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Physical Review A

Experimental and theoretical investigations of the optogalvanic effect in a Hg-Ar discharge

Wayne H. Richardson, Lute Maleki, E. Garmire

Published Dec 1, 1987• 11 citations
Publisher

Abstract

The optogalvanic effect induced in a Hg-Ar discharge by light at 546.1 nm is studied experimentally and theoretically. The plasma and its response to a disturbance in the atomic-level populations are described entirely in terms of electron collision cross sections and other fundamental parameters. In a detailed description of the laser interaction with the atomic system, it was necessary to take into account velocity-changing collisions (VCC). These collisions (VCC) strongly modify the interaction between the Doppler-broadened system and the optical field. In some Doppler-free optogalvanic spectra the dramatic influence of VCC on the velocity distribution of atomic-level population is evident. Measurements of the magnitude of the optogalvanic signal confirm most of the theoretical results. Some anomalous features of the impedance change due to a weak beam were observed when a strong counterpropagating beam was introduced.

DOI
10.1103/physreva.36.5713

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The optogalvanic effect induced in a Hg-Ar discharge by light at 546.1 nm is studied experimentally and theoretically. The plasma and its response to a disturbance in the atomic-level populations are described entirely in terms of electron collision cross sections and other fundamental parameters. In a detailed description of the laser interaction with the atomic system, it was necessary to take into account velocity-changing collisions (VCC). These collisions (VCC) strongly modify the interaction between the Doppler-broadened system and the optical field. In some Doppler-free optogalvanic spectra the dramatic influence of VCC on the velocity distribution of atomic-level population is evident. Measurements of the magnitude of the optogalvanic signal confirm most of the theoretical results. Some anomalous features of the impedance change due to a weak beam were observed when a strong counterpropagating beam was introduced.
The phase-conjugate signal observed in resonant degenerate four-wave mixing on the 6/sup 3/P/sub 2/ to 7/sup 3/S/sub 1/ transition of atomic mercury in a Hg-Ar discharge is investigated. At a fixed argon pressure the variation of the signal with pump powers is explained by a model that includes the effects of velocity-changing collisions (VCCs). As the argon pressure was varied from 0 to 1 torr an increase in the phase conjugate signal was observed and is ascribed to a change in the discharge dynamics with argon pressure and to the influence of VCCs. To further clarify the role of collisions and optical pumping, degenerate four-wave mixing spectra are examined as a function of pump power. Line shapes are briefly discussed.>
The authors present an investigation of the modification caused by an intense laser field to the rates of m J changing collisions for Hg perturbed by argon. They have studied experimentally the competition between radiative and collisional coupling between m J states with a strong laser field tuned to the Hg J=1 to J=1 transition at lambda 435.8 nm. Population is pumped into the strongly dressed manifold from the ground state with a weak UV 'probe' laser detuned slightly from the lambda 253.7 nm intercombination line. The fluorescence observed at lambda 404.7 nm depends critically on whether the excitation proceeds via collisional redistribution or resonance through Stark shifted levels. The authors present data that clearly exhibit the effects of this competition. The data show in particular how one can produce excitation to transient Stark shifted levels in the presence of collisions. This will be critical in future experiments on collisions in strong fields.
Reports the optogalvanic detection of observables induced by optical pumping in metastable states of neon. As with optical methods signals are obtained which are related to electronic and nuclear orientation or alignment. Many optical pumping experiments may thus be performed using optogalvanic techniques, including the Hanle effect and 'velocity selective optical pumping' described. Different mechanisms are discussed from which optogalvanic signals related to non-scalar observables may originate. Profiles obtained from frequency sweeps of two polarisation-modulated counterpropagating pump beams indicate the dominant mechanism is a second-order pumping effect which induces a modulated depletion of the metastable state. No effect has been observed arising from a variation of the Penning collision cross section when the two metastable colliding atoms are oriented. The analogy between signals produced by optogalvanic and optical methods are pointed out.
Experimental and theoretical investigation of the coherent propagation and amplification of polychromatic broadband laser pulses in optically dense resonant media without population inversion was carried out for the cases of plasma in a Ne glow discharge and a pulse discharge in Hg-Ar mixture. High density of Ne and Hg atoms in the metastable state (about 1012 cm-3), which was chosen to be the ground state of the two-level system under consideration, leads to the collective behavior of the atomic system in resonant electromagnetic field. The broadband probe field attenuation and the sideband amplification of the probe under the action of the coherent pump were observed. The amplification dependence on the pump intensity was obtained. The model of the parametrically enhanced collective interactions was developed and comparison the experimental data was made.