${\mathrm{H}}^{-}$ and ${\mathrm{D}}^{-}$ densities are measured in low-pressure plasmas by photodetachment to confirm and extend previous work done with probes. Measurements indicate no isotope dependence, with $n_{-}$ rising in proportion to ${n_e}^3$ in the density range below $n_e={10}^{10}$ ${\mathrm{cm}}^{-3\mathrm{}}$. This functional dependence is consistent with ${\mathrm{H}}^{-}$ production by a nonlinear process such as dissociative attachment to excited hydrogen molecules and ${\mathrm{H}}^{-}$ loss by diffusion.

• Received 2 January 1979

DOI:https://doi.org/10.1103/PhysRevLett.42.1538