Ionizing Collisions of Metastable Helium Atoms in Gases

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Abstract
Ionization cross sections for collisions of metastable 23S and 21S He atoms in Ar, Kr, Xe, H2, O2, N2, and CO have been measured using a thermal‐energy atomic beam in which a fraction of the helium atoms are excited by electron bombardment. Ions produced in these gases by inelastic collisions were detected at a metal electrode, and unscattered or elastically scattered metastable atoms were detected by observing the electrons they ejected from metal surfaces. Within the experimental error of the measurements, the cross sections for the triplet and singlet states of helium were the same in all cases except that of H2. For the rare gases, the results indicated that the size of the struck particle is the primary factor in determining the cross section for ionization, rather than the energy excess. In the case of the diatomic molecules an additional consideration, that of the number and type of ionization processes possible, was also found to be important.