Energy measurement of the lowestP01Feshbach resonance inH−

We use the 800-MeV LAMPF ${\mathrm{H}}^{\mathrm{-}}$ beam to Doppler-shift the energies of photons from a Nd:YAG laser (where YAG denotes yttrium aluminum garnet) up to the 10–13-eV range. Using these Doppler-shifted photons and calibrating our apparatus by observing the Lyman resonances in ${\mathrm{H}}^0$, we find that the energy of the first Feshbach resonance below n=2 in the ${\mathrm{H}}^{\mathrm{-}}$ ion is 10.9264(6) eV. This energy is in good agreement with predictions and appears to indicate that the one-electron reduced-mass rydberg, rather than the infinite-mass rydberg, is appropriate to use for this Feshbach resonance.