Photofragmentation of ketene to CH sub 2 ( tilde X sup 3 B sub 1 ) + CO. 2. Rotational-state distributions of product CO

Ketene cooled in a supersonic jet is photolyzed at 1,120, 190, and 30 cm{sup {minus}1} above and at 50 cm{sup {minus}1} below the top of the barrier to dissociation on the triplet surface. The barrier is 1,330 cm{sup {minus}1} above CH{sub 2}({tilde X}{sup 3}B{sub 1}) + CO. The rotational-state distributions of CO produced from triplet ketene are measured by vacuum-UV laser-induced fluorescence. These distributions cannot be described adequately by a Boltzmann temperature or by phase space statistical models. An average of 22.4 {plus minus} 1% of the total available energy goes to CO rotation as predicted by an impulsive model using the ab initio transition-state geometry. This indicates that the energy partitioning is determined by the geometry of the transition state with the repulsive force directed along the breaking bond. The fraction of energy going to CO rotation does not depend on the photolysis wavelength. The Gaussian shape of the CO rotational-state distribution can be understood quantitatively in terms of the zero-point motion of the C-C-O bending vibration of the transition state. Even when the excess energy above the top of the barrier is comparable to the barrier height itself, the energy release appears to be completely dynamically controlled.