Rayleigh scattering in dense fluid helium

TL;DR

This study provides an accurate evaluation of Rayleigh scattering in dense fluid helium, considering atomic polarizability, density effects, and spatial correlations, with implications for white dwarf atmospheres.

Contribution

It extends previous work by incorporating detailed atomic polarizability calculations, density-dependent effects, and Monte Carlo simulations to model scattering in dense helium.

Findings

Rayleigh scattering decreases due to collective effects in dense helium.

Collision-induced scattering dominates at densities above 1-2 g/cm$^3$.

Analytical fits for cross sections are provided for practical calculations.

Abstract

Iglesias et al. (2002) showed that the Rayleigh scattering from helium atoms decreases by collective effects in the atmospheres of cool white dwarf stars. Their study is here extended to consider an accurate evaluation of the atomic polarizability and the density effects involved in the Rayleigh cross section over a wide density-temperature region. The dynamic dipole polarizability of helium atoms in the ground state is determinated with the oscillator-strength distribution approach. The spectral density of oscillator strength considered includes most significant single and doubly excited transitions to discrete and continuum energies. Static and dynamic polarizability results are confronted with experiments and other theoretical evaluations shown a very good agreement. In addition, the refractive index of helium is evaluated with the Lorentz-Lorenz equation and shows a satisfactory agreement with the most recent experiments. The effect of spatial correlation of atoms on the Rayleigh scattering is calculated with Monte Carlo simulations and effective energy potentials that represent the particle interactions, covering fluid densities between 0.005 and a few g/cm$^3$ and temperatures between $1000$ K and $15000$ K. We provide analytical fits from which the Rayleigh cross section of fluid helium can be easily calculated at wavelength $\lambda>505.35$ \AA. Collision-induced light scattering was estimated to be the dominant scattering process at densities greater than 1-2 g/cm$^3$ depending on the temperature.