Forbidden Continuum: Free-Bound Transitions in Hydrogen Plasmas

It is shown that local electric fields in ionized gases can lead to forbidden free-bound transitions. The photodetachment of electrons from ${\mathrm{H}}^{-}$ is considered, and specifically the cross section for detachment into continuum $s$ waves is calculated from a binary-collision model. The corresponding forbidden radiative attachment is computed. The calculations indicate that in hydrogen plasmas with ion densities of ${10}^{15}$ ${\mathrm{cm}}^{-3\mathrm{}}$ or above, the forbidden continuum should make a large contribution to the near-infrared emission. The effect may also appear in alkali plasmas.