Vibrational relaxation of a triatomic molecular impurity: D2O in vitreous As2S3

Measurements of the relaxation of the D2O stretch mode in vitreous As2S3 are presented. Because the bending mode of the molecule offers an intra-molecular decay channel for the stretch mode, the decay scheme of the D2O molecule is more complex than that of diatomic molecules. The asymmetric stretch mode of D2O has a frequency of 2680 cm-1. To study the relaxation of this mode we applied a pump-probe technique, using intense psec; pulses of the Stanford Free Electron Laser. Due to the small cross-section of the vibrational mode, successful efforts were made to improve the signal to noise ratio by using a laser stabilization system and a tightly focused beam to increase the intensity, by averaging the signal with a kHz repetition rate and by using samples with an optimized D2O concentration. A rapid relaxation rate on the order of 5 x 109 sec-1 at low temperature is found that increases with temperature. Recalling that the bending mode of the D2O molecule has a frequency of 1170 cm-1, one would expect a decay in a third order process, involving two quanta of the bending mode plus a vibrational host quanta with a frequency of 340 cm-1, which coincides with a fundamental frequency of the pyramidal building blocks of the glassy As2S3 host. Instead, we find from the temperature dependence of the relaxation rate that the D2O stretching mode relaxes in a higher order process. This indicates that the relaxation dynamics of small molecules is more complex than generally assumed