Second and Third Virial Coefficients of Collision-Induced Absorption and Light Scattering

Abstract

We consider the virial expansion of the zeroth and first spectral moments of collision-induced absorption and light scattering spectra. For the case of absorption the gases and gas mixtures helium-argon, hydrogen-helium, hydrogen-argon, and hydrogen are considered. The measured second virial spectral coefficients are in close agreement with computations based on first principles. The third virial coefficients of the complexes He-Ar-Ar, He-He-Ar, H2-He-He, H2-H2-He, H2-H2-H2, and H2-Ar-Ar, have also been computed from first principles, using the most dependable pair dipole and pair potential functions available. Measured ternary moments are consistently greater than calculated ones, especially at the higher temperatures. The observed excess must be due to a positive ternary spectral component of collision-induced absorption due to irreducible dipole components. In the case of collision-induced, depolarized light scattering, a similar study of the 2- and 3-body effects in compressed hydrogen gas suggests at low temperatures (50 K) the presence of a positive irreducible component of a magnitude of roughly 40% of the pairwise-additive component. Theoretical estimates of the long-range (classical) irreducible anisotropy of the polarizability of 3-body complexes suggest a positive spectral contribution which, however, amounts to about 1/4 of the difference observed between measurement and theory. At the high temperature (297 K) a negative irreducible component is seen in the measurements which must be non-classical, of an overlap (short-range) nature. Quantum corrections of the Wigner-Kirk wood type are found to be significant.