Rotationally resolved photofragment alignment and dissociation dynamics in H2O and D2O

Abstract

Rotationally resolved photofragment alignments and population distributions are reported for OH(A) and OD(A) fragments, generated through photodissociation of H₂O and D₂O by atomic resonance radiation at 130.4, 129.5, 123.6 and 121.6 nm. Comparison of the rotational population distributions (corrected for polarisation effects at high N′), the alignments (corrected for angular momentum coupling at low N′) and the parent molecular absorption spectra allows the contributions from excitation into the B¹A₁, C¹B₁ or D¹A₁ states in the parent molecules to be resolved. Quantitative calculations of the degradation of OH, OD(A→X) fluorescence polarisation caused by electron and nuclear spin-rotation coupling are included in an Appendix. The corrected rotational alignments tend to a maximum at levels N′, close to the limit imposed by energy conservation. At levels above this limit, where the additional energy is provided by thermally populated rotational states in the photoexcited molecule, both the populations and the alignments fall sharply. It is suggested that the rotationally assisted pathway is favoured for molecules with J≈K_a.