Ultrafast anisotropy dynamics of water molecules dissolved in acetonitrile

Abstract

Infrared pump-probe experiments are performed on isolated H2 O 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 ∼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 H2 O stretching mode relaxation and therefore can compete with the intermolecular energy transfer in bulk water. © 2007 American Institute of Physics.