Cross Sections for the De-Excitation of Helium Metastable Atoms by Collisions with Atoms

The lifetimes of metastable helium atoms $\mathrm{He}\left(2\mathrm{}{}_{}{}^3{}_{}{}^{}S\right)$ and $\mathrm{He}\left(2\mathrm{}{}_{}{}^1{}_{}{}^{}S\right)$ were measured by a time-resolved optical absorption technique in the afterglow of a pulsed helium discharge at 10-mm pressure with small concentrations of various impurity gases added. The following de-excitation cross sections in units of ${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ have been determined for $\mathrm{He}\left(2\mathrm{}{}_{}{}^3{}_{}{}^{}S\right)$: Ne—0.28, Ar—6.6, Kr—10.3, Xe—13.9, ${\mathrm{N}}_2$—6.4, and ${\mathrm{H}}_2$—6.0. The uncertainties are estimated to be ±20% for the rare gases and ±50% for the molecular gases. $\mathrm{He}\left(2\mathrm{}{}_{}{}^1{}_{}{}^{}S\right)$ cross sections were measured to be Ne—4.1, Ar—55, Kr—64, and Xe—103, all times ${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$. The neon cross section is believed reliable to ±20% but the remaining singlet cross sections may be too high by as much as a factor 2 or 3. The cross sections refer to de-excitation by ionizing collisions except in the case of neon where neon excitation occurs. Competing destruction processes render the singlet cross-section analysis somewhat uncertain except in the case of neon.