Structure of hydrogen bonds in liquid water at subpicosecond time scales

A theory is presented showing how infrared pump–probe spectroscopy can be used to measure subpicosecond variations of the oxygen–oxygen distribution function in liquid water, ΔgOO(R, t). For times t > tc, where tc > 100 − 200 fs is the pump–probe time delay, the profiles of OH stretching bands of HDO/D2O can be brought into a full coincidence with ΔgOO(R, t). Infrared pump–probe spectroscopy thus plays the role of time-resolved X-ray diffraction in this particular case. This method has many advantages. Not only it permits to obtain structural data concerning the OH⋯ O bonds at subpicosecond time scales, but also is non destructive in its essence; this is particularly important when studying biological water. Finally, pump–probe spectroscopy is very economical as compared with time-resolved X-ray techniques. Although theoretical, the present work may have an important impact in experimental biology.