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We have measured optically detected cyclotron resonance using farinfrared radiation on an exceptionally pure sample of GaAs in fields up to 15.5 T. This relatively new experimental technique is shown to offer high resolution of free and donor impurity-bound electron transitions without the reproducibility problems of photoconductivity. The data confirm the existence of metastable donor states and provide a detailed picture of chemical shifts. The optically detected cyclotron resonance signal represents an interaction between the donor bound electron states which are influenced by the far-infrared radiation and the donor bound exciton states which are responsible for the photoluminescence. Attenuation of the luminescence intensity under far-infrared illumination is primarily the result of a photothermal effect. At high fields, there is indication of an interaction between the electron and excitonic energy levels.

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