Multicolor IR Spectroscopy of Pure Liquid Water

Abstract. Multicolor infrared ultrafast spectroscopy is applied to investigate the vibrational relaxation dynamics in liquid water at room temperature with both the stretching and the bending mode being photoexcited and probed. A unified model, capable of the reproduction of as much as 150 transients, yielded cross-sections and relaxation times for the stietching and bending modes. It is demonstrated, that the energy from the initially excited stretching vibration is partitioned to the bending modes of approximately two water molecules. Recent advances in femtosecond infrared (fs-IR) spectroscopy have brought about considerable progress in unravehng various molecular dynamical phenomena in water [1,2], in particular vibrational energy relaxation (VER). In neat liquid water VER is quite complex because of numerous intra- and intermolecular resonances (Fig. la): the stretching mode is resonant with the first bending overtone of the same molecule (Fermi resonance) and to two bending quanta partitioned over pairs of molecules. Furthermore, stretching and bending vibrations of different molecules are coupled via a hydrogen bond network, and their coupling t