PENNING IONIZATION OF HCHO, CH2CH2, AND CH2CHCHO BY COLLISION WITH HE-ASTERISK(2(3)S) METASTABLE ATOMS

Penning ionization of formaldehyde (HCHO), ethylene (CH2CH2), and acrolein (CH2CHCHO) upon collision with metastable He*(2(3)S) atoms was studied by collision-energy-resolved Penning ionization electron spectroscopy. Collision energy dependence of Penning ionization cross sections (CEDPICS) indicates that interaction potentials are highly anisotropic between He*(2(3)S) atoms and the planar molecules investigated. Attractive interactions around the oxygen atom of the C=O group in the aldehydes were found to be stronger in the molecular plane than out-of-plane. In the case of ethylene, local interaction potentials with He* atoms were characterized more precisely. The regions of pi electrons undergo attractive interactions, whereas intraplane potentials are highly repulsive; the intraplane direction to the middle of the methylene unit gives a softer repulsion than the collinear direction to the H atom in the CH bond.