Anisotropic interaction of halogen atom in C₂H₅X (X = Cl, F) with He*(2³S) as probed by two-dimensional penning ionization electron spectroscopy

Penning ionization of C₂H₅X (X = Cl, F) upon collision with metastable He*(2³S) atoms was studied by two-dimensional (collision-energy/electron energy resolved) Penning ionization electron spectroscopy. Partial ionization cross sections are found to be larger for ionization from orbitals having n_x characters. Collision energy dependence of the partial Cross sections, which reflects interaction potential energy between the molecule and He(2³S), indicates anisotropic interaction potentials around the molecule. As elucidate with the aid of calculated energy surfaces for the chemically related systems Li-C₂H₅X (X = Cl, F), a different trend was found in the interaction around C-X axis: for the former the attractive interaction was dominated around the perpendicular directions to the C-Cl bond axis, while for the C-F bond the attractive interaction was localized around the collinear axis.