Photoionization dynamics of the tetraoxo complexes OsO4 and RuO4
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) and compared to accurate experimental measurements. Schio et al. Inorg.Chem. (2020)
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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. |
Measured photoionization cross-section branching ratios for the first five PE bands of OsO4 are reported in the upper panel of Fig. 2, where DFT and TDDFT results are shown together with the experimental data. Apart from the description of the autoionization resonances, which is given only by the TDDFT method, whose predicted magnitudes appear strongly damped in the experiment, the most clear case where TDDFT is needed for a quantitative agreement with the experimental cross sections is for the ionizations of the d metal-based MOs 1e and 2t2 in the 50–100 eV photon energy range. Compared to the other outer valence ion states, the partial cross section for the ionization of the 2a1 state shows a distinctly different behavior for both the magnitude as well as for the presence of a maximum at around 45 eV in the DFT profile. We ascribe this resonant enhancement to the occurrence of a shape resonance in the t2 continua.
The experimental β parameters for the outer valence ionizations of OsO4 obtained in this work are shown in the lower panel of Fig. 2 together with the calculated theoretical values. The agreement between the DFT/TDDFT estimates and the experimental data is overall very satisfactory for all PE bands.
Comparison of our theoretical results with those based on a plane waves representation of the outgoing electron, also reported in Fig. 2, demonstrate that the plane waves method provides a poor description of the photoionization dynamics of OsO4.
Cooper minima, which are present in the ionization from Os 5d and Ru 4d AOs, do not affect ionizations from MOs that are involved in the formation of the covalent M–O bonds and have substantial metal d orbital character. A Cooper minimum is a purely atomic effect and occurs in the ionization from AOs with radial nodes.
Due to the presence of such minimum in the atomic d orbital ionizations, the cross section of this AO exhibits in the low energy region a steeper decrease tha nthat shown by AOs without radial nodes, e.g., the O 2p orbitals. The computed cross section of atomic d orbitals shows, in the low energy region, a decrease larger than that of the TDDFT cross sections of MOs with pronounced d metal character, namely 1e and 2t2, caused by the significant O 2p AO’s contributions to the MO character (Gelius’ model). The cross section for the 1e orbital ionizations is characterized, in both OsO4 and RuO4, by a somewhat larger decrease than that corresponding to the 2t2 MOs. This is due to the larger metal 5d (4d) contribution to the 1e MOs. The absence of a clear minimum in the TDDFT cross sections is, however, a clear indication of the metal– oxygen mixed character of the 1e and 2t2 MOs due to the covalent M–O bonds formation.
Overall, the photoionization dynamics of the two complexes displays signatures of both single particle and manybody effects. Purely single particle effects include the presence of shape resonances in selected ionization channels (2a1 orbital ionization). Many body effects include the super Coster– Kronig decay of np → nd giant resonances that profoundly affect the ionization dynamics around 60 eV photon energy and whose effects are predicted to be stronger in RuO4 compared to OsO4.
The occurrence of a Cooper minimum in ionization from MOs with d metal character has been ruled out by our theoretical predictions, highlighting the strong hybridization of the d metal orbitals with ligand orbitals
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Photoionization dynamics of tetraoxo complexes OsO4 and RuO4. |
Ultima modifica il Sabato, 10 Settembre 2022 17:55
