Anisotropic interaction of halogen atom in C2H5X (X = Cl, F) with He*(2(3)S) as probed by two-dimensional Penning ionization electron spectroscopy

Penning ionization of C2H5X (X = Cl, F) upon collision with metastable He*( 2(3)S) atoms was studied by two-dimensional (collision-energy/electron ener gy resolved) Penning ionization electron spectroscopy. Partial ionization c ross sections are found to be larger for ionization from orbitals having n( x) characters. Collision energy dependence of the partial cross sections, w hich reflects interaction potential energy between the molecule and He(2(3) S), indicates anisotropic interaction potentials around the molecule. As el ucidate with the aid of calculated energy surfaces for the chemically relat ed systems Li-C2H5X (X = Cl, F), a different trend was found in the interac tion around C-X axis: for the former the attractive interaction was dominat ed around the perpendicular directions to the C-CI bond axis, while for the C-F bond the attractive interaction was localized around the collinear axi s.