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 nx 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.
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