Photoionization of the H atom in strong electric fields by resonant two-photon excitation

Photoionization of a hydrogen atom in a strong electric field has been investigated experimentally. Two-step resonant excitation of the atoms in the electric field was performed using tunable VUV laser light at the Lyman-alpha line for the first resonant step, and tunable UV laser light for the second ionizing step: H(n = 1) + VUV to H(n = 2) + UV to H(+) + e. Photoionization spectra were obtained using parallel and perpendicular polarization of the laser light. The spectra show discrete line structures of quasi-stable states superimposed on a continuum in the classical field ionization region. Oscillatory structures in the photoionization cross section near the zero field ionization limit were detected. 15 references