RAMAN-SPECTRUM OF WATER - TRANSVERSE AND LONGITUDINAL ACOUSTIC MODES BELOW ALMOST-EQUAL-TO 300 CM-1 AND OPTIC MODES ABOVE ALMOST-EQUAL-TO 300 CM-1

Liquid water behaves like a moderately rigid, isotropic, elastic solid at terahertz (1012 Hz) frequencies. The nominal 60-and 175-cm-1 Raman peaks, ≈1.8 and ≈5.3 THz, correspond to transverse spherical acoustic shear, S, and longitudinal spherical acoustic dilatational, P, waves, respectively, and involve H2O center-of-mass motions. The ≥8-9 Å shear wavelength corresponds to the high-distance tail of the O-third-O (or O-fourth-O) X-ray RDF peak - the structural correlation length (SCL), where G(r) ≈ 1. The ≈ 5-Å dilatational wavelength corresponds approximately to the density-related O-second-O distance. Optic and multiphonon modes above ≈330 cm-1 mainly involve proton motion. This acoustic-optic division correlates with amorphous and crystalline ice; e.g., the TA and LA, zone-edge phonon frequency range, respectively, is 65-70 cm-1 and 164-181 cm-1, for ice Ic and Ih. © 1990 American Chemical Society.