Low velocity experiments for collision energy dependence of partial ionization cross-sections of C ₂H ₂ with He*(2 ³S) metastable atoms

Collision energy dependence of partial ionization cross-sections of C ₂H ₂ by collision with He*(2 ³S) metastable atoms in low collision energy regions (20-50 meV) was first observed using a liquid N ₂ cooled discharge nozzle source. Negative collision energy dependence was found for ionizations from σ _CH molecular orbitals, while positive collision energy dependence has been obtained in higher collision energy region of 100-360 meV. These results can be explained by the anisotropic interaction of the present system, and especially the low collision energy feature should indicate that a shallow attractive well exists around hydrogen atom.