Deuterium isotope effects on ultrafast polarisability anisotropy relaxation in methanol

The ultrafast polarisability anisotropy relaxation of methanol and three deuterated derivatives were recorded by means of the optically heterodyne detected optical Kerr effect. The data beyond 1 ps are non-exponential. On this timescale the effect of deuteration is to slow the relaxation, the effect being largest for exchange of the hydroxyl hydrogen. These data are shown to be consistent with recent studies of isotope effects on solvation dynamics. On the sub-picosecond timescale the effect of deuteration is small. Exchange of the hydroxyl hydrogen leads to a minor red shift in the spectral density. This confirms that the most isotope sensitive intermolecular modes of methanol lie above 300 cm−1. The data are discussed with reference to recent MD simulations of methanol.