State-to-state molecular photodissociation dynamics
The water molecule, rotationally state selected in the third and fourth streching overtone (|04>- and |05>-) and stretch-bend combination (|04>|2>-) levels, has been photodissociated at γ ≈ 282 nm via the Ã state. The OH photofragment rotational distributions, determined by OH(A-X) laser-induced fluoresence (LIF), are found to differ from those reported previously by Andresen and coworkers (H2O|01>- + 193 nm), Grim and coworkers (H2O|04>- + 239.5 nm) and Rosenwaks and coworkers (H2O|01>+ + 193 nm). These variations become more apparent with increasing angular momentum in the parent water molecule and with an increasing number of OH stretching quanta in the intermediate vibrational overtone. The Franck-Condon model of Balint-Kurti is able to qualitatively reproduce the observed trends, provided that dissociation at lower photolysis photon energies and via higher intermediate overtone states is assumed to occur preferentially from extended RH-OH configurations. The calculations suggest that the variation in the photofragment rotational distributions lies in a gradual change in the inertial properties of the bound state water molecule as the H-OH bond is stretched. In a second study, the partially deuterated water molecule, rotationally state selected in the third and fourth OH stretching vibrational overtone levels have been photodissociated via the Ã state at γ ≈ 288 nm. A branching ratio betweem the H + OD and D + OH dissociation channels is estimated from OD and OH (A-X) LIF measurements to be φ(OD)/φ(OH) > 20; this compares well with the previous measurements of Grim and coworkers, and the theoretical work of Imre and coworkers. The small shift in the centre of mass in the water molecule arising from the substitution of of a deuterium atom for one of the hydrogen atoms is shown to have a marked effect on the rotational distributions of the OD photofragment. Calculations using a modified Franck-Condon model, which includes an approximate exit-torque, are able to reproduce qualitatively the experimental OD rotational distributions at sensible values of RR-OD(~ 1.4 Å). In addition to being sensitive to the dynamics of the parent molecule on the ground state potential, the product OD state distributions are shown to be very sensitive to even the smallest exit channel torque on the excited potential surface.