Evidence for steric effect in methyl chloride ionization by metastable argon atoms

The orientation dependence of methyl chloride ionization by collision with metastable argon atoms at an average collision energy of 0.09 eV has been investigated using an electric hexapole field selector followed by an orienting field. The steric opacity function of the process has been determined. The highest reactivity has been found for approaches of the metastable atom towards the Cl-end of the molecule, while the lowest corresponds to the opposite approaches, close to the CH3-end. The stereo-selectivity is discussed in terms of spatial distribution of the highest occupied molecular orbital of CH3Cl, mainly localized on the chlorine atom.