Penning ionization of phenylacetylene and diphenylacetylene upon collision with metastable He*(23S) atoms was studied by collision-energy-/electron-energy-resolved two-dimensional Penning ionization electron spectroscopy (2D-PIES). On the basis of the collision energy dependence of partial ionization cross-sections (CEDPICS) obtained from 2D-PIES as well as ab initio molecular orbital calculations for the approach of a metastable atom to the target molecule, anisotropy of interaction between the target molecule and He*(23S) was investigated. For the calculations of interaction potential, a Li(22S) atom was used in place of He*(23S) metastable atom because of its well-known interaction behavior with various targets. The results indicate that attractive potentials localize in the π regions of the phenyl groups as well as in the π-conjugated regions of the acetylene group. Although similar attractive interactions were also found by the observation of CEDPICS for ionization of all π MOs localized at the C≡C bond, the in-plane regions have repulsive potentials. Rotation of the phenyl groups about the C≡C bond can be observed for diphenylacetylene because of a low torsion barrier. So the examination of measured PIES was performed taking into consideration the change of ionization energies for conjugated molecular orbitals.
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