FAR-INFRARED STUDY OF LOW-FREQUENCY VIBRATIONAL-STATES IN AS2S3 GLASS

Far-infrared absorption and reflection spectra of As2S3 glass have been measured in the frequency range 10-60 cm-1 at room temperature. The absorption coefficient alpha(v) has been determined as a function of frequency v and found to deviate from the quadratic frequency dependence. A broad absorption peak corresponding to a boson peak is clearly found at about 27 cm-1. Infrared coupling coefficient C(ir)(v) is determined experimentally from alpha(v) and the density of vibrational states previously reported. It is found that C(ir)(v) shows the frequency dependence similar to that of the Raman coupling coefficient C(r)(v) obtained in the same manner for As2S3 glass and is not described by the Martin-Brenig model. Both C(ir)(v) and C(r)(v) show no peak and the frequency dependence close to that predicted by a fractal model. The far-infrared absorption peak may be interpreted as a result of the phonon-fraction crossover.