Diffusion ofHe(2S13)in Helium Gas;2S13−1S01Interaction Potentials at Long Range

Abstract

The existence at intermediate separations of a repulsive barrier in the interaction potentials between a $\mathrm{He}\left(2\mathrm{}{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}\right)$ metastable atom and a $\mathrm{He}\left(1\mathrm{}{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}\right)$ ground-state atom has been well established both experimentally and theoretically. The present investigation deals with the quantitative nature of this repulsive interaction for large internuclear separations. The diffusion coefficient, and thus the diffusion cross section, for $\mathrm{He}\left(2\mathrm{}{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}\right)$ atoms in helium gas has been measured over the temperature range from 1 to 300°K by means of a pulsed afterglow technique. The diffusion cross sections so determined are 82, 58, 46, and 34 (${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$) ±5% at 4.2, 20, 77, and 300°K, respectively. For purposes of comparison, diffusion cross sections were also calculated quantum mechanically using adjustable long-range potentials of a form consistent with previous theoretical calculations. A comparison of the calculated and measured diffusion cross sections clearly indicates that the previous theoretically determined interaction potentials are much too strongly repulsive at large internuclear separations. Accordingly the parameters in the adjustable potentials were varied until agreement with the experimental diffusion cross sections was obtained. The adjusted interaction potentials were then used to calculate total and excitation transfer cross sections, and the results are shown to be in excellent agreement with previous and independent experimental measurements.