MANY-BODY TIME CORRELATIONS FOR THE FAR-INFRARED ABSORPTION OF LIQUIDCS2

Computer simulations of various contributions to the spectral time cor relation functions (tcfs) for the quadrupole-induced dipole (QID) abso rption in the far infrared are reported for liquid CS2 at several poin ts along the liquid-vapour coexistence curve. The quadrupole-induced d ipole moment is separated into contributions from induction via the is otropic and anisotropic parts of the molecular polarizability tenser. The component two-, three- and four-body autocorrelations for these tw o induced dipoles and their cross correlation have been evaluated. Sum s of the component tcfs involving correlations of the isotropically in duced dipole with both the isotropic and the anisotropic induced dipol es show partial cancellation in the limit of t = 0. This cancellation is absent in the autocorrelations of the anisotropically induced dipol es. At long times, all the induction mechanisms give tcfs that show st rongly destructive interference between the tails of the component man y-body correlation functions. However, the amplitudes of the two-, thr ee- and four-body tails differ significantly from the 1:-2:1 ratio fou nd in interaction-induced light scattering. At moderate to large times , it was found that the ratios of the magnitudes of the various QID tc fs approach values that happen to be characteristic of the perfect sol id and the decay rates all become the same. Consequently, each of the collective induced dipole tcfs separately approaches zero due to cance llation in this time regime. A possible explanation for this behaviour is proposed.