Analysis of the population of continuum states in wave packet propagation calculations

We present a 'virtual detector' method which allows the extraction of the momentum distributions in continuum states from time-dependent wave packet propagation studies with explicitly time-dependent Hamiltonians. It is based on the approach developed by Balint-Kurti et al (1990 J. Chem. Soc. Faraday Trans. 86 1741) for quantum chemistry applications and consists in measuring a signal in the asymptotic region. In that sense, it is close to the semiclassical method proposed recently in this journal (Feuerstein and Thumm 2003 J. Phys. B: At. Mol. Opt. Phys. 36 707). The advantage of the present approach is that it is based on a rigorous quantum mechanical treatment, robust, easy to handle and requires a much smaller grid size. The method is illustrated with the 3D example of electron detachment from the H^- ion colliding with a free-electron metal surface.