An analysis has been made of the optical absorption edge of single-crystal trigonal selenium. Results show a distinct difference between measurements obtained with the electric-field vector perpendicular and parallel to the crystal c axis. An exciton accounts for the Urbach rule observed for E⊥c, and the results are discussed in terms of Toyozawa's theory. Indirect transitions are observed for E∥c.
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Roberts et al. (1968) studied this question.
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