Development of a cooled He* (2 ³ S) beam source for measurements of state-resolved collision energy dependence of Penning ionization cross sections: Evidence for a stereospecific attractive well around methyl group in CH ₃CN

A low-temperature discharge nozzle source with a liquid- N2 circulator for He* (2 S3) metastable atoms has been developed in order to obtain the state-resolved collision energy dependence of Penning ionization cross sections in a low collision energy range from 20 to 80 meV. By controlling the discharge condition, we have made it possible to measure the collision energy dependence of partial ionization cross sections (CEDPICS) for a well-studied system of CH3 CN+ He* (2 S3) in a wide energy range from 20 to 350 meV. The anisotropic interaction potential energy surface for the present system was obtained starting from an ab initio model potential via an optimization procedure based on classical trajectory calculations for the observed CEDPICS. A dominant attractive well depth was found to be 423 meV (ca. 10 kcalmol) at a distance of 3.20 Å from the center of mass of CH3 CN in the N-atom side along the CCN axis. In addition, a weak attractive well (ca. 0.9 kcalmol) surrounding the methyl group (- CH3) has been found and ascribed to the interaction between an unoccupied molecular orbital of CH3 CN and 2s atomic orbital of He* (2 S3).