Depolarized interaction-induced light scattering in dense phases

Abstract

Depolarized interaction-induced light scattering (DILS) spectra of low density gases are now fully understood. Experimental data from different laboratories are in agreement within experimental error and the comparison among different experimental techniques (namely the depolarization ratio and the second virial Kerr effect) is finally positive (D. A. Dunmur, M. R. Manterfiel and D. J. Robinson, Mol. Phys., 1983, 50, 573). From the pair spectra empirical models of polarizability anisotropy can be derived, which can be used as an input in molecular dynamics (MD) simulation programs in order to extend the comparison between theory and experiment to the dense phase. This has already been done for argon, and the results are that, within one or two standard deviations, the pairwise additivity approximation, either in the total potential-energy function or in the total polarizability anisotropy, accounts for the DILS spectral moments over the whole density range of the fluid phase (F. Barocchi, G. Spinelli and M. Zoppi, Chem. Phys. Lett., 1982, 90, 22; M. Zoppi and G. Spinelli, Phys. Rev. A, 1986, 33, 939). On the other hand, DILS spectra are also present in the solid phase, but they are mixed with other effects such as phonon scattering. The analysis of the available experimental data shows a substantial difference between the correlation functions associated with DILS spectra, in the liquid and solid phase. This difference is attributable in part to the structure of the solid (long-range spatial order) and in part to simple density effects. The aim of this work is to present some preliminary experimental results which could be used to gain a better understanding, on a quantitative basis, of the relative weight of the kinetic and collective behaviour of dense hydrogen, near the liquid-solid transition.