Absorption Studies of Helium Metastable Atoms and Molecules

Abstract

The lifetimes of the metastable states of the helium atom and molecule in pure helium have been determined by using a time-sampling technique for the measurement of the time-varying optical absorption caused by the various metastables. The measured loss of helium singlet metastable atoms indicates destruction after diffusion to the wall, upon collisions with single atoms, and by conversion into triplet metastable atoms in collisions with thermal electrons. The triplet metastable atoms are destroyed upon diffusion and by collisions with two neutral atoms which result in the formation of molecules in the $2{{}_{}{}^3{}_{}{}^{}\Sigma _u^{}{}_{}{}^{}}^{+}$ state. These molecules have a natural lifetime of at least 0.05 second and are destroyed upon diffusion to the wall and by collisions with slow electrons or other metastables. The product of the diffusion coefficient and the helium atom density at 300°K is 1.5×${10}^{19}$ (${\mathrm{cm}}^2$/sec) (atom/cc) for the atomic metastables and 1.0×${10}^{19}$ (${\mathrm{cm}}^2$/sec) (atom/cc) for the molecular metastable. The cross sections for the destruction of the singlet atomic metastables are 3×${10}^{-14\mathrm{}}$ ${\mathrm{cm}}^2$ and 3×${10}^{-20\mathrm{}}$ ${\mathrm{cm}}^2$ for collisions with thermal (300°K) electrons and neutral atoms, respectively. The three-body combination coefficient for the conversion of triplet atomic metastables into molecular metastables is 2.5×${10}^{-34\mathrm{}}$ ${\mathrm{cc}}^2$ ${\mathrm{atom}}^{-2\mathrm{}}$ ${\sec }^{-1\mathrm{}}$ at 300°K.