One- and two-electron photoejection from ${\mathrm{H}}^{-}$: A multichannel $J$-matrix calculation

The multichannel $J$-matrix technique is used to solve the pseudostate close-coupling equations for ${\mathrm{H}}^{-}$ photodetachment using only $L^2$ basis functions and standard configuration-interaction techniques. The problem of linear dependence which arises when the same $L^2$ basis is used to expand both the target and free electrons is discussed and exactly solved using a matrix partitioning. Results for total and partial photodetachment cross sections are given from threshold to 70-eV photon energy (1664 to 18 nm) with emphasis on the single-photon two-electron ejection cross section, and the ${}_{}{}^1{}_{}{}^{}P$ shape resonance just above the hydrogenic $n=2$ threshold.