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Journal of Physics B Atomic Molecular and Optical Physics

Stimulated radiative recombination of and

Michael Rogelstad, Farook Bashir Yousif, T. J. Morgan, J. B. A. Mitchell

Published Sep 14, 1997• 22 citations
Publisher

Abstract

Stimulated radiative recombination has been demonstrated experimentally in and collisions using a merged electron - ion beams apparatus with field ionization detection of the excited neutral products. Enhancement of the recombination over spontaneous recombination to form the n=11, 12 and 13 levels of atomic hydrogen and the n = 11 and 12 levels of atomic helium by factors of between 1000 and 3000 have been found using a laser power of 8 W. Evidence for the resolution of fine-structure levels has been seen for the case of helium.

DOI
10.1088/0953-4075/30/17/018

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Stimulated radiative recombination has been demonstrated experimentally in and collisions using a merged electron - ion beams apparatus with field ionization detection of the excited neutral products. Enhancement of the recombination over spontaneous recombination to form the n=11, 12 and 13 levels of atomic hydrogen and the n = 11 and 12 levels of atomic helium by factors of between 1000 and 3000 have been found using a laser power of 8 W. Evidence for the resolution of fine-structure levels has been seen for the case of helium.
The problem of stimulated recombination at a resonance is investigated by a time-dependent, non-perturbative approach. The number of pairs recombined by a laser pulse is computed as a function of the intensity, of the pulse duration, and of the initial electron-energy distribution (as well as of the parameters which characterize the atomic resonance and its field-induced coupling to the final bound state). The role of spontaneous emission (or other paths) from the recombined state (if it is excited) is also discussed. Numerical results are given including an example of recombination to the ground state in the Sr atom.
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.
By photoionizing samples of laser-cooled atoms with laser light tuned just above the ionization limit, plasmas can be created with electron and ion temperatures below 10 K. These ultracold neutral plasmas have extended the temperature bounds of plasma physics by two orders of magnitude. Table-top experiments, using many of the tools from atomic physics, allow for the study of plasma phenomena in this new regime with independent control over the density and temperature of the plasma through the excitation process. Characteristic of these systems is an inhomogeneous density profile, inherited from the density distribution of the laser-cooled neutral atom sample. Most work has dealt with unconfined plasmas in vacuum, which expand outward at velocities of order 100 m/s, governed by electron pressure, and with lifetimes of order 100 μs, limited by stray electric fields. Using detection of charged particles and optical detection techniques, a wide variety of properties and phenomena have been observed, including expansion dynamics, collective excitations in both the electrons and ions, and collisional properties. Through three-body recombination collisions, the plasmas rapidly form Rydberg atoms, and clouds of cold Rydberg atoms have been observed to spontaneously avalanche ionize to form plasmas. Of particular interest is the possibility of the formation of strongly coupled plasmas, where Coulomb forces dominate thermal motion and correlations become important. The strongest impediment to strong coupling is disorder-induced heating, a process in which Coulomb energy from an initially disordered sample is converted into thermal energy. This restricts electrons to a weakly coupled regime and leaves the ions barely within the strongly coupled regime. This review will give an overview of the field of ultracold neutral plasmas, from its inception in 1999 to current work, including efforts to increase strong coupling and effects on plasma properties due to strong coupling.
Abstract We present a theoretical study of energetic electron-beam excitation of ‘six-level helium atom’ embedded in a laser field. The angular distribution of the scattered electron-beam, which contains all information about the combined interactions, is analyzed and exhibits a resonant peak in the vicinity of the 1 sns – 1 snp ( n = 2, 3) transitions. Here, we show that this resonant feature can be stimulated by means of the transferred momentum between the collision partners during the scattering process. By manipulating the latter parameter, one can enhance the resonant peak or lead to its attenuation and even to its cancellation. Our findings are supported by additional calculations based on the time-dependent perturbation theory, where a spurious divergence is expected at a resonant region.