Mode-selective bond fission: Comparison between the photodissociation of HOD (0,0,1) and HOD (1,0,0)

The 193 nm photodissociation of individual rotational levels of HOD molecules excited with one quantum of O-H or O-D stretching vibrational energy is described. Stimulated Raman excitation and coherent anti-Stokes Raman scattering are used to prepare and detect, respectively, the (0,0,1) (O-H stretch) or (1,0,0) (O-D stretch) vibrationally excited HOD. The OD and OH fragments are detected by laser-induced fluorescence. In the photodissociation of HOD (0,0,1), the yield of both fragments is enhanced [relative to HOD (0,0,0)], but the yield of OD is increased 2.5 ± 0.5 times more than that of OH. In the photodissociation of HOD (1,0,0), no enhancement of the yield of the fragments is obtained. Our results show that even the very lowest possible level of vibrational excitation can be "leveraged" to effect selective bond breaking. Also, these results demonstrate that bond cleavage does not necessarily occur on the weakened bond and they agree with theoretical calculations indicating that the yield of OD and OH fragments depends on the Franck-Condon overlap of the vibrational wave function with the repulsive surface of the upper state.