This paper describes the results of a theoretical and experimental investigation of the tensor dielectric susceptibility of a plasma in which some of the participating atoms exhibit population inversion. The measurement of the real part of the tensor dielectric susceptibility in the vicinity of an optical resonance has previously been very difficult due to the large absorption near resonance; however, by examining a resonance for which the levels exhibit population inversion we have obtained a direct measure of the real part of the susceptibility by observing the "negative" Faraday rotation, i.e., "negative" circular birefringence associated with the resonance. We also observed for levels with inverted populations the complement of the inverse Zeeman effect, that is, we observed a Zeeman splitting of the gain curve in the amplifying medium. The high-gain $5d\left[\frac{3}{2}\right]_1^{}{}_{}{}^0-6p{\left[\frac{3}{2}\right]}_1$ transition of atomic xenon at 2.026 μ in a helium-xenon mixture was used for these measurements. The observed results were consistent with the theory.

• Received 5 November 1962

DOI:https://doi.org/10.1103/PhysRev.129.2577