The linear-Stark-effect approximation is employed to investigate the effects of electric fields on autoionizing doubly excited atomic states corresponding to relatively large values of the outer-electron principal quantum number
n. By assuming that a statistical distribution of the inner-electron magnetic sublevels has been established, simple expressions are derived which relate the autoionization and radiative transition rates in the presence of an electric field to the respective field-free rates. It is pointed out that the multiphoton laser excitation of the doubly excited states can give rise to a nonstatistical distribution of the magnetic sublevels and to complex interference terms involving transition amplitudes associated with different values of the outer-electron angular momentum quantum number
l. The results of calculations are presented for the autoionization and radiative-transition rates which play the most important role in the dielectronic recombination of the Li-like
Fe+23 ion through the $2p\ensuremath{\rightarrow}2s$ inner-electron transition, where the contributions from large-
n values are particularly important. In these calculations it is assumed that the magnetic sublevels are statistically populated and only the simple expressions for the transition rates are employed. The effects of the electric field on the dominant (inner-electron) radiative decay rate can be neglected, but the electric field mixing of the outer-electron
l substates substantially increases the autoionization rates from the higher-
l sublevels.
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.
It should be possible to separate experimentally the contributions to dielectronic recombination (DR) of energetically unresolved intermediate autoionizing Rydberg
nl states using electric fields. This notion is based on two essential ideas. First, electric fields enhance the DR rate by Stark-mixing low-
l states with high autoionization rates with high-
l states with low autoionization rates. Second, the field at which an
l state becomes Stark mixed is determined by its quantum defect, a known function of
l. Consequently, the electric-field dependence of the DR rate should reflect the
l dependence of the autoionization rates and thus the contributions of the zero-field
nl states to the DR rate. This notion cannot be tested experimentally by examining true DR. However, it can be tested by studying DR from a continuum of finite bandwidth (CFB), for in this case the intermediate Rydberg
nl states are restricted to a single value of
l. Specifically, we have examined the electric-field dependence of DR from two CFB's, the
Ba6p3∕211d and
6p3∕28g states. In these two cases the intermediate autoionizing Rydberg states are restricted to the
Ba6p1∕2nd and
6p1∕2ng states
(l=2and4), which have quantum defects of 0.25 and 0.02, respectively. For the same
n they are Stark mixed at fields differing by an order of magnitude. We show experimentally that enhancement of the DR rate occurs at fields differing by a factor of 10 for
nd and
ng states of the same
n, as expected, confirming that the field dependence of DR can be used to extract information about the contributions of energetically unresolved
l states to the zero-field DR rate.
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.
Using a three-step laser saturation excitation technique, the saturation effects on the Ba 6p n s ( J = 1) and 6p n d ( J = 1, 3) autoionization spectra are observed systemically in zero field. These saturation spectra are introduced to determine the high n members of 6p nl ( l = 0, 2) autoionizing series and are used to analyse the channel interactions among the autoionizing series in zero field. Furthermore, the saturation excitation technique is applied to the electric field case, in which the saturation spectra of Ba 6p nk (| M | = 0, 1) autoionizing Stark states are measured. Most of these saturation spectra are observed for the first time so far as we know, which indicate the mixing of the autoionizing states in the electric fields.
Dielectronic recombination from a continuum of finite bandwidth has been examined down to very low electric fields, 60 mV/cm, with very high resolution. A tunable laser is used to excite atoms to the continuum of finite bandwidth, the broad
Ba6p3/211d autoionizing state, which straddles the
Ba+6p1/2 limit. Atoms which make transitions into the high-lying
Ba6p1/2nd states and radiatively decay to the bound
6snd states are detected by field ionization. The recombination, integrated over energy, has been measured for fields from 60 mV/cm to 28 V/cm, showing a peak at 0.5 V/cm, in agreement with expectation based on calculations done for other systems. The final-state distribution has been measured, showing that the outer electron remains a spectator in the radiative decay. At fields below 1.0 V/cm and binding energies less than 12 ${\mathrm{cm}}^{\ensuremath{-}1}$ below the
Ba+6p1/2 limit we find a deviation of our experimental results from the isolated resonance approximation.