Development of a cooled He*(2S3) 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 CH3CN

Abstract

A low-temperature discharge nozzle source with a liquid-N(2) circulator for He*(2(3)S) 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 CH(3)CN+He*(2(3)S) 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 kcal/mol) at a distance of 3.20 A from the center of mass of CH(3)CN in the N-atom side along the CCN axis. In addition, a weak attractive well (ca. 0.9 kcal/mol) surrounding the methyl group (-CH(3)) has been found and ascribed to the interaction between an unoccupied molecular orbital of CH(3)CN and 2s atomic orbital of He*(2(3)S).

Anisotropic Interaction and Stereoreactivity in a Chemi-Ionization Process of OCS by Collision with He*(23S) Metastable Atoms

“…As described previously, 30 the OE method is able to describe effectively anisotropic potentials by expanded terms using overlap integrals between target MOs φ i and an atomic orbital χ. The expansion coefficients C i in eq 5 and the exponent ζ in eq 6 are parameters to be optimized in the potential corrections; however, in our previous studies [30][31][32][33] the optimized values of ζ in eq 6 have been found to be very close to the exponent of the 2s orbital of He* atom, which is 0.575 bohr -1 determined by Slater's rule. According to this fact, the parameter of ζ was fixed at the above value.…”

“…In the occupied MOs, the coefficients C i except for C 2 show positive values. As discussed in the previous studies, [30][31][32][33] contributions of the positive coefficients for occupied MOs supplement 2s-2p hybridization effects, 21,[57][58][59] which are underestimated in the Li model potential V 0 due to the larger energy gap between the 2s and 2p orbitals of the Li atom (1.848 eV) than that of He*(2 3 S) (1.114 eV). The relatively large negative value (-1195 meV) of the optimized coefficient C 2 is somewhat confusing; however, it is supposed that this feature is caused by a subtle balance of many factors in evaluating highly anisotropic interaction as the present system.…”

“…The increasing trend for K (i) with respect to the ionization potentials for the respective ionic states has been discussed in the previous studies. [30][31][32][33]60 Figure 3 shows a contour map of the optimized V* for OCS + He*(2 3 S). In the contour map, solid and dashed lines show positive and negative energy area, respectively.…”

“…In this work, by means of a liquid N 2 cooled discharge source for He*(2 3 S) metastable beams, 31 CEDPICS of OCS has been recorded in a wide collision energy range from 20 to 350 meV. Interaction potential V* for the present system has been obtained by the experimental data and the theoretical methods mentioned above.…”

“…On the other hand, CEDPICS for molecular targets was first calculated by classical trajectory simulations based on ab initio interaction potentials and approximated ionization widths. , Reliable potentials for molecular targets, however, have not been obtained by ab initio treatment, because of difficulties in constructing highly anisotropic interaction of A* + M systems embedded in an ionization continuum higher than the IP of M. To compare the calculated results with the observed ones quantitatively, interaction potentials employed in trajectory calculations need to be modified based on an optimization procedure. An overlap expansion method improving anisotropic potential has been developed very recently, and then for several targets, such as N 2 , CO, CH 3 CN, C 2 H 2 , C 2 H 4 , and C 6 H 6 , spatial anisotropy of V * with He*(2 3 S) and stereodynamics of the collisional ionization upon V * has been investigated in detail.…”