Negative-ion photodetachment in an electric field

A multichannel study of the photodetachment of a negative ion such as ${\mathrm{S}}^{\mathrm{-}}$ in the presence of a constant electric field is presented. The analysis is carried out as a series of frame transformations for the outgoing photoelectron. An earlier analysis in the absence of the external field is now augmented by the further transformations from spherical to cylindrical symmetry that are required. The interplay between the various thresholds due to fine structure in the negative ion and residual atom and the effects of the external field are analytically treated. The net effect is to eliminate the thresholds, to induce modulations in the detachment cross section, and to introduce field-assisted-tunneling contributions to the photodetachment. Numerical illustrations are provided. Our treatment closely parallels similar treatments of photoionization of neutral atoms in an electric field and of photodetachment in a magnetic field.