We report the enhancement of dielectronic recombination from a continuum of finite bandwidth in combined electric and magnetic fields. Specifically, we have examined the process Ba $6{p}_{3/2}8g\ensuremath{\rightarrow}6{p}_{1/2}ng\ensuremath{\rightarrow}6{s}_{1/2}ng+h\ensuremath{\nu}$ for $32\ensuremath{\le}n\ensuremath{\le}47$ in combined fields ranging from 0 to 7 V/cm and 0 to 250 G. The results demonstrate that it is the component of $B\ensuremath{\perp}E$ that leads to enhancement. The experimental results indicate that the
B field coupling must exceed the radiative rate, the autoionization rate, and the Stark splitting of the intermediate autoionizing Rydberg states.
Dielectronic recombination (DR) from a continuum of finite bandwidth in Ba atoms has been studied in electric fields of 0--5 V/cm and magnetic fields of 0--240 G for both $\stackrel{\ensuremath{\rightarrow}}{B}\ensuremath{\parallel}\stackrel{\ensuremath{\rightarrow}}{E}$ and $\stackrel{\ensuremath{\rightarrow}}{B}\ensuremath{\perp}\stackrel{\ensuremath{\rightarrow}}{E}.$ In electric fields of \ensuremath{\geqslant}0.1 V/cm, the presence of a perpendicular magnetic field increases the integrated DR rate by up to 60%, with the peak enhancement moving towards higher magnetic fields as the electric field is increased. The general behavior of the enhancement for $\stackrel{\ensuremath{\rightarrow}}{B}\ensuremath{\perp}\stackrel{\ensuremath{\rightarrow}}{E}$---as well its absence for $\stackrel{\ensuremath{\rightarrow}}{B}\ensuremath{\parallel}\stackrel{\ensuremath{\rightarrow}}{E}$ is in agreement with theoretical expectations. A simple model is presented to provide insight into the results, and reasons for its shortcomings are presented.
A small electric field is shown to increase the dielectronic recombination, via autoionizing Rydberg states, of an electron from a continuum of finite bandwidth. The continuum of finite bandwidth is a broad autoionizing state which is part of a series converging to a higher limit, and the field enhancement of the rate occurs because the field converts the nl Rydberg states to nk Stark states, increasing the number of contributing recombination paths. The experimental results are in excellent agreement with the predictions of an isolated resonance approximation treatment and show clearly both the positive effect of Stark mixing and the negative effect of field ionization on dielectronic recombination.
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.
We have observed dielectronic recombination of
Ba+ and ${e}^{\ensuremath{-}}$ from a continuum of finite bandwidth in the presence of microwave fields of frequencies 8.08 and 12.05 GHz and amplitudes of up to 2 V/cm. There are sharp resonant enhancements when the microwave frequency matches the $\ensuremath{\Delta}n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1$, 2, and 3 resonances of the intermediate autoionizing Rydberg states, and we attribute the enhancements to resonant microwave Stark $\ensuremath{\ell}$ mixing. The microwave field provides a simple and powerful way to enhance the recombination rate for incident electrons of a specific energy.
The presence of an electric field converts the intermediate autoionizing Rydberg $n\ensuremath{\ell}$ states, through which dielectronic recombination passes, into
nk Stark states, which have autoionization and capture rates in excess of the radiative decay rates and contribute to dielectronic recombination. In zero field the high $\ensuremath{\ell}$ states do not contribute to dielectronic recombination, but the conversion to Stark states makes it possible and raises the dielectronic recombination rate. However, an electric field can also result in coupling to loss channels which locally reduce the dielectronic recombination rate. We have observed holes in the spectrum of dielectronic recombination from the Ba
6p3∕28g continuum of finite bandwidth via the intermediate
6p1∕2ng states. The holes appear when an electric field is applied, and we attribute them to interaction with rapidly decaying $6{p}_{3∕2}8\ensuremath{\ell}$ states, which diverts flux from dielectronic recombination.