New aspects of the photodissociation of water in the first absorption band: How strong is excitation of the first triplet state?

We consider the possibility of the $X̃ 1A1→3B1$ excitation of water at wavelengths near 193 nm, i.e., in the red tail of the first absorption band. The corresponding excited-state potential-energy surface is calculated by quantum mechanical methods and the dynamics calculations are performed in the time-dependent representation. It is shown that an absorption cross section for exciting the triplet state ³$B1,$ that at its maximum is about five hundred times (or more) smaller than the corresponding cross section for excitation of the ¹$B1$ state, might explain the surprisingly small H+OD/D+OH branching ratio in the photodissociation of HOD at 193 nm measured by Plusquellic et al. (the foregoing paper). The singlet–triplet transition dipole moment estimated in this way also explains the unexpectedly small cross section ratio for $H2O$ and $D2O$ measured at 193 nm.