Observation of Two-Electron Photoionization of theH−Ion near Threshold

Photoejection of both electrons from the ${\mathrm{H}}^{-}$ ion by a single incident photon has been observed with use of a crossed-relativistic-beam technique. The dipole selection rules for a single photon pick out the ${}_{}{}^1{}_{}{}^{}P_{}^{\circ }{}_{}{}^{}$ final state. The relative cross section was measured over a 0.30-eV range of photon energies near threshold in steps of 0.007 eV with a resolution of 0.007 eV full width at half maximum. The energy dependence of the cross section in this region was consistent with a Wannier threshold law.