Abstract
The photoionization dynamics of OsO4 and RuO4, chosen as model systems of small-size mononuclear heavy-metal complexes, has been theoretically studied by the time-dependent density functional theory (TDDFT). Accurate experimental measurements of photoionization dynamics as a benchmarking 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. The theoretical results are in good agreement with the available experimental data. The observed dynamical behavior of partial cross sections and asymmetry parameters has been related to both the coupling to the continuum of discrete excited states, giving strong modulations in the photon energy dependency, and the atomic composition of the initial ionized states, which determines the rate of decay of ionization probability for increasing excitation energies. Overall, an extensive analysis of the photoionization dynamics for valence and core orbitals is presented, showing good agreement with all the available experimental data. This provides confidence for the validity of the TDDFT approach in describing photoionization of heavy transition element compounds, with the perspective of being used for larger systems. Further experimental work is suggested for RuO4 to gather evidence of the sensitivity of the theoretical method to the nature of the metal atom.
Highlights
Mononuclear and polynuclear organometallic clusters are a class of interesting and technologically relevant complexes due to their widespread use in catalysis.[1]
Valence photoelectron (PE) spectra of the OsO4 molecule were recorded at fixed photon energies spanning the 17−90 eV range using the ARPES-TPES end station, equipped with a designed rotatable hemispherical analyzer to detect photoelectrons (ARPES) and threshold photoelectrons (TPES) emitted by highly reactive and chemically aggressive gaseous species.[25−30] The analyzer was operated in constant pass energy mode selecting 10 and 15 eV pass energies for photoelectron spectroscopy (PE) spectra recorded below and above 22 eV, respectively
Earlier measurements of the He−I PE spectra of both tetraoxo complexes were reported by Diemann et al.[44] and Burroughs et al.,[45] while a later work by Green et al.,[16] using synchrotron radiation, was instrumental in resolving long-standing controversies[43] concerning the energy ordering of the outer valence molecular orbitals (MOs) and the assignment of the outermost bands of the PE spectrum of OsO4 and RuO4
Summary
Mononuclear and polynuclear organometallic clusters are a class of interesting and technologically relevant complexes due to their widespread use in catalysis.[1]. The present work has been motivated by the results of preliminary calculations of photoionization parameters of polynuclear heavy metal complexes, such as Ru3(CO)[12] and Os3(CO)[12], which are currently performed by our group with the aim to elucidate the correlation between the electronic structure and the photoionization observables. This task proves quite challenging due to the complex electronic structure of these systems,[4,5] which makes even the assignment of the experimental spectra difficult. Computational methods to describe photoionization observables are well established for relatively simple and medium-sized organic molecules.[12−15] 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 ideal model cases, since they are relatively rigid and small-sized systems for which the assignment of the valence photoelectron spectrum has been elucidated in detail on a firm basis.[16−19]
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