Collision-Induced Spectra of the Helium Isotopes

Gas-phase Raman spectra of the diatoms ${}_{}{}^3{}_{}{}^{}\mathrm{He}_2^{}{}_{}{}^{}$ and ${}_{}{}^4{}_{}{}^{}\mathrm{He}_2^{}{}_{}{}^{}$ are measured, the former for the first time. The depolarized spectra of the collisional pairs are in near agreement with computations based on a recent configuration-interaction calculation of the helium-diatom polarizability (anisotropic part). The polarized spectra, however, require a somewhat more substantial trace than the configuration-interaction prediction. Measurements of the second virial dielectric coefficient support this conclusion.