Predissociation of HONO upon excitation into the S1 state: An ab initio and dynamics study

We have investigated the vibrational predissociation of nitrous acid in the first excited singlet state, HONO (S1) → OH (X̃ 2Π) + NO (X̃ 2Π), by calculating the potential energy surface as a function of the pertinent coordinates. Of the ab initio methods tested in this context, the multiconfigurational second-order perturbation theory (CASPT2) was found to deliver the best trade-off between accuracy and expense of computer time. We solved the nuclear dynamics for a three dimensional model system treating the OH group as a pseudo atom and obtained the absorption spectrum, the lifetimes and the partial cross sections of the NO fragment state distributions. The OH rotational state distribution was assessed with a model treating the NO group as a pseudo atom and keeping the OH bond frozen. Special attention was given to the mechanistic features of the decay such as the coupling effects among the degrees of freedom and the state specific lifetimes, and the assignment of the S0→S1 absorption spectrum of anti- and syn-HONO.