The authors report a study of the stepwise photoionisation of ytterbium atoms near the first ionisation threshold in the presence of a DC external electric field. Several spectra corresponding to ionisation from the levels 4f146s7s 1S0 and 3S1, 4f146s6p 1P10 and 4f146s5d 1D2 and to various polarisation schemes of the exciting light beams were recorded by detecting the ion signal. Modulations of the photoionisation cross sections above the field-free ionisation limit were observed and investigated: for the 6s7s 1S0 and 3S1 initial levels, the spectra exhibit modulations only for pi polarisation of the last excitation, spectra obtained via the 6s6p 1P10 level exhibit modulations for the pi pi polarisation scheme only. An experimentally unprecedented result is obtained for the 6s5d 1D2 level: in that case the largest modulation rate is obtained for sigma polarisation of the last excitation. The behaviour of the rate and of the period of modulation as a function of energy were studied. It was observed that the period of modulation as a function of energy were studied. It was observed that the period of modulation at the ionisation limit varies with the electric field intensity F according to a F3/4 law. The structure of the Stark spectra below the field-free ionisation limit was also investigated. Narrow resonances, all having a symmetrical shape, were recorded. They could be identified by calculating their energies using perturbation theory. A number of doubly excited levels were recorded, their influence on the spectra was analysed.
Experimental results on the photoionization spectra of excited rubidium atoms in a well-defined
ml=1 state in the presence of an electric field are reported. Spectra corresponding to a definite polarization of the ionizing light exhibit characteristic resonances which are ascribed either to the non-Coulombic part of the central potential or to the spin-orbit interaction. The different line profiles are interpreted as being due to interferences between discrete states and continua, the observed structures depending directly on the density of oscillator strengths and not simply on the density of continuum states.
The first detailed observation of the perturbations by an electric field of the Rydberg autoionizing resonances of a rare-gas atom is reported. The Stark spectroscopy of autoionizing Rydberg states converging to the second ionization limit $4{p}^{5}^{2}P_{\frac{1}{2}}^{o}$ in Kr I is performed in an atomic beam with the use of a two-step laser excitation from the
4p55s[23]20 metastable state. In addition to the verification of the ${({n}^{*})}^{\ensuremath{-}3}$ dependence of the zero-field width of two autoionizing series and of the linear dependence of the Stark-manifold splittings with the electric field, we have observed an
(n)0.7 law for the dependence of the Stark-manifold splitting along the series, an important narrowing and dissymmetrization of the $n{d}^{\ensuremath{'}}$ resonances, and a nonlinear-field behavior of Stark-manifold components in the vicinity of the first anticrossing. Discussions are carried out with the use of the recent work on Rydberg-Stark spectroscopy and on autoionizing states in two-electron atoms.
The Stark spectroscopy of the hydrogen atom has been investigated near the ionization threshold in static homogeneous external electric fields from \ensuremath{\sim}1000 to \ensuremath{\sim}8000 V/cm with use of a two-step excitation H(n=1)+\ensuremath{\Elzxh}${\ensuremath{\omega}}_{\mathrm{vuv}\ensuremath{\rightarrow}}$H(n=2)+\ensuremath{\Elzxh}\ensuremath{\omega} $_{\mathrm{uv}\ensuremath{\rightarrow}}$
H∗ with vuv and uv laser radiation, independently tunable and linearly polarized either parallel (\ensuremath{\pi}) or perpendicular (\ensuremath{\sigma}) 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 (
n1,
n2,\ensuremath{\Vert}
ml\ensuremath{\Vert}) have been prepared in the n=2 state with sub-Doppler resolution. Photoionization spectra have been obtained with \ensuremath{\pi}- and \ensuremath{\sigma}-polarized uv radiation from the four possible sublevels of the n=2 Stark manifold: \ensuremath{\Vert}1,0,0〉 \ensuremath{\Vert}\ifmmode\pm\else\textpm\fi{}1/2〉, \ensuremath{\Vert}0,1,0〉 \ensuremath{\Vert}\ifmmode\pm\else\textpm\fi{}1/2〉, \ensuremath{\Vert}0,0,\ifmmode\pm\else\textpm\fi{}1〉 \ensuremath{\Vert}\ifmmode\pm\else\textpm\fi{}1/2〉, and \ensuremath{\Vert}0,0,\ensuremath{\mp}1〉 \ensuremath{\Vert}\ifmmode\pm\else\textpm\fi{}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, ${E}_{\mathrm{SP}=\mathrm{\ensuremath{-}}2\phantom{\rule{0ex}{0ex}}\mathrm{\ensuremath{\surd}}\mathrm{F}}$ 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 \ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}
1011 ${\mathrm{sec}}^{\mathrm{\ensuremath{-}}1}$. 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 \ensuremath{\Vert}1,0,0〉 \ensuremath{\Vert}\ifmmode\pm\else\textpm\fi{}1/2〉 sublevel excited with the uv \ensuremath{\pi}-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
F3/4, with slight but definite deviations. Theoretical analysis of the oscillations yields quantitative agreement with the experiment.
We report experimental results on the photoionization spectra of excited sodium atoms in a well-defined
ml=1 substate in the presence of a dc electric field. The various spectra corresponding to a definite polarization of the ionizing light exhibit characteristic features. It appears in addition that they are strikingly perturbed by the Earth's magnetic field.
We report the first observation of significant perturbations by external fields of the autoionizing resonance structure of a two-electron atoms. The effects of electric fields up to 20 kV/cm on autoionizing resonances in Sri were observed by three-photon ionization spectroscopy in an atomic beam. In addition to effects analogous to those for bound Stark states of a one-electron Rydberg atom, we observed a new effect: The characteristic shapes and widths of autoionizing resonances (Fano-Beutler profiles) can be strongly influenced by electric fields.