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

The lifetimes of metastable helium atoms He(2³S) and He(2¹S) 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 cm² have been determined for He(2³S): Ne---0.28, Ar---6.6, Kr---10.3, Xe---13.9, N₂---6.4, and H₂---6.0. The uncertainties are estimated to be ±{}20% for the rare gases and ±{}50% for the molecular gases. He(2¹S) cross sections were measured to be Ne---4.1, Ar---55, Kr---64, and Xe---103, all times 10^-16 cm². 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.