Deuterium isotope effect on the solvation dynamics of a dye molecule in methanol and acetonitrile

Solvation dynamics of a dye molecule in methanol, acetonitrile, and their deuterated analogs has been measured by the three-pulse stimulated photon echo peak shift. The effects of deuterium substitution on the stationary fluorescence spectra and the time domain data are found to be significant in both solvents. Moreover, the deuterium isotope effects are contrasted in methanol and acetonitrile. The ultrafast component, found to be 200-260 fs, is not slowed down by the deuterium substitution of the hydroxyl group in methanol, and it is even slightly shortened by deuteration in acetonitrile. The isotope dependence and the time scales suggest that the ultrafast component is not originated from the inertial free rotation of the solvent molecules but from collective dispersive solvent motion, at least for the present system. In addition to the ultrafast and ps solvation time scales usually observed in methanol and acetonitrile, a large amplitude ns component is observed in all solvents. The amplitudes of the ns component are substantially different in isotopomers, showing the opposite trend in methanol and acetonitrile. (C) 1999 American Institute of Physics. [S0021-9606(99)51722-4].