We have observed fluorescence line narrowing in the LO-phonon-assisted intrinsic exciton emission of $\mathrm{Cd}{\mathrm{S}}_x{\mathrm{Se}}_{1-x}$, selectively excited over a range of energies below a critical energy $E_{\mathrm{em}}$, on the low-energy side of the exciton absorption line. The sharp emission lines disappear rather abruptly as the exciting energy increases through $E_{\mathrm{em}}$ or as the temperature is raised above 10 K. We argue that this sharp line emission arises from excitons localized, on the time scale of the radiative lifetime (about 1 nsec), by alloy fluctuations. Its abrupt disappearance may be evidence for an effective mobility edge. A sideband on each of the sharp LO-phonon-assisted emission lines is also observed. The shape of this sideband can be accounted for by a model of spectral diffusion by acousticphonon-assisted tunneling between localized states.

• Received 4 November 1981

DOI:https://doi.org/10.1103/PhysRevB.25.3828