Many-body time correlations for the far infrared absorption of liquid CS_2

摘要

Computer simulations of various contributions to the spectral time correlation functions (tcfs) for the quadrupole-induced dipole (QID) absorption in the far infrared are reported for liquid CS_2 at several points along the liquid-vapour coexistence curve. The quadrupole-induced dipole moment is separated into contributions from induction via the isotropic and anisotropic parts of the molecular polarizability tensor. The component two-, three- and four-body autocorrelations for these two induced dipoles and their cross correlation have been evaluated. Sums of the component tcfs involving correlations of the isotropically induced dipole with both the isotropic and the anisotropic induced dipoles show partial cancellation in the limit of t = 0. This cancellation is absent in the autocorrelations of the anisotropically induced dipoles. At long times, all the induction mechanisms give tcfs that show strongly destructive interference between the tails of the component many-body correlation functions. However, the amplitudes of the two-, three- and four-body tails differ significantly from the 1: — 2:1 ratio found in interaction-induced light scattering. At moderate to large times, it was found that the ratios of the magnitudes of the various QID tcfs approach values that happen to be characteristic of the perfect solid and the decay rates all become the same. Consequently, each of the collective induced dipole tcfs separately approaches zero due to cancellation in this time regime. A possible explanation for this behaviour is proposed.