Signatures of non-adiabatic dynamics in the fine-structure state distributions of the OH((X)over-tilde/(A)over-tilde) products in the B-band photodissociation of H2O

A detailed quantum mechanical characterization of the photodissociation dynamics of H2O at 121.6 nm is presented. The calculations were performed using a full-dimensional wave packet method on coupled potential energy surfaces of all relevant electronic states. Our state-to-state model permits a detailed analysis of the OH((X) over tilde/(A) over tilde) product fine-structure populations as a probe of the non-adiabatic dissociation dynamics. The calculated rotational state distributions of the two Lambda-doublet levels of OH((X) over tilde, v = 0) exhibit very different characteristics. The A' states, produced mostly via the (B) over tilde -> (X) over tilde conical intersection pathway, have significantly higher populations than the A '' counterparts, which are primarily from the (B) over tilde -> (A) over tilde Renner-Teller pathway. The former features a highly inverted and oscillatory rotational state distribution, while the latter has a smooth distribution with much less rotational excitation. In good agreement with experiment, the calculated total OH((X) over tilde) rotational state distribution and anisotropy parameters show clear even-odd oscillations, which can be attributed to a quantum mechanical interference between waves emanating from the HOH and HHO conical intersections in the (B) over tilde -> (X) over tilde non-adiabatic pathway. On the other hand, the experiment-theory agreement for the OH((A) over tilde) fragment is also satisfactory, although some small quantitative differences suggest remaining imperfections of the ab initio based potential energy surfaces. (c) 2015 AIP Publishing LLC.