Time-Resolved Infrared Spectroscopy of Water. Relation between the OH Stretching Frequency and the OO Distance

It is shown how infrared pump-probe spectroscopy can be used to measure subpico second variations of the oxygen-oxygen distribution function in liquid water. A diluted solution HDO/D2O rather than pure H2O is considered to switch off resonant vibrational interactions between water molecules; the local structure remains unchanged in this substitution. The present study is limited to times superior to 100–200 fs. This permits to avoid problems generated by hard sphere type collisions between water mole ules, as well as the interference between ultrafast pump and probe pulses. It is then shown that the Novak-Mikenda type relations between the OH stretching frequency and the OO distance largely sur vive when going from standard to ultrafast infrared spectroscopy. More over, the infrared pump-probe profiles of OH stretching bands closely parallel the oxygen-oxygen distribution functions in this time do main. Infrared pump-probe spectroscopy is thus a use ful substitute of time-resolved X-ray diffraction in this exceptional case.