Measurements of transient photoinduced optical absorption in $a$-${\mathrm{As}}_2$${\mathrm{Se}}_3$ show that carriers thermalize within a distribution of localized states in the band gap. Photocurrent as well as photoinduced optical absorption transients are explained in a unified way with the multiple-trapping mechanism. Under such circumstances, transient photoconductivity provides a spectroscopy of the density of localized states.

• Received 19 August 1980

DOI:https://doi.org/10.1103/PhysRevLett.46.1421