Semiclassical photodissociation cross section for H2O

We investigate semiclassically the photodissociation of water in the first absorption band. Experimental absorption spectra show weak undulatory structures, which are related to the vibrational symmetric stretch motion on the excited (A) over tilde-state potential-energy surface (PES). Using the cycle expansion resummation we demonstrate that there are only small corrections due to other fundamental periodic orbits and negligible contributions from longer orbits. The error in the positions of the semiclassical resonances is of the same order or less as the variations when using different Born-Oppenheimer potential surfaces. For this light molecule the background term is strongly influenced by quantum effects. We discuss estimates for the importance of quantum corrections and a way to incorporate them.