Probing anisotropic interaction potentials of unsaturated hydrocarbons with He*(2S3) metastable atom: Attractive-site preference of σ-direction in C2H2 and π-direction in C2H4

Abstract

State-resolved collision energy dependence of Penning ionization cross sections of acetylene (C2H2) and ethylene (C2H4) with He*(2 3S) metastable atoms was observed in a wide collision energy range from 20 to 350 meV. A recently developed discharge nozzle source with a liquid N2 circulator was employed for the measurements in the low-energy range from 20 to 80 meV. Based on classical trajectory calculations for the energy dependence of the partial ionization cross sections, anisotropic potential energy surfaces for the present systems were obtained by optimizing ab initio model potentials for the chemically related systems Li+C2H2 and C2H4. In the case of C2H2, the global minimum was found to be located around the H atom along the molecular axis with a well depth of 48 meV (ca. 1.1 kcal/mol). On the other hand, a dominant attractive well with a depth of 62 meV (ca. 1.4 kcal/mol) was found in the piCC electron region of C2H4. These findings were discussed in connection with orbital interactions between molecular orbitals of the target molecules and atomic orbitals of the metastable atom. It is concluded that sigma-type unoccupied molecular orbitals of C2H2 and a piCC-type highest occupied molecular orbital of C2H4 play a significant role for the attractive-site preference of sigma direction in C2H2 and pi direction in C2H4, respectively.

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.…”

“…The coefficients C i for the unoccupied MOs (C 5 -C 7 ) are found to be positive, which are consistent with the previous studies. [30][31][32][33] The relative ratio between the optimized K (i) parameters associated with the ionization width for producing respective ionic states Γ (i) in eq 7 is found to be K (X) :K (A) :K (B) :K (C) ) 1.0: 2.2:7.1:15.8. The increasing trend for K (i) with respect to the ionization potentials for the respective ionic states has been discussed in the previous studies.…”

“…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 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.…”

A New Insight on Stereo-Dynamics of Penning Ionization Reactions

“…The basic novelty consists on the following two main points: (i) The radial dependence of the real part of the potential [V t in the Equation (1)], both in the entrance and exit channels, is represented by an Improved Lennard Jones Potential model (ILJ) defined by few parameters related to basic physical properties of the interacting particles (Pirani et al, 2008). Its reliability, tested on high resolution scattering and spectroscopic experiments as well as by ab initio calculations, has been found to be comparable and even better with respect to that of multiparameter functions, widely used in the past, as the TT model (Falcinelli et al, 2017); (ii) The description of the anisotropy in both channels in terms of the same components related to the selective dependence of CT on the half-filled P orbital alignment within the intermediate collision complex; (iii) The internally consistent representation of both real V t and imaginary Γ parts of the optical potential [see Equation (1)] which, for the first time, has been considered not independent (as it has been done until now) but interdependent being related to the selectivity of CT. Other relevant works in the stereodynamical investigation of auto-ionization processes have been carried out by Ohno and coworkers on the He * -molecule systems where the He * is an isotropic reagent and the dependence on the features of the involved molecular orbitals has been characterized (Ohno, 2004; Yamazaki et al, 2005, 2007; Horio et al, 2006; Khishimoto and Ohno, 2007). Important contributions have been also provided by the Kasai group on the dependence of reactions on the orientation of symmetric top molecules investigated in detail considering Ar * as an isotropic collisional partner (Yamato et al, 2000; Brunetti et al, 2001; Ohoyama et al, 2001).…”