LONG-RANGE, COLLISION-INDUCED DIPOLES OF T-D-D-INFINITY-H MOLECULE PAIRS - THEORY AND NUMERICAL RESULTS FOR CH4 OR CF4 INTERACTING WITH H-2, N-2, CO2, OR CS2

Compressed gases and liquids containing molecules of T-d and D-infinit y h symmetry absorb far-infrared radiation, due to transient dipole mo ments induced during molecular collisions. In earlier theoretical work on far-infrared absorption by CH4/N-2 mixtures, good agreement was ob tained between calculated and experimental spectra at low frequencies, but at higher frequencies-from 250 to 650 cm(-1)-calculated absorptio n intensities fell significantly below the experimental values. In thi s work, we focus on an accurate determination of the long-range, colli sion-induced dipoles of T-d...D-infinity h pairs, including two polari zation mechanisms not treated in the earlier line shape analysis: disp ersion and nonuniformity in the local field gradient acting on the Td molecule. Since these mechanisms produce transitions with Delta J= +/- 3 or +/-4 for CH4 and Delta J=0 or +/-2 for N-2, their inclusion is ex pected to increase the calculated absorption intensities in the high f requency wings for CH4/N-2 mixtures. This should improve agreement wit h the experimental spectra, and permit more accurate determination of anisotropic overlap terms in the collision-induced dipole. We give num erical values for the long-range dipole coefficients of CH4 or CF4 int eracting with H-2, N-2, CO2, or CS2; the dipole coefficients have been derived with spherical-tenser methods and evaluated using single-mole cule moments and susceptibilities from recent ab initio calculations o r experiments. The dispersion dipoles are given rigorously in terms of integrals involving the imaginary-frequency polarizability alpha(i om ega) and the hyperpolarizabilities beta(0;i omega,-i omega) and B(0;i omega,-i omega). To obtain numerical estimates for the dispersion dipo les, we have developed constant-ratio approximations that require only the static susceptibilities and Cg van der Waals coefficients. (C) 19 98 American Institute of Physics. [S0021-9606(98)02433-7].