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Research supported in part by the Once of Naval Research. )The work presented here forms a portion of the thesis of John J. Quinn, submitted to the faculty of the University of Maryland in partial fulfillment of the requirements for the Ph. D. degree in Physics. The results have been reported at the Washington Meeting of the American Physical Society t J. J. Quinn and R.

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A theory of charged excitons X − in a dilute 2D electron gas in a high magnetic field is presented. In contrast to previous calculations, three bound X − states (one singlet and two triplets) are found in a narrow and symmetric GaAs quantum well. The singlet and a "bright" triplet are the two optically active states observed in experiments. The bright triplet has the binding energy of about 1 meV, smaller than the singlet and a "dark" triplet. The interaction of bound X − 's with a dilute 2D electron gas is investigated using exact diagonalization techniques. It is found that the short-range character of the e-X − interactions effectively isolates bound X − states from a dilute e-h plasma. This results in the insensitivity of the photoluminescence spectrum to the filling factor ν, and an exponential decrease of the oscillator strength of the dark triplet X − as a function of ν −1 .71. 35.Ji, 71.35.Ee, 73.20.Dx