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Physical Review A

Enhancement of dielectronic recombination by an electric field

Lung Ko, Victor Klimenko, Thomas F. Gallagher

Published Mar 1, 1999• 18 citations
Publisher

Abstract

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 $\mathrm{Ba} {6p}_{3/2}11d$ autoionizing state, which straddles the ${\mathrm{Ba}}^{+} {6p}_{1/2}$ limit. Atoms which make transitions into the high-lying $\mathrm{Ba} {6p}_{1/2}\mathrm{nd}$ 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 ${\mathrm{Ba}}^{+} {6p}_{1/2}$ limit we find a deviation of our experimental results from the isolated resonance approximation.

DOI
10.1103/physreva.59.2126

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Physical Review A•Apr 29, 2003•1 citation
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\mathrm{Ba} {6p}_{3/2}11dBa6p3/2​11d autoionizing state, which straddles the Ba+6p1/2{\mathrm{Ba}}^{+} {6p}_{1/2}Ba+6p1/2​ limit. Atoms which make transitions into the high-lying Ba6p1/2nd\mathrm{Ba} {6p}_{1/2}\mathrm{nd}Ba6p1/2​nd states and radiatively decay to the bound 6snd6snd6snd 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 cm\ensuremath−1{\mathrm{cm}}^{\ensuremath{-}1}cm\ensuremath−1 below the Ba+6p1/2{\mathrm{Ba}}^{+} {6p}_{1/2}Ba+6p1/2​ limit we find a deviation of our experimental results from the isolated resonance approximation.
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.
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\mathit{B}B field coupling must exceed the radiative rate, the autoionization rate, and the Stark splitting of the intermediate autoionizing Rydberg states.
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 nln\mathcal{l}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+{\mathrm{Ba}}^{+}Ba+ 6p1/2{6p}_{1/2}6p1/2​ limit.
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.
The presence of an electric field converts the intermediate autoionizing Rydberg $n\ensuremath{\ell}$ states, through which dielectronic recombination passes, into nknknk 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∕28g6{p}_{3∕2}8g6p3∕2​8g continuum of finite bandwidth via the intermediate 6p1∕2ng6{p}_{1∕2}ng6p1∕2​ng 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.
The presence of a static electric field substantially enhances the dielectronic recombination (DR) rate below the classical field ionization limit and suppresses DR entirely above it. The addition of a small microwave field polarized orthogonally to the static field leads to a substantial DR signal above the classical ionization limit. We have examined DR from a continuum of finite bandwidth in static fields from 0 to 130 V/cm and with 8--11 GHz microwave fields of amplitudes up to 5 V/cm. We attribute the microwave-induced DR above the classical limit to the transfer of atoms by the microwave field from autoionizing the m=0m=0m=0 Rydberg intermediate states to higher m states. Since higher m states are more stable against field ionization, these states are more likely to decay radiatively to stable bound Stark states, thereby completing DR. This m-changing process most likely occurs by resonantly driving the $\ensuremath{\Delta}n=0,$ $\ensuremath{\Delta}m=1$ transitions of the autoionizing Rydberg-Stark states with the microwave field.