The CN- vibration and the bend/libration combination mode of water in the 2100 cm-1 spectral region have been investigated by three pulse heterodyned echo experiments. The 2D-IR spectra of both systems show that they undergo spectral diffusion on ultrafast time scales. For the CN- vibration, peak-shift measurements and the shape changes of the correlation spectra provide information on the fast local environment fluctuations which mimic the structure fluctuations of the solvation shell. The main spectral diffusion is sub-picosecond. For neat water, a rapid decay of the echo signal is observed on a 150fs coherence time scale. The inhomogeneous frequency distribution of the combination mode is found to disappear in less than 100fs. The heterodyne grating signal of neat water driven at different frequencies shows evolution that elucidates the underlying dynamics of the combination mode of neat water. An approximately linear relationship between the index change and the grating efficiency in the probed regions was found. The relaxation of the combination mode leads to a faster heating of the water compared with excitation of the background absorbing states in a similar frequency region. The grating diffraction maximizes before the deposited energy is uniformly distributed.

Acknowledgements: Supports from NSFCHE and NIH RR01348.