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Physical Review A

Resonant enhancement of dielectronic recombination by a high-frequency microwave field

E. S. Shuman, Thomas F. Gallagher

Published Mar 24, 2005• 0 citations
Publisher

Abstract

Dielectronic recombination via the Ba $6{p}_{3∕2}n\ensuremath{\ell}$ states from a continuum of finite bandwidth is resonantly enhanced by a 38.8-GHz microwave field at the energies for which the $\ensuremath{\Delta}n=1,\phantom{\rule{0.2em}{0ex}}2,\phantom{\rule{0.2em}{0ex}}\dots{},\phantom{\rule{0.2em}{0ex}}k$ transitions of the intermediate $6{p}_{1∕2}n\ensuremath{\ell}$ states are resonant with the microwave frequency. The mechanism is resonant Stark mixing of high and low $\ensuremath{\ell}$ states by the microwave field. While static field enhancement of dielectronic recombination occurs at fields scaling as ${n}^{\ensuremath{-}5}$, the field required for enhancement of the $\ensuremath{\Delta}n=k,\phantom{\rule{0.2em}{0ex}}k\ensuremath{\geqslant}2$ resonances is roughly $n$ independent.

DOI
10.1103/physreva.71.033415

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Dielectronic recombination via the Ba 6p3∕2n\ensuremathℓ6{p}_{3∕2}n\ensuremath{\ell}6p3∕2​n\ensuremathℓ states from a continuum of finite bandwidth is resonantly enhanced by a 38.8-GHz microwave field at the energies for which the \ensuremathΔn=1,2,…,k\ensuremath{\Delta}n=1,\phantom{\rule{0.2em}{0ex}}2,\phantom{\rule{0.2em}{0ex}}\dots{},\phantom{\rule{0.2em}{0ex}}k\ensuremathΔn=1,2,…,k transitions of the intermediate 6p1∕2n\ensuremathℓ6{p}_{1∕2}n\ensuremath{\ell}6p1∕2​n\ensuremathℓ states are resonant with the microwave frequency. The mechanism is resonant Stark mixing of high and low \ensuremathℓ\ensuremath{\ell}\ensuremathℓ states by the microwave field. While static field enhancement of dielectronic recombination occurs at fields scaling as n\ensuremath−5{n}^{\ensuremath{-}5}n\ensuremath−5, the field required for enhancement of the \ensuremathΔn=k,k\ensuremath⩾2\ensuremath{\Delta}n=k,\phantom{\rule{0.2em}{0ex}}k\ensuremath{\geqslant}2\ensuremathΔn=k,k\ensuremath⩾2 resonances is roughly nnn independent.
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.
We have observed dielectronic recombination of Ba+{\mathrm{Ba}}^{+}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 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.
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 ${\mathrm{cm}}^{\ensuremath{-}1}$ 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.
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.
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.