The Stark spectroscopy of the hydrogen atom has been investigated near the ionization threshold in static homogeneous external electric fields from ∼1000 to ∼8000 V/cm with use of a two-step excitation H(n=1)+ħ${\omega }_{\mathrm{vuv}\to }$H(n=2)+ħω ${}_{\mathrm{uv}\to }{}^{}{}_{}{}^{}$${\mathrm{H}}^{\mathrm{*}}$ with vuv and uv laser radiation, independently tunable and linearly polarized either parallel (π) or perpendicular (σ) with respect to the external field. Different from other, nonhydrogenic atoms, the hydrogen atom is a peculiar case insofar as the initial states of H(n=2), from which excitation takes place, are essentially of parabolic character at the field strengths employed in this study.

Individual sublevels ($n_1$,$n_2$,‖$m_l$‖) have been prepared in the n=2 state with sub-Doppler resolution. Photoionization spectra have been obtained with π- and σ-polarized uv radiation from the four possible sublevels of the n=2 Stark manifold: ‖1,0,0〉 ‖±1/2〉, ‖0,1,0〉 ‖±1/2〉, ‖0,0,±1〉 ‖±1/2〉, and ‖0,0,∓1〉 ‖±1/2〉. All spectra show sharp line structures of quasistable field-ionizing states superimposed on continua in the energy region between the classical saddle point, $E_{\mathrm{SP}=\mathrm{-}2\mathrm{\surd }\mathrm{F}}$ a.u., and the zero-field ionization threshold E=0 a.u. Employing existing theoretical treatments, the quasistable spectral structures are quantitatively completely analyzed and accounted for, as long as their field ionization rate is less than ∼5×${10}^{11}$ ${\sec }^{\mathrm{-}1}$. Field-ionization rates of quasistable states have been measured and are compared with theory. Oscillatory resonance structures of the ionization cross section near the zero-field threshold are observed with pronounced modulation degree only in the spectrum from the ‖1,0,0〉 ‖±1/2〉 sublevel excited with the uv π-polarized. The energy spacing of the oscillations has been measured as a function of the electric field strength F and found to be nearly proportional to $F^{3/4}$, with slight but definite deviations. Theoretical analysis of the oscillations yields quantitative agreement with the experiment.

• Received 14 February 1985

DOI:https://doi.org/10.1103/PhysRevA.33.301