LOCAL SUBSTRUCTURES OF WATER STUDIED BY TRANSIENT HOLE-BURNING SPECTROSCOPY IN THE INFRARED - DYNAMICS AND TEMPERATURE-DEPENDENCE

The spectral substructure of the OH-stretching band of the isotopic mi xture HDO in D2O is demonstrated in the temperature range of 273-343 K , using two-color IR spectroscopy with tuneable subpicosecond and pico second pulses. We derive from time-resolved spectra three major compon ents peaked at approximately 3330 cm(-1) (I), 3400 cm(-1) (II), and 34 50-3500 cm(-1) (III). In contrast to I and II, species III displays a distinct temperature dependence of position and bandwidth. The latter varies in the range 90-140 cm(-1), representing inhomogeneous broadeni ng above 290 K, as indicated by novel hole-burning observations with a hole width of 45 cm(-1) and a lifetime of the holes of approximate to 1 ps. The species I-III are also characterized by different values of the reorientational time constant in the range of 3-15 ps, depending on temperature, and are attributed to different preferred local enviro nments in the hydrogen-bonded network. Component I observed with decre asing amplitude up to 343 K is close to a frequency characteristic for the ice structure I-h and provides evidence for approximately tetrahe dral local geometries in liquid water. From the measured cross-relaxat ion among the spectral species, a structural relaxation time of 1.5-0. 8 ps is deduced in the range 273-343 K. The populational lifetime of t he first excited state of the OH-stretching vibration of component LI is measured to be 1.0 +/- 0.2 ps at room temperature.