Two pronounced dips in the ${\mathrm{H}}^{-}$ photodetachment cross section have been observed at photon energies of 12.650±0.004 and 12.837±0.004 eV, just below the hydrogenic $n=3\mathrm{}$ threshold. These energies were reached by crossing a uv laser beam with a relativistic ${\mathrm{H}}^{-}$ beam. The dips are interpreted as the first two members of a ${}_{}{}^1{}_{}{}^{}P$ Feshbach resonance series, of the "+" type, converging on $n=3\mathrm{}$. Fano line shapes have been fitted to these structures to extract their intrinsic widths and line profile indices.

• Received 5 September 1979

DOI:https://doi.org/10.1103/PhysRevLett.43.1715