Observation of narrow resonances in the H - photodetachment cross section near the n=3 threshold

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