An experimental and theoretical study of the bond selected photodissociation of HOD

Experimental and theoretical studies of the photodissociation of single vibrational states in HOD provide a qualitative and quantitative understanding of the dissociation dynamics and bond selectivity of this process. Vibrationally mediated photodissociation, in which one photon prepares a vibrational state that a second photon dissociates, can selectively cleave the O-H bond in HOD molecules containing four quanta of O-H stretching excitation. Dissociation of HOD(4ν_OH) with 266 or 239.5-nm photons produces OD fragments in at least a 15 fold excess over OH, but photolysis of the same state with 218.5-nm photons produces comparable amounts of OH and OD. Wave packet propagation calculations on an ab initio potential energy surface reproduce these observations quantitatively. They show that the origin of the selectivity and its energy dependence is the communication of the initial vibrational state with different portions of the outgoing continuum wave function for different photolysis energies.