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Sharp luminescence lines from indirect-free-exciton decay are observed in selenium. These permit for the first time an unambiguous identification of the indirect gap. A unique feature of the band structure and symmetry is the existence of two almost degenerate types of excitons with different wave vectors, leading to a superposition of two different indirect-exciton luminescence spectra.
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B. Moreth (1979) studied this question.
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