Abstract
The photoluminescence excitation spectra in some arsenic chalcogenide semiconductors have been explored to low energies, from which absorption coefficients down to 10−1 cm−1 can be derived. New evidence is presented for a weak absorption tail in both crystals and glasses, and the data are discussed in terms of an optical transition involving a localized defect state. It is shown that the data support a recent model in which the photoluminescence recombination centre is a shallow state that has a strong electron-phonon coupling.