ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
Transient, collision-induced changes in polarizability Δα 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 coefficients. For pairs of D∞h molecules, this work gives the long-range contributions to Δα 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 Δα 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 ΔJ 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 ab initio results for permanent multipoles and susceptibilities to evaluate the classical induction contributions to Δα. For the dispersion contributions, we have derived exact results in the form of integrals of the polarizability and γ hyperpolarizability over imaginary frequencies, and we have approximated these numerically in terms of the static α and γ values, together with van der Waals energy coefficients. For the pairs and configurations studied, the first-order DID terms give the dominant contributions to Δα; the agreement between these terms and the full long-range results tends to be better for the anisotropic collision-induced polarizability ΔαM2 than for the isotropic part Δα00, particularly for T-shaped pairs.The relative contributions to Δα 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 der Waals minimum.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.466834
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