonic free jet is photolyzed in the ultraviolet range. The appearance rate of fragment CO from triplet ketene as a function of ultraviolet photon energy is measured by vacuum-ultraviolet laser-induced fluorescence. A tunneling-corrected RRKM theory is used to fit the observed dissociation rate of ketene using the ab initio vibrational frequencies of the transition state. The density of states inferred from the RRKM fit is a factor of 2 greater than the density of states of the ground electronic state (S{sub 0}) and 300 times greater than that of T{sub 1}. Tunneling corrections are necessary to fit the dissociation rate below the top of the exit barrier. The experimental rates show a sharp change in slope at 28,290-cm{sup {minus}1} photolysis frequency. The height of the exit barrier is determined to be 3.79 {plus minus} 0.06 kcal/mol above ground-state fragments. The imaginary frequency that fits the data is (150 {plus minus} 25)i cm{sup {minus}1}, much lower than the ab initio frequency of 523i cm{sup {minus}1}. At higher energies the RRKM theory predicts faster rates than are observed. It is possible that the intersystem crossing rate is relatively independent of energy and becomes the rate-determining step for formation of triplet products at higher energies.