MULTIPHOTON DETACHMENT OF H- AND THE APPLICABILITY OF THE KELDYSH APPROXIMATION

We report results of calculations of photodetachment rates and photoelectron energy spectra for the negative hydrogen ion irradiated by a low-frequency field. We have carried out calculations in both one-electron and two-electron models (with electron-electron correlation included in the initial channel in the two-electron model). Our main focus is on exploring the differences between various approximations that arise from different treatments of the electron-field interaction, rather than from different treatments of the electron-electron interaction. Thus, for the most part, we describe the active electron by the one-electron model. Within this framework, the Keldysh theory yields results in remarkably good agreement with those obtained using Floquet theory. Furthermore Keldysh theory reproduces well the minima and discontinuous slopes of the rate at thresholds. The accuracy of the Keldysh approximation can be understood by examining the parameters that characterize the strength of the electron-field interaction. The Faisal-Reiss theory also gives good agreement with Floquet theory (unless the intensity is very high); the reason for this is more technical, having to do with the short range of the one-electron atomic potential. We have generalized the Keldysh and Faisal-Reiss theories to include electron-electron correlation by representing the ground-state ion with an accurate two-electron wave function. Correlation leads to a long-range interaction of the active electron with the atomic core, and thereby to more significant discrepancies between the Keldysh and Faisal-Reiss theories. We compare results obtained using the two-electron Keldysh theory with results, obtained by others, using two-electron perturbation theory and two-electron Floquet theory. © 1990 The American Physical Society.