Photodissociation dynamics of water in the second absorption band. I. Rotational state distributions of OH(2Σ) and OH(2Π)

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Abstract
The photodissociation of H2O and D2O in the second band (λ≳120 nm) is investigated using two-dimensional (translation and rotation) classical trajectories. The calculations include all electronic states which are involved in the dissociation dynamics, i.e., B̃ 1A1, X̃ 1A1, and à 1B1. The nonadiabatic transitions B̃→X̃ and B̃→à near linearity are modeled in a very simple way, which does not yield the OH(2Σ)/OH(2Π) branching ratio. The rotational distributions for OH(2Σ) and OD(2Σ) agree qualitatively very well with the measurements. They are highly inverted and peak close to the highest accessible state. Comparing the OH(2Π) rotational distributions with recent experimental results we conclude that B̃→X̃ is probably the main dissociation pathway, although contributions from a B̃→à transition cannot be excluded. The OH(2Π) distribution is also highly inverted with a peak near j∼43 in excellent agreement with experiment. The majority of trajectories on all three potential energy surfaces is direct. The shape of the various rotational distributions is determined by the first step of the dissociation from the FC region up to linearity where the crossing to the X̃ or the à state might occur. As envisioned a long time ago the strong angular force near the FC region on the B̃ potential energy surface is responsible for the extremely high degree of rotational excitation for OH(2Σ) as well as for OH(2Π).