Shape Resonances in the Hydrogen Stark Effect in Fields up to 3 MV/cm

We observe the Stark effect in hydrogen by photoionizing a relativistic hydrogen beam prepared by photodetachment of the 800-MeV ${\mathrm{H}}^{-}$ beam at the Clinton P. Anderson Meson Physics Facility. Photoexcitation of ${\mathrm{H}}^0$ is performed in a magnetic field (up to 6.5 kG) that appears in the atomic frame as an electric field up to 3 MV/cm. Resonances observed with an angle Doppler-tuned laser beam match calculated energies. At sufficiently high field, near the Stark-Coulomb potential-barrier peak, we observe shape resonances with an apparent elevation of the base line on the high-energy side.