Ultrafast anisotropy dynamics of water molecules dissolved in acetonitrile

Infrared pump-probe experiments are performed on isolated H2O molecules diluted in acetonitrile in the spectral region of the OH stretching vibration. The large separation between water molecules excludes intermolecular interactions, while acetonitrile as a solvent provides substantial hydrogen bonding. Intramolecular coupling between symmetric and asymmetric modes results in the anisotropy decay to the frequency-dependent values of similar to 0-0.2 with a 0.2 ps time constant. The experimental data are consistent with a theoretical model that includes intramolecular coupling, anharmonicity, and environmental fluctuations. Our results demonstrate that intramolecular processes are essential for the H2O stretching mode relaxation and therefore can compete with the intermolecular energy transfer in bulk water. (c) 2007 American Institute of Physics.