Anomalous Isotope Effect in the Relaxation of the SD and SH Vibrational Stretch Modes in Amorphous As2S3

The vibrational relaxation rate of the SD stretching mode in amorphous A${\mathrm{s}}_2$ ${\mathrm{S}}_3$ is found to be comparable to that of the SH molecule in the same material. This result is in marked contrast to the behavior expected for multiphonon decay into vibrational quanta of the amorphous host. The lack of an isotope effect in the relaxation rate is shown to be due to an efficient decay channel into a high frequency local vibrational mode of the impurity molecule. Our results demonstrate that local modes cannot only influence but can dominate the decay of molecular impurities in solids.