COMPARISON OF REAGENT STRETCH VS BEND EXCITATION IN THE H+D2O REACTION - AN EXAMPLE OF MODE-SELECTIVE CHEMISTRY

A state-to-state study of the reaction H + D2O (nu1,nu2,nu3) --> OD(v,N) + HD was carried out to compare the effects of reagent stretch and bend excitation. D2O was prepared by infrared excitation in either the asymmetric stretch fundamental state, denoted D2O(0,0,1), or the bend-asymmetric stretch combination state, denoted D2O(0,1,1). Fast H atoms are generated by photolysis of HI at 266 nm, and the OD product is probed quantum-state-specifically by laser-induced fluorescence (LIF). This system exhibits mode-selective chemistry; excitation of the asymmetric stretch in D2O enhances the reaction rate by at least a factor of 5 (and possibly a factor of 20), whereas excitation of the D2O bend does not affect the reaction rate within experimental uncertainty. For both D2O(0,0,1) and D2O(0,1,1) the OD product is produced with little vibrational excitation [OD(v=1): OD(v=0) almost-equal-to 1:20]. This observation suggests that the D2O asymmetric stretch state is best thought of as a linear combination of local mode stretches, with the H atom reacting preferentially with the stretched OD bond. The measured OD rotational distributions from vibrationally excited D2O are nearly the same as the distribution from ground-state D2O. A-doublet ratios are also reported.