Collision-energy dependence of partial Penning ionization cross-sections (CEDPICS) of OCS was obtained from collision-energy/electron-energy-resolved two-dimensional spectrum with a highly sensitive electron energy analyzer. The negative CEDPICS for X(2)Pi and C(2)Sigma(+) states indicates attractive interaction around S and O atoms with He*(2(3)S), where the corresponding 3pi(S-3p-O-2p) and 8sigma(sigma(CO)) orbitals extend, respectively. Utilizing classical trajectory calculations for CEDPICS and optimization of model potential energy surface, a large ring-like attractive well with a depth of ca. 120 meV was found for perpendicular directions around C=S axis and smaller interaction was found in the collinear direction of the C=O axis with well depth of ca. 70 meV around the oxygen atom in the lone pair region by sp(2) hybridization. (C) 2003 Elsevier B.V. All rights reserved.