Collisional Relaxation Processes for the n=3 States of Helium. II. Associative Ionization

Abstract

Rate constants for the associative ionization reaction ${\mathrm{He}}^{*}+\mathrm{He}\to {\mathrm{He}}_2^{+}\mathrm{+\; e}$ have been measured, where He^* represents the n=3 states of helium. By line absorption from the metastable 2¹S and 2³S states, the n=3 states in the positive column of a dc discharge in helium were selectively and periodically modulated. Using phase‐sensitive detection, the changes in excited‐state populations were determined spectroscopically while the corresponding in‐phase variations in the concentration of ${\mathrm{He}}_2^{+}$ were measured with a quadrupole mass filter. The cross sections for associative ionization in units of 10⁻¹⁶ cm² were determined to be: ${\text{σ (3}}^3{\text{P) = 1.6 ± 0.1,\hspace{0.17em}σ (3}}^1{\text{P) = 3.1 ± 1,\hspace{0.17em}σ (3}}^3\text{D) = 4.5 ± 0.5,}$ and ${\text{σ (3}}^1{\text{D) = 20 ± 4.\hspace{0.17em}σ (3}}^3\mathrm{S)}$ and σ(3¹S) were apparently zero.