The polarizability of a pair of helium atoms

Lineshape and Moment Analysis in Isotropic and Anisotropic Interaction-Induced Light Scattering Spectra of Gaseous Helium

Calibrated Raman spectra of the helium gas diatom at room temperature (294.5 K) have recently been measured over a wide range of intensities, with polarizations of the incident beam parallel and perpendicular to the direction of observation. From the resulting spectra, the isotropic and anisotropic components are separated for the helium diatom. For direct comparison with the measurements, quantum mechanical lineshapes are computed using recent model of the interatomic potential and models of the diatom polarizability tensor invariants as input. New empirical trace and anisotropy parameters, consistent with the spectroscopic measurements, are thus obtained for the helium gas. We compare these where possible with the existing computations based on classical physics, utilizing the first few even moments of the isotropic and anisotropic spectra. Good agreement with ab initio results in the literature is obtained and profiles calculated with these models are in excellent agreement with experiments.

The scattering of light. II. The complex refractive index of a molecular fluid

We treat the complex refractive index of a finite molecular fluid on the basis of a classical many-body theory; the surface problem is handled through surfacedependent propagators. We develop a density expansion generalizing the Lorentz-Lorenz relation and sum all two-body terms to a closed form with intermolecular correlations determined by a Lennard-Jones pair potential; the dependence on density, temperature and frequency is discussed and the case when the frequency is near a molecular resonance is considered. The refractive index and the extinction coefficient are compared with experiments for gases. We also derive a generalization of the macroscopic relation of Onsager and Bottcher from the many-body theory, essentially as an expansion in an effective polarizability of a molecule in the many-body system. Exact microscopic expressions for an effective polarizability and for a reaction field are identified, and it is shown that they are related like the Bottcher polarizability and the Onsager reaction field in a well-defined decorrelation approximation. The relation with, and validity of, the macroscopic formulae of Lorentz and Bottcher are analysed in depth.