Uniform semiclassical calculation of the direct part of the photodissociation cross section of water

The photodissociation cross section of water in the first absorption band n aturally splits into two parts, a rather dominant and broad background, and a series of isolated resonances. We focus here on the semiclassical calcul ation of the background term. We compare various semiclassical methods of c alculation, including the reflection approximation, the Wigner-Weyl series, Airy function approximations, and higher-order approximations in the time domain, and find that only the latter gives quantitative agreement. They ar e numerically efficient as they can be reduced to a 2-D spatial integration together with a Fourier transform. (C) 1999 American Institute of Physics. [S0021-9606(99)00224-X].