Multiphoton ionization of one-electron diatomic molecules: the discretization approach for the two-centre problem

We propose a finite-element approach with prolate spheroidal coordinates to calculate, for one-electron diatomic molecules, ac Stark shifts and multiphoton ionization cross-sections in lowest-order perturbation theory. We use B-spline expansions to represent both angular and radial parts of the wavefunctions. The method takes advantage of the fact that the Schrodinger equation is separable with prolate spheroidal coordinates. The effective completeness of the basis set leads to an accurate representation of the molecular spectrum. Partial cross-sections, associated with each open continuum, are calculated. Particular attention is placed on the problem of the normalization of the discretized continua. We show that the proper normalization is simply obtained from a density of states, thus the explicit evaluation of the continua at large distances is not necessary. Accurate values of energies, shifts, one-, four- and six-photon ionization cross-sections are obtained in lowest-order perturbation theory and they are compared with other calculations when available.