Effect of an Electric Field upon Resonances in theH−Ion

Two resonances in the photodetachment cross section of ${\mathrm{H}}^{-}$ have been studied in a variable dc electric field by exploiting a relativistic colliding-beam technique. The Feshbach resonance, at a photon energy of 10.930 eV, splits into two and, possibly, a smaller third component for field strengths of about ${10}^5$ V/cm, disappearing entirely for fields greater than 2×${10}^5$ V/cm. The shape resonance, at a photon energy of 10.98 eV, begins to diminish in amplitude and broaden only for fields above 5×${10}^5$ V/cm.