Photodissociation dynamics of jet-cooled H2O and D2O in the non-Franck–Condon regime: Relative absorption cross sections and product state distributions at 193 nm

Abstract

Quantum state distributions for nascent OH and OD fragments generated by Franck–Condon “forbidden” 193 nm photodissociation of H2O and D2O are reported, with the two isotopomers initially prepared in their zero-point vibrational and lowest ortho/para nuclear spin allowed rotational states (i.e., JKaKc=101 and 000 in a 3:1 ratio for H2O and 1:2 ratio for D2O) by cooling in a slit supersonic expansion. Product state distributions are probed via OH/OD laser-induced fluorescence (LIF) with cylindrical mirror collection optics optimized for the slit expansion geometry, which makes photodissociation studies feasible with cross sections as low as ≈10−26 cm2. The OH and OD fragments are formed exclusively in v=0, but with highly structured quantum state distributions in rotational, Λ-doublet, and fine structure levels (2Π3/2+, 2Π1/2+, and 2Π3/2−) that exhibit qualitatively different trends than observed in previous jet photolysis studies at 157 nm in the Franck–Condon “allowed” regime. The relative OH/OD fragment yields at 193 nm indicate a 64±10 times greater propensity for OH vs OD bond cleavage in H2O than D2O, which is more than three-fold smaller than predicted from full three-dimensional quantum scattering calculations on ground (X̃ 1A1) and first excited state (Ã 1B1) potential surfaces. One-dimensional semiclassical calculations of the Franck–Condon overlap matrix elements confirm these discrepancies to be considerably outside uncertainties associated with the ground and excited state potential surfaces. These results indicate that the photodissociation dynamics for this benchmark system are not yet fully understood and suggest either non-Born–Oppenheimer effects or contributions from other electronic surfaces may be important in the extreme non-Franck–Condon photolysis regime.