Ultrafast vibrational dynamics of liquid water

Multidimensional IR spectroscopy has the power to demystify molecular dynamics in the liquid phase. It allows a glimpse beneath the broadened spectral lineshapes of molecular liquids giving insight into the internal mechanisms of energy transfer and decoherence. An excellent candidate of this technique is liquid H2O, whose importance to chemistry and biology cannot be understated. Little is known about the time resolved dynamics of this liquid and the hydrogen bonding network which is responsible for its anomalous properties. This lack of previous experimental work is due to the inherent difficulty of performing ultrafast studies on the vibrational transitions of H2O in the condensed phase. In this work, two-dimensional infrared photon echo measurements of the OH stretching vibration in liquid water are performed at various temperatures. The temperature dependence of energy dynamics is of particular interest because it can isolate the effect of the hydrogen bonding network on the intermolecular dynamics. New insight into the hydrogen bonding network are revealed by the spectroscopic behaviour of this hydrogen bonded liquid. It is found that within the pure 'liquid spectral diffusion and resonant energy transfer occur on a time scale much shorter than the average hydrogen bond lifetime. Room temperature measurements show a loss of frequency and, thus, structural correlations on a 50 fs timescale. Weakly hydrogen bonded OH stretching oscillators absorbing at high frequencies undergo slower spectral diffusion than strongly bonded oscillators. With decreasing temperature the loss in memory slows down. Near freezing the frequency correlations in the OH stretch vibration persist beyond sim; 200 fs, pointing to a reduction in dephasing by librational excitations. Polarization resolved purnp-probe studies give a resonant intermolecular energy transfer time of 80 fs which is unaffected by temperature. At low temperature, structural correlations persist longer than the energy transfer time, suggesting new evidence for a delocalization of OH stretching excitations over many water molecules and exciton-like behaviour for the primary excitation of water, a distinctly quantum mechanical effect.