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 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.
We report the results of a study of the effects of combined electric and magnetic fields on dielectronic recombination (DR) from a continuum of finite bandwidth. Specifically, we have examined the process $\mathrm{Ba}\phantom{\rule{0.3em}{0ex}}6{p}_{3∕2}8g\ensuremath{\rightarrow}\mathrm{Ba}\phantom{\rule{0.3em}{0ex}}6{p}_{1∕2}nk\ensuremath{\rightarrow}\mathrm{Ba}\phantom{\rule{0.3em}{0ex}}6{s}_{1∕2}nk+h\ensuremath{\nu}$ in the presence of electric fields from
0to7V∕cm and magnetic fields from
0to250G. Our observations elucidate the requirements for magnetic field enhancement of the DR rate. In particular, they demonstrate that the magnetic coupling must be comparable not only to the electric field splitting of the intermediate autoionizing Rydberg states, but also to their decay rates.
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.
Dielectronic recombination of Ba from a continuum of finite bandwidth in microwave fields of 4, 8, and 12 GHz exhibits a resonant enhancement when the microwave frequency matches a $\ensuremath{\Delta}n=1,$ 2, or 3 transition in the intermediate autoionizing
nl Rydberg states. The origin of the enhancement is resonant Stark mixing of high- and low-l states of different n. Surprisingly, there is little difference between linear and circular polarization of the microwave field, which is attributed to the fact that more than two n levels are coupled in both cases. The dependence of the widths of the resonances on the microwave field is related to how many n states are coupled, and the threshold field for the appearance of the resonances is due to the requirement that low-l states with nonzero quantum defects be mixed with the hydrogenic high-l states. Finally, recombination has been observed from energies in excess of the
Ba+ 6p1/2 limit.
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.