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

The photoionization of the H atom in strong electric fields, F, by resonant two-photon excitation, H(1)+VUV→H(2)+UV→H++e, has been investigated at energies from the classical field ionization saddle point, E/sub s//sub p/ = 2 √F a.u., through the zero field ionization limit, E = 0, into the continuum, E>0. The atoms have been excited to single Stark levels in n = 2 with tunable pulsed VUV laser light around the Lyman-α line in an atomic beam with sub-Doppler resolution. The ionization from selected Stark levels by the UV was observed as a function of the UV wavelength. The VUV and UV were linearly polarized either perpendicular, sigma, or parallel, π, to the electric field. Ionization spectra have been taken with (π,π), (π,sigma), (sigma,π), and (sigma,sigma) polarization and investigated at field strengths from 2-7 KV/cm. All spectra exhibit quasi-stable state structure superimposed on a continuum at energies 0>E>-Vertical BarE/sub s//sub p/Vertical Bar. For the first time, also oscillatory structures have been observed with the H atom, extending well into the continuum region, E>O, and below the zero field limit, E<0. Their field dependence follows proportionalF/sup 3/4/, with a deviation of a few percent. The oscillations appeared pronounced in (π,π) excited spectra, while not observable with the other polarization combinations