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Journal of Physics B Atomic Molecular and Optical Physics

Quenching of Einstein coefficients by photons

F. Aumayr, W Lee, C. H. Skinner, S. Suckewer

Published Nov 14, 1991• 4 citations
Publisher

Abstract

Experimental evidence is presented for the change of Einstein's A coefficients for spontaneous transitions from the upper laser level of an argon in laser discharge due to the presence of a high-intensity laser flux. To demonstrate that this quenching effect cannot be attributed to a reduction in self-absorption of the strong spontaneous emission line, absorption and line profile measurements have been performed. Computer modelling of the reduction of self-absorption due to Rabi splitting also indicated that this effect is too small to explain the observed quenching of spontaneous line emissions.

DOI
10.1088/0953-4075/24/21/005

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Experimental evidence is presented for the change of Einstein's A coefficients for spontaneous transitions from the upper laser level of an argon in laser discharge due to the presence of a high-intensity laser flux. To demonstrate that this quenching effect cannot be attributed to a reduction in self-absorption of the strong spontaneous emission line, absorption and line profile measurements have been performed. Computer modelling of the reduction of self-absorption due to Rabi splitting also indicated that this effect is too small to explain the observed quenching of spontaneous line emissions.
For atomic systems under strong influence of electromagnetic fields, the values of Einstein A and B coefficients have been observed to change from those of isolated atoms. However in our previous experiments which demonstrated the changes of A-coefficients, the self absorption of spontaneous emission was very strong and might the experimental result. In order to reduce the absorption drastically so that its effect can be neglected, we conducted a new experiment. In our new experiment, we measured the spontaneous emission from a very short discharge tube where the absorption was less than 4%. We also use a multichannel detector to monitor a number of spontaneous emission lines simultaneously. We observed that, as the incident photon flux increased, the intensity ratio of two spontaneous emission lines from the same upper level decreased. Any possible instrumental effect has been checked and eliminated. Therefore we can conclude that such observed decrement can only due to the change of Einstein A-coefficients.
Stimulated absorption of spontaneous emission of a gas-discharge plasma can cause a significant increase in the population of the emitting states, compared to their population determined by inelastic collisions and spontaneous decay from upper levels. By imposing a magnetic field, this self-saturation is reduced and its contribution to the level population can be identified. The magnetic-field-dependent changes in Doppler profiles due to radiative transitions caused by saturating spontaneous emission are analyzed in the case of absorption of a weak monochromatic wave.
New interesting effects arise, when three-level atoms interact with the plasma, laser field, or a heat bath. If the atoms inside the plasma are excited by the polarized laser pulse or an electron beam, the line wings of the radiation are completely polarized, while the line core is only partially polarized. The Einstein coefficients are generalized to the case, when the atomic decay channels are subject to quantum interference. Such atoms may develop nonzero quantum coherence between upper levels, if they are in contact with the heat bath in thermal equilibrium. This coherency changes the emissivity of the medium and hence can be experimentally observed. In the case when three-level atoms interact with the laser field, spontaneous emission can be suppressed over the whole frequency spectrum.
Abstract The Einstein A and B coefficients for atom–photon reactions are defined: they describe absorption, stimulated emission, and spontaneous emission. We calculate B and consequently find A by the Einstein trick. The procedure is validated by application of detailed balance. Separating out frequencies permits introduction of the spectral lineshape function (normalized line profile). A reformulation describes transitions involving single atomic states and single field modes. This points to a link with quantum electrodynamics.