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.
Laser investigation of the photoionization spectra in the presence of an electric field has been performed in $\ensuremath{\pi}$ and $\ensuremath{\sigma}$ polarizations on an ytterbium atomic beam from the particular level
4f146s7s1S0 which was populated by a two-step excitation. Broad modulations (in $\ensuremath{\pi}$ polarization) and Stark resonances in the vicinity of the zero-field ionization limit are interpreted satisfactorily as in the case of alkali atoms. However, the photoionization spectra are deeply affected by the presence of two perturbing levels belonging to the configuration
4f135d26s. This phenomenon is explained by strong interference effects between the structured Stark continuum and the field-induced autoionizing states corresponding to the doubly excited levels.
The authors have studied phenomena related to the ionisation of indium Rydberg atoms by an external electric field. The indium atoms are excited in an atomic beam apparatus by pulsed dye laser light. The high-lying states are populated in one step from the ground state by an ultraviolet, frequency-doubled dye laser or by stepwise excitation using two dye lasers in the visible region. Shortly after the excitation, an electric field is applied in the interaction region which causes ionisation of the highly excited atoms. The authors have investigated (i) the lowest adiabatic ionisation threshold over a wide range of n values and compared the results with model predictions, (ii) the structure in the adiabatic part of the ionisation spectrum, and (iii) the transition from adiabatic to diabatic ionisation as a function of the slew rate of the ionising field pulse and the quantum defect and quantum numbers of the atomic state concerned. The ionisation signal is demonstrated to shift considerably towards higher field strengths with increasing slew rate. A qualitative explanation is presented and consequences for the detection efficiency are mentioned. Differences in the field ionisation behaviour between nearly degenerate excited states are demonstrated and exploited to achieve selective detection of fine-structure and even hyperfine-structure states.
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.
A theory is developed of multistage isotopically selective ionisation of ytterbium ions. This theory is based on the rate equations for the level populations and on the transfer equation for the laser line intensities. An analysis of the time characteristics of multistage photoionisation is used to find the optimal laser radiation intensities for different ionisation stages. A multilevel model of multistage photoionisation developed for a natural mixture of isotopes is used to identify the ranges of wavelengths where the photoionisation selectivity is the highest. A mechanism of degradation of a short but powerful laser pulse is considered and the length of an efficiently excited medium is found.
The phenomenon of diabatic ionisation of indium Rydberg atoms by an external electric field has been investigated. The indium atoms are excited in an atomic beam apparatus to high lying p states by stepwise laser excitation. The authors have studied the dependence of the ionisation processes on the slew rate of the applied electric field. The observation of a new diabatic ionisation feature for high slew rates is reported and an explanation is presented. Consequences for the ionisation efficiency are mentioned.