Photo-Ionization Cross Section of Beryllium near Threshold

The ${}_{}{}^1{}_{}{}^{}P$ continuum wave functions of beryllium with energies from 0 to ∼3.5 eV above the $2s$ threshold are calculated with the inclusion of auto-ionizing lines by configuration interaction. These functions are then combined with a ten-configuration ground-state function to compute the photo-ionization cross section which is found to be dominated by auto-ionization from the $2pns$ and the $2pnd$ series. The energy region studied contains the first three $2pns$ lines, which have widths of a significant fraction of a volt, comparable to their separation; and the first two $2pnd$ lines, which are about a thousand times narrower. There are no experimental measurements of the cross section for comparison; however, the computed phase shift agrees reasonably well with recent close-coupling calculations.