The potential energy surface of benzene (C 6H 6) with a He *(2 3S) atom was obtained by comparison of experimental data in collision-energy-resolved two-dimensional Penning ionization electron spectroscopy with classical trajectory calculations. The ab initio model interaction potentials for C 6H 6+He *(2 3S) were successfully optimized by the overlap expansion method; the model potentials were effectively modified by correction terms proportional to the overlap integrals between orbitals of the interacting system, C 6H 6 and He *(2 3S). Classical trajectory calculations with optimized potentials gave excellent agreement with the observed collision-energy dependence of partial ionization cross sections. Important contributions to corrections were found to be due to interactions between unoccupied molecular orbitals and the He *2s orbital. A C 6H 6 molecule attracts a He *(2 3S) atom widely at the region where π electrons distribute, and the interaction of -80meV (ca. -1.8kcal/mol) just cover the carbon hexagon. The binding energy of a C 6H 6 molecule and a He * atom was 107 meV at a distance of 2.40 Å on the sixfold axis from the center of a C 6H 6 molecule, which is similar to that of C 6H 6+Li and is much larger than those of the C 6H 6+[He,Ne,Ar] systems.
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