Outer valence ionic states of ferrocene were investigated by means of Penning ionization electron spectroscopy upon collision with metastable He *(23S) excited atoms. By two-dimensional measurement combining electron spectroscopy and collision-energy-resolved technique of the metastable atomic beam, ionic-state-resolved measurements of collision energy dependence of partial Penning ionization cross sections (CEDPICS) were carried out. Since the partial Penning ionization cross sections can be connected with spatial extension of corresponding molecular orbitals (MOs) outside the boundary surface for the collision with He* atoms, different slope values of CEDPICS were related with stereodynamics in Penning ionization as well as anisotropic interaction around the ionization region: attractive interaction around ligand π orbitals and repulsive interaction around ligand σ orbtials and the metal atom. The observed negative collision energy dependence of ionization cross sections for the first (2E2') and second (2A1′) ionic states was consistent with configuration interactions suggested by ab initio MO calculations [J. Chem. Phys. 2002, 117, 6533], which is due to the strong electron correlation effects beyond the one-electron MO description.
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