Quantum state-selected photodissociation dynamics in H2O and D2O

Two photon excitation, tunable near 248·5 nm, has been used to dissociate H 2 O/D 2 O via the [Ctilde] 1 B 1 and [Btilde] 1 A 1 states. Rotationally resolved OH/OD(A 2 Σ + ) photofragment excitation spectra are reported, following excitation to predissociated levels of [Ctilde] 1 B 1 . Rotational resolution of the OH/OD(A 2 Σ + → X 2 Π) fluorescence, generated from individual J′ K a K c levels of [Ctilde] 1 B 1 , allows full quantum state selection in both the entry and exit channels. The OH/OD(A 2 Σ + ) fragment is formed rotationally hot as a result of the large change in bond angle in going from [Xtilde] 1 A 1 (or [Ctilde] 1 B 1 ) to the linear dissociative [Btilde] 1 A 1 surface. Product alignment measurements allow assignment of the two photon continuum absorption to [Btilde] 1 A 1 : a-axis rotation in [Ctilde] 1 B 1 destroys product alignment from these levels. Electronic branching from ⪷B 1 A 1 to à 1 B 1 (and/or [Xtilde] 1 A 1 ) during the dissociation forms ground state OH/OD(X 2 Π). Relative branching ratios are obtained and display a strong K′ a dependence.