Bond-selective photofragmentation of jet-cooled HOD at 193 nm: Vibrationally mediated photochemistry with zero-point excitation

Abstract

Photofragment yields are reported for supersonically cooled H2O, D2O, and HOD via one photon, 193 nm photolysis in a slit jet expansion, with OH and OD fragments monitored by laser induced fluorescence methods. Detailed analysis of the dependence of OH vs OD photofragment signals on isotopic composition is used to extract relative photolysis cross sections and branching ratios for bond-selective cleavage in HOD, H2O, and D2O samples. Specified relative to the 193 nm cross sections for H2O→H+OH, the ratios are 0.392(20), 0.032(20), and 0.0157(19) for (i) HOD→H+OD, (ii) HOD→D+OH and D2O→D+OD channels, respectively. Specifically, these results indicate a propensity for H–OD vs D–OH bond cleavage in HOD of 12(8):1. This strong H/D isotopic selectivity reflects extreme non-Franck–Condon photolysis out of classically of forbidden regions of the ground-state wave function, i.e., bond-selective photochemistry mediated solely by zero-point vibrational excitation. However, when compared with theoretical predictions from full three-dimensional quantum scattering calculations on the ground (X̃ 1A1) and excited (Ã 1B1) potential-energy surfaces (PES) of water, the observed HOD branching ratio is found to be too low by an order of magnitude. These results provide additional evidence that photodissociation of water in the extreme non-Franck–Condon region is not adequately explained by current theoretical models and suggest that contributions from other electronic surfaces may be important.