Ionization of the hydrogen atom in strong magnetic fields: beyond the adiabatic approximation

Abstract

High magnetic fields in neutron stars, B ~ 10^{11} - 10^{13} G, substantially modify the properties of atoms and their interaction with radiation. In particular, the photoionization cross section becomes anisotropic and strongly polarization dependent. In a number of previous works based on the adiabatic approximation the conclusion was drawn that the transverse cross section vanishes for frequencies smaller than the electron cyclotron frequency. In other works (which employed a different form of the interaction operator) appreciable finite values were obtained. An adequate interpretation of the neutron star thermal-like radiation requires a resolution of this controversy. In this work the atomic wave functions for both discrete and continuum states are calculated by solving the coupled channel equations allowing the admixture between different Landau levels, which provides much higher accuracy than the adiabatic approximation. This enables to resolve the above contradiction in favour of the finite transverse cross sections. The non-adiabatic treatment of the continuum includes coupling between closed and open channels, which leads to the autoionization of quasi-bound energy levels associated with the electron cyclotron excitations and gives rise to Beutler - Fano resonances of the photoionization cross section. Autoionization widths of these quasi-bound levels are calculated and compared with the radiative widths. The results are important for investigations of the radiation emergent from the surface layers of neutron stars.