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Journal of Physics B Atomic Molecular and Optical Physics

The effects of dissipation of plasmas on the spontaneous radiation intensity of ionized atoms

Chang-qi Cao, Hui W. Cao

Published Nov 14, 1993• 5 citations
Publisher

Abstract

The effect of dissipation of plasmas on the spontaneous radiation of an ionized atom is investigated by Langevin equations and the modified Weisskopf-Wigner approximation to consider the quenching effect of the Einstein A coefficient. The dissipation of field in plasmas results from inverse bremsstrahlung of free electrons and absorption of the ions of the same kind as the radiator. Under certain conditions the Einstein A coefficient may be reduced significantly. The lifetime of the excited ion will be changed correspondingly.

DOI
10.1088/0953-4075/26/21/028

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The effect of dissipation of plasmas on the spontaneous radiation of an ionized atom is investigated by Langevin equations and the modified Weisskopf-Wigner approximation to consider the quenching effect of the Einstein A coefficient. The dissipation of field in plasmas results from inverse bremsstrahlung of free electrons and absorption of the ions of the same kind as the radiator. Under certain conditions the Einstein A coefficient may be reduced significantly. The lifetime of the excited ion will be changed correspondingly.
Spontaneous radiation by three-level atoms in a dissipative medium (such as plasma) has been investigated by Langevin equations and the modified Weisskopf-Wigner approximation. General formulae for calculating spontaneous emission coefficients, i.e. Einstein A coefficients, are presented. Numerical calculation shows that the dissipation of plasmas can modify the spontaneous emission coefficients significantly and causes a pit in the coefficient-density curves for relatively low temperature plasmas, and also a pit in the coefficient-temperature curves.
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.
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.
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.
An investigation is made of the effect on the dynamics of spontaneous emission of the Wigner-Weisskopf atom in an infinite-system limit of various choices of the coupling function or form factor describing the atom's interaction with the spectrum of the radiation field. This is carried out both for the exact solution to the problem of spontaneous emission, obtained in the earlier papers in the series, and for some approximate solutions, also previously considered, in particular one based on the Schrödinger equation of the problem and one based on the weak-coupling Prigogine-Résibois master equation. The details of the form factor are found, by numerical computation of the solutions, to be critical in determining the nonexponential parts of the solutions, and these parts are seen to be capable in some cases of dominating the exponential parts, which are given only by the values of the form factor near the resonance energy. The approximate solutions discussed are found to vary widely in their worth, and one, which yields the exact solution for the Wigner-Weisskopf problem, is singled out as being of probable use in the statistical-mechanical description of more complicated systems.