Helium Ions at 76 °K: Their Transport and Formation Properties

The time dependences of the ion wall currents in a 76 ^∘{}K helium afterglow were measured with a quadrupole mass-filter system. The species observed were He⁺, He₂⁺, He₃⁺, He₄⁺, and metastable atoms. The results of the present work are a zero-field mobility, μ₀(He⁺), of 14.4 ±{} 0.3 cm²{V}^{{-}1}sec^-1, and a three-body conversion frequency of He⁺ into He₂⁺ of (1.44±0.14)×10^-31×N² sec^-1, where N is the density in atoms/cm³. The diffusion coefficient that describes the late-time behavior of He₃⁺ was found to be 95.3 ±{} 8.6 cm²{sec}^{{-}1}Torr. The metastable helium atoms at 76 ^∘K were found to have a diffusion coefficient given by{D}ₘ{p}₀=146$ ${cm}²sec^-1 Torr. Evidence is presented for the presence of helium negative ions in the 76 ^∘{}K helium afterglow.