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Transient Changes in Polarizability for Centrosymmetric Linear Molecules Interacting at Long Range: Theory and Numerical Results for H2...H2, H2...N2, and N2...N2
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| Journal of Chemical Physics |
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Transient, collision-induced changes in polarizability Da on the subpicosecond time scale affect Rayleigh and rototranslational Raman scattering by diatomic molecules in dense gases and liquids, induced birefringence, impulsive stimulated scattering, and dielectric and refractivity virial coefficient. For pairs of Dooh molecules, this work gives the long-range contributions to Da complete through order R^-6 in the intermolecular separation R, including the first- and second-order dipole-induced-dipole (DID) interactions, higher-multipole induction, effects of the nonuniformity in the local field, hyperpolarization, and dispersion. We have used spherical tensor methods to cast Da into the symmetry-adapted form needed for spectroscopic line shape analysis. The polarization mechanisms included here give rise to isotropic rototranslational Raman scattering and to simultaneous rotational transitions on two interacting molecules; both are collision-induced phenomena. Transitions with DJ up to +4 are produced by the R^5 and R^6 polarization mechanisms treated in this work (and the isotropic part of the pair potential). For the pairs H2...H2, H2...N2, and N2...N2, we have used ad initio results for permanent multipoles and susceptibilities to evaluate the classical induction contributions to Da. For the dispersion contributions, we have derived exact results in the form of integrals of the polarizability and g hyperpolarizability over imaginary frequencies, and we have approximated these numerically in terms of the static a and g vales, together with van deer Walls energy coefficients. For the pairs and configurations studied, the first-order DID terms give the dominant contributions to Dal the agreement between these terms and the full long-range results tends to be better for the anisotropic collision-induced polarizability Da2^M than for the isotropic part Da0^0, particularly for T-shaped pairs. The relative contributions to Da from the other polarization mechanisms reach as high as 29% for second-order DID terms, 27% for E-tensor terms (higher-multipole induction and local field nonuniformity), 21% for dispersion, and 8% for hyperpolarization effects, for R values ~0.5-1.0 a.u. outside the isotropic van deer Walls minimum.
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