Density dependence of the electric-field-gradient induced birefringence of the helium, neon and argon gases.

An ab initio investigation of the density dependence of the electric field gradient induced birefringence (EFGB) for the noble gases helium, neon and argon is presented. To determine the second coefficient in the virial expansion of the molecular EFGB constant _mQ, the effect of two-body interactions has been studied by computing the internuclear dependence of molecular quadrupole moment and of the dipole--dipole--quadrupole and dipole--magnetic dipole--dipole hyperpolarizabilities of the van der Waals dimers. A full-configuration interaction approach as well as the coupled cluster singles and doubles and the coupled cluster singles and doubles plus perturbative triples approximations have been adopted, and extended basis sets including midbond functions have been employed. A semiclassical integration yields the virial coefficients. The effect of density for standard experimental conditions is found to be in the order of a few tens of ppm for helium and neon, and in the order of a few parts per thousands for argon at low temperatures, and thus not detectable apparently with current apparatus.