Response of thePo1resonance nearn=3 in theH−continuum to external electric fields

The response to external electric fields of the ${}_{}{}^1{}_{}{}^{}\mathrm{P}_{}^{\mathrm{o}}{}_{}{}^{}$ resonance ‘‘dip’’ in the ${\mathrm{H}}^{\mathrm{-}}$ photodetachment continuum cross section below the n=3 hydrogenic excitation threshold is investigated. Using the relativistic (β=0.806) 650-MeV ${\mathrm{H}}^{\mathrm{-}}$ beam at the Clinton P. Anderson Meson Physics Facility (LAMPF) in Los Alamos, the fourth harmonic (266 nm) of a Nd:YAG laser (where YAG denotes yttrium aluminum garnet) is Doppler shifted to provide a continuously tunable photon beam in the rest frame of the ions. The magnetic field from pulsed Helmholtz coils, surrounding the photon-${\mathrm{H}}^{\mathrm{-}}$ interaction point provides a Lorentz-transformed barycentric electric field. As the magnitude of the external field is increased, there is no observed shift in the photon energy to excite the resonance $E_o$, 12.650±0.001 eV, until quenching occurs at a field strength of 2.36 MV/cm. The effective strength of the resonant state is consistent with a linear decrease with increasing field. The line shape index q is constant within statistical uncertainty, until the resonance disappears. There is evidence that the width of the resonance remains constant at fields below 1.25 MV/cm. It then broadens at higher fields before the resonance quenches.