Quantum state-selected photodissociation dynamics in H2O and D2O

Two photon excitation, tunable near 248.5 nm, has been used to dissociate H2O/D2O via the C-tilde 1B1 and B-tilde 1A1 states. Rotationally resolved OH/OD(A2Σ+) photofragment excitation spectra are reported, following excitation to predissociated levels of C-tilde1B1. Rotational resolution of the OH/OD(A2Σ+ → X2 PI) fluorescence, generated from individual J'sub(Ksub(a)Ksub(c)) levels of C-tilde1B1, allows full quantum state selection in both the entry and exit channels. The OH/OD(A2Σ+) fragment is formed rotationally hot as a result of the large change in bond angle in going from X-tilde1A1(or C-tilde1B1) to the linear dissociative B-tilde1A1 surface. Product alignment measurements allow assignment of the two photon continuum absorption to B-tilde1A1: α-axis rotation in C-tilde1B1 destroys product alignment from these levels. Electronic branching from B-tilde1A1 to A-tilde1B1 (and/or X-tilde1A1) during the dissociation forms ground state OH/OD(X2 PI). Relative branching ratios are obtained and display a strong K'sub(a) dependence. (author)