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.
We have studied the Stark effect for the bound Rydberg states of xenon
(2P3/2 core) in the vicinity of n=17. These are the first detailed measurements of Rydberg-state Stark structure in a heavy rare-gas atom. The experiment was performed by one-photon laser spectroscopy from the metastable 6s'
[(1/2)]0 state in an atomic beam. Laser polarization can be chosen to excite pure M=0 or \ensuremath{\Vert}M\ensuremath{\Vert}=1 levels. Stark maps are presented for electric fields from 0 to 2000 V/cm. The major low-field features are a prominent linear manifold for states with l\ensuremath{\ge}3 and an isolated group of states arising from mixing within the overlapping p and d terms. A simple model of the p,d complex, showing strong mixing at fields as low as 200 V/cm, is used to correlate observed states with the zero-field states. The spectra become exceedingly complex for fields greater than 1000 V/cm because of mixing within the extensive fine structure of xenon.
We have studied experimentally and theoretically two-step photoionization through the 3P state of sodium in an electric field of up to 3.90 kV/cm. The apparatus uses an atomic beam crossed by two pulsed laser beams, one at 589 nm for the first step, and the other swept through the zero field threshold Eo near 409 nm for the ionization. We observe broad undulations in the photocurrent above Eo that narrow with decreasing energy and eventually sharpen into well-defined but asymmetric transitions to Stark-shifted Rydberg states. We calculate the oscillator strength density for the appropriate transitions using modified hydrogenic wave functions and find excellent agreement for the position and relative strength of all the spectral features. We model the asymmetries as Fano-type interferences between quasi-discrete and continuum ionization channels. Using appropriate core potentials, and taking into account core-induced coupling of both discrete-to-continuum and continuum-to-continuum states, we obtain estimates of Fano's q and Ɖ that agree reasonably well with our measurements.
Stark spectra of barium Rydberg states above the classical field-ionization limit have been recorded at high density of states employing high-resolution laser spectroscopy. The spectra, taken at a binding energy of about \ensuremath{\varepsilon}\ensuremath{\approxeq}-55 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ with respect to zero-field threshold and at electric-field strengths F\ensuremath{\approxeq}100 V/cm exhibit dramatic variations in line profiles with increasing external field strength. The WKB quantum-defect theory of the Stark effect of nonhydrogenic atoms was used to analyze the recorded spectra, and excellent agreement between measured and calculated line profiles was achieved. Core-induced coupling of closed and open hydrogenic Stark channels was found to contribute dominantly to the linewidth of the recorded Stark resonances. In addition, a reformulation of this theory by McNicholl, Ivri, and Bergeman (unpublished) in terms of a configuration-interaction model was employed to analyze a particular avoided crossing of several Stark components. In this way recorded line profiles were parametrized within a limited model to obtain resonance energies, linewidths, and Fano q parameters.
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.
The nonresonant, three-photon ionization spectrum of atomic Xe in the region between the
Xe+ 23/2 and
21/2 fine-structure limits is reported. Using linearly polarized light, four autoionizing Rydberg series are observed converging to the
Xe+ 21/2 limit, corresponding to
(2P\ifmmode^\circ\else\textdegree\fi{}
1/2)ns'[(1/2)]\ifmmode^\circ\else\textdegree\fi{}
1, (2P\ifmmode^\circ\else\textdegree\fi{}
1/2)nd'[(3/2)]\ifmmode^\circ\else\textdegree\fi{}
1,
(2P\ifmmode^\circ\else\textdegree\fi{}
1/2)nd'[(5/2)]\ifmmode^\circ\else\textdegree\fi{}
3, and
(2P\ifmmode^\circ\else\textdegree\fi{}
1/2)ng'[(7/2)]\ifmmode^\circ\else\textdegree\fi{}
3; as expected, using circularly polarized light only the J=3 series are observed. These results are in contrast to earlier experimental work in which no resonant structure was observed. Photoelectron angular distributions are reported for the
(2P\ifmmode^\circ\else\textdegree\fi{}
1/2)9s'[(1/2)]\ifmmode^\circ\else\textdegree\fi{}
1 and the
(2P\ifmmode^\circ\else\textdegree\fi{}
1/2)7d'[(3/2)]\ifmmode^\circ\else\textdegree\fi{}
3 autoionizing resonances. The results are compared with the recent theoretical calculations by P. Gangopadhyay et al. [Phys. Rev. A 34, 2998 (1986)].