The collision energy and orientation-angle-resolved Penning ionization cross sections were determined for the Ar (P-3)+CH3I reaction. A remarkable depletion was recognized in the collision- energy dependence of the orientation-angle-resolved Penning ionization cross section. The depletion could be interpreted as the competitive neutral dissociation from the Rydberg states formed by an electron exchange mechanism related to a 3a(1) molecular orbital. The stereo selectivity for the Penning ionization and neutral dissociation were discussed. From the series of our studies on Ar (P-3)+CH3X (X=Cl,Br,I) reactions, it was substantiated that the orientation angle and collision-energy-resolved Penning ionization cross section give an important information about the branching mechanism to the Rydberg states that are linked to the neutral dissociation channels. (C) 2003 American Institute of Physics.