MODE-SPECIFICITY IN PHOTODISSOCIATION OF FUNDAMENTAL STRETCHINGS OF HOD

Abstract
Stimulated Raman excitation and coherent anti-Stokes Raman scattering prepare and detect, respectively, HOD molecules with one quantum of vibrational excitation in the O-H or O-D stretch vibrations. An ArF excimer laser at 193 nm promotes vibrationally excited molecules to the first electronic surface A(¹B₁) where they dissociate to produce OH + D or OD + H fragments. The photodissociation products OD and OH are detected via laser induced fluorescence. The photodissociation cross section of HOD (0,0,l) vibrationally excited molecules with one quantum of vibration in the O-H stretch is greatly enhanced over that for HOD (0,0,0). The dissociation of this vibrationally excited molecule is also very selective, the OD + H yield being 2.5±0.5 times greater than the OH + D yield. In the photodissociation of the O-D stretch, excited HOD (l,0,0), no enhancement of the yield of the fragments is obtained. 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 ground state vibrational wavefunction with the continuum wavefunction on the repulsive surface of the upper state. Our results show that even the very lowest possible level of vibrational excitation can be "leveraged" to effect selective bond breaking.