Signatures of non-adiabatic dynamics in the fine-structure state distributions of the OH(X^~/A^~) 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^~/A^~) product fine-structure populations as a probe of the non-adiabatic dissociation dynamics. The calculated rotational state distributions of the two Λ-doublet levels of OH(X^~, v = 0) exhibit very different characteristics. The A′ states, produced mostly via the B^~→X^~ conical intersection pathway, have significantly higher populations than the A″ counterparts, which are primarily from the B^~→A^~ 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^~) 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^~→X^~ non-adiabatic pathway. On the other hand, the experiment-theory agreement for the OH(A^~) fragment is also satisfactory, although some small quantitative differences suggest remaining imperfections of the ab initio based potential energy surfaces