Valence ionized states of iron pentacarbonyl and $η5$-cyclopentadienyl cobalt dicarbonyl studied by symmetry-adapted cluster-configuration interaction calculation and collision-energy resolved Penning ionization electron spectroscopy

Valence ionized states of iron pentacarbonyl $Fe(CO)5$ and $η5$-cyclopentadienyl cobalt dicarbonyl $Co(η5-C5H5)(CO)2$ have been studied by ultraviolet photoelectron spectroscopy, two-dimensional Penning ionization electron spectroscopy (2D-PIES), and symmetry-adapted cluster-configuration interaction calculations. Theory provided reliable assignments for the complex ionization spectra of these molecules, which have metal-carbonyl bonds. Theoretical ionization energies agreed well with experimental observations and the calculated wave functions could explain the relative intensities of PIES spectra. The collision-energy dependence of partial ionization cross sections (CEDPICS) was obtained by 2D-PIES. To interpret these CEDPICS, the interaction potentials between the molecules and a Li atom were examined in several coordinates by calculations. The relation between the slope of the CEDPICS and the electronic structure of the ionized states, such as molecular symmetry and the spatial distribution of ionizing orbitals, was analyzed. In $Fe(CO)5$⁠, an attractive interaction was obtained for the equatorial CO, while the interaction for the axial CO direction was repulsive. For $Co(η5-C5H5)(CO)2$⁠, the interaction potential in the direction of both Co–C–O and Co–Cp ring was attractive. These anisotropic interactions and ionizing orbital distributions consistently explain the relative slopes of the CEDPICS.