Mononuclear and polynuclear organometallic clusters are a class of interesting and technologically relevant complexes due to their widespread use in catalysis. They display a complex electronic structure, bearing the signature of strong correlation and relativistic effects. Moreover, their nontrivial nuclear dynamics leads to a high degree of fluxionality and complex fragmentation dynamics. Photoelectron spectroscopy has established itself as one of the principal tools for the investigation of molecular electronic structure, providing both ionization energies and dynamical observables such as partial ionization cross sections and asymmetry parameters β. In particular, the angular distribution of photoelectrons has scarcely been investigated for organometallic systems.

Computational methods to describe photoionization observables, based on ground-state density functional theory (DFT) and linear response time-dependent DFT (TDDFT), are well established for relatively simple organic molecules. To benchmark and assess the quality of the theoretical approach in the case of heavy metal atom containing systems, we have here investigated the tetraoxo complexes OsO4  and RuO4  as model cases. Accurate experimental measurements of photoionization dynamics as test for the theory are reported for the photoelectron asymmetry parameters of outer valence ionizations of OsO4, measured in the 17–90 eV photon energy range.

Major upgrades of the ARPES-TPES end station, namely a fast 2D-PositionSensitive electron detector and a novel asymmetric fringing field corrector, made the spectrometer, which was specifically designed to study highly reactive and chemically aggressive gaseous species, very efficient to investigate photoionization observables of OsO4.
The valence electronic configuration of OsO4  and RuO4 is as follows: (1a1 )2(1t2 )6(1 e)4(2t2 )6(2a1 )2(3t2 )6(1t1 )6. The valence PE spectrum of OsO4  recorded in this work at hν = 40 eV and θ = 54.7, the “magic angle”, is shown in Fig 1. The first band, A, arises from the ionization from the 1t1 molecular orbitals (MOs). Bands B and C, separated by 0.4 eV in OsO4, are assigned to the spin–orbit components associated with ionization from the 3t2  MOs with strong contribution of O 2p atomic orbitals (AOs), namely the 2T2  ion state. The fourth band, D, is assigned to the 2A1  ion state, while the fifth band, E, encompasses the remaining outer valence MO ionizations (2E and 2T2  ion states). A main contribution to the broadening of band D is associated to the breakdown of the one-particle picture.