One- and two-photon ionization of model atoms: The sphericalδ-shell potential

One- and two-photon ionization cross sections for the negative hydrogen ion ${\mathrm{H}}^{\mathrm{-}}$ are evaluated by using a one-electron model where the effective intra-atomic potential energy is assumed to be of a very-short-range character and approximated by a ‘‘spherical δ shell.’’ The adjustment of the two parameters involved in the model potential so as to reproduce either the photodetachment energy or the behavior of the one-photon ionization cross section over a substantial range of frequencies beyond the ionization threshold leads to conjecture of a reasonable estimate of the two-photon ionization cross section over the proper range of frequencies of the ionizing field.