Molecular dynamics simulation study of water near critical conditions. II. Dynamics and spectroscopy

摘要

The dynamic and spectroscopic properties of water with density d >= 0.66 g cm~(-3) at elevated temperatures (T) are studied with the Molecular Dynamics computer simulation technique. The electronic polarizability and hyperpolarizability effects and related many-body interactions are explicitly incorporated into the simulations via the truncated adiabatic basis-set representation. The rotational and translational dynamics associated with individual and collective motions of water molecules are examined. Under the thermodynamic conditions studied here, dielectric relaxation becomes accelerated with increasing T and decreasing d, while water librations become weaker and slower. The far-IR absorption, depolarized Raman scattering (DRS) and optically heterodyne detected optical Kerr effect (OKE) spectroscopies are analyzed. The DRS structure near 50-60 cm~(-1) observed in the supercritical phase is attributed to the collective excitations, i.e., acoustic wave propagation in the short wavelength limit. Its asymmetric band shape is mainly due to the water librational motions. As for OKE, the nuclear Kerr response becomes slower and less oscillatory with growing T and lowering d. Spectroscopic scales couched in the spectral shift of far-IR and DRS librational bands are introduced to characterize hydrogen bonding. Comparison with the results of the preceding paper [B. D. Bursulaya and H. J. Kim, J. Chem. Phys. 110, 9646 (1999)] shows that simple geometric criteria could significantly overestimate the extent and strength of H-bonding in supercritical water.