Observation of transition-state vibrational thresholds in the rate of dissociation of ketene.

Rate constants for the dissociation of highly vibrationally excited ketene (CH(2)CO) have been measured at the threshold for the production of CH(2)((3)B(1)) and CO((1)Sigma(+)). The rate constant increases in a stepwise manner with increasing energy, consistent with the long-standing premise that the rate of a unimolecular reaction is controlled by flux through quantized transition-state thresholds. The data give the energies of the torsional and C-C-O bending vibrations of the transition state.