TRANSLATIONAL AND ORIENTATIONAL DYNAMICS OF A WATER CLUSTER (H2O)(108) AND LIQUID WATER - ANALYSIS OF NEUTRON-SCATTERING AND DEPOLARIZED LIGHT-SCATTERING

Molecular dynamics simulations were performed to analyze effects of the translational and orientational motions on the coherent and the incoherent neutron scattering and the depolarized light scattering (DLS) spectra of the water cluster (H2O)108 and liquid water. In the neutron scattering of the water cluster and liquid water, there exist two modes, the high and the low frequency modes, in the collective longitudinal current fluctuations for oxygen atoms. The low frequency mode is almost independent on wave number above 0.6 Å 1, while the velocity of the high frequency mode is faster than 3000 m/s, as experimentally observed in liquid water. The nature of this high frequency mode is analyzed by changing the molecular interaction parameters characterizing the hydrogen bond structure of the system. It was found that the high frequency mode is very sensitive to hydrogen bond structure and a three dimensional network structure of the hydrogen bonds is essential for its existence. In order to characterize the water dynamics reflected in the neutron scattering, the density fluctuations of water are classified into two kinds of dynamics; the local oscillatory dynamics around local potential minima (intrabasin dynamics) and the large amplitude dynamics associated with the structure change of hydrogen bond network (inherent structure transitions; interbasin dynamics). We also analyzed the collective and individual longitudinal current fluctuations for hydrogen atoms. For the depolarized light scattering, the contribution of the cross correlation is examined and compared with those in the collective orientational relaxation in the far infrared spectrum. The interaction-induced component plays a dominate role below 300 cm-1 while the permanent component does above 300 cm-1 in DLS of the cluster and liquid water. The induced component relaxes very slowly in the cluster and yields almost an identical relaxation to that of the dipole-dipole interaction tensors. It was found that the power spectrum of the DLS base line of liquid water obtained from the molecular dynamics calculation is in good agreement with experimentally observed spectrum by Walrafen et al. © 1995 American Institute of Physics.