Velocity dependence for the chemi-ionization of Ar, Kr, N2, CO, and O2 by He(2 1S) and He(2 3S) metastable atoms

Abstract

The velocity dependence of the title reactions was measured using crossed molecular beams and time-of-flight velocity dispersion of the metastable helium atoms. The relative collision energy ranged from 6–10 to 600–800 meV. The singlet cross section energy dependences are all of the ’’repulsive’’ type, i.e., increasing with energy at low energies, implying that the interaction potential well depths are ≲6–8 meV. The triplet cross sections are also generally repulsive, but the Ar, Kr, and O2 cases show evidence of minimums at the lowest energies. These observations are in qualitative agreement with the available interaction potential data. The cross section curves were put on an absolute basis by normalization to thermal quenching rate constants. The cross sections tend to approach the same magnitude at high energies and diverge at low energies. The singlet cross sections increase more rapidly than the triplet cross sections at low energies, with the singlet Ar, Kr, and N2 curves having especially steep slopes. Theoretical cross section curves were calculated for the He(1S,3S)/Ar, Kr cases using interaction potentials available from differential scattering studies. The results are in good agreement with experiment, and confirm the need for unusual low energy repulsive structure in the singlet potentials to explain the collisional energy dependence of chemi-ionization.