Abstract
The authors show from a comparison of optical absorption and emission spectra of high-quality single crystals that the characteristic narrow doublet in the edge luminescence is produced by the decay of excitons bound to neutral acceptors from the J=5/2 and J=3/2 exciton states, with the former lying lowest. The further J=1/2 bound exciton state expected on a j-j coupling model is now firmly identified with a weak higher-energy line previously reported for both materials. These assignments are consistent with the more positive of the conclusions obtained from Zeeman analysis of these transitions, although some less certain features conflict.