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 mixture HDO in D₂O is demonstrated in the temperature range of 273-343 K, using two-color IR spectroscopy with tuneable subpicosecond and picosecond pulses. We derive from time-resolved spectra three major components peaked at approximately 3330 cm^-1 (I) 3400 cm^-1 (II), and 3450-3500 cm^-1 (III). In contrast to I and II, species III delays a distinct temperature dependence of position and bandwidth. The latter varies in the range 90-140 cm^-1, representing inhomogeneous broadening 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 ≈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 environments in the hydrogen-bonded network. Component I observed with decreasing amplitude up to 343 K is close to a frequency characteristic for the ice structure I_h and provides evidence for approximately tetrahedral local geometries in liquid water. From the measured cross-relaxation 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 the first excited state of the OH-stretching vibration of component II is measured to be 1.0 ± 0.2 ps at room temperature.