We have measured cross sections for chemi-ionization in thermal energy collisions of $\mathrm{He}\left(2^{1}S\right)$ and $\mathrm{He}\left(2^{3}S\right)$ metastable atoms with argon using both crossed-beams and gas-cell techniques. The results indicate that several earlier measurements of these cross sections were in error.

• Received 26 June 1972

DOI:https://doi.org/10.1103/PhysRevLett.29.321