Stability of the 1P o shape resonance in H- in moderate electric fields.

The $^{1}\mathrm{P}^{\mathrm{o}}$ shape resonance of ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ has been studied in moderate motional electric fields by Doppler-tuning the photon energy absorbed by a 650-MeV ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ beam crossed by the fourth-harmonic beam of a Nd:YAG laser (YAG denotes yttrium aluminum garnet). The resonance parameters are remarkably insensitive to electric fields in the range from 120 to 240 kV/cm. Our measurements reveal that the width of the shape resonance may decrease from its zero-field value, on average, by (3.3\ifmmode\pm\else\textpm\fi{}0.9)% in this electric field regime. This observation contradicts the (naive) expectation that perturbation of the resonance by an electric field should promote more rapid decay, and hence greater width. Interpretations are given in terms of (a) Stark mixing with narrower singlet states and (b) tunneling through the potential barrier in hyperspherical coordinates.