Vibrational and reorientational dynamics of water molecules in liquid matrices

Polarization-resolved double-resonance spectroscopy with tunable picosecond pulses in the infrared is used to study the vibrational and reorientational relaxation of monomeric water molecules in various organic solvents. After excitation of the antisymmetric OH stretching vibration ν₃ a rapid population redistribution with the neighboring stretching vibration ν₁ is observed, followed by a much slower and strongly solvent-dependent population decay of this vibrational ensemble. The findings are explained by energy transfer to the overtone 2ν₂ of the bending mode and the influence of weak hydrogen bonds between water and solvent molecules. The latter govern the energy difference between the initial (ν₁) and final (2ν₂) state of the relaxation process. A theoretical discussion of polarization effects is presented taking into account the varying directions of the transition dipole moments of different modes in the molecular coordinate frame. It is shown that measurements of the induced dichroism facilitate the interpretation of our transient infrared spectra and allow the determination of reorientational relaxation times. The reorientational time constants also strongly depend on the weak hydrogen bonds between solute and solvent molecules.