A review of the theoretical developments to predict various kinds of soft X-ray spectra has been published by us recently. It describes how different observables can be extracted from the theoretical calculations: absorption (XAS), photoionization (PES or XPS), resonant inelastic scattering (RIXS), and Auger. The main focus is on L-edge spectra of transition metals and in particular on multi-reference methods as they appear to be essential for this type of systems. However, the review also contains a brief overview of other methods and applications. Enjoy!
Density functional theory is an indispensable theoretical method to study moderate to large systems due to its computational efficiency. Being physically sound, it relies on approximations since the explicit form of the density functional is not known. Most of the standard functionals available on the market have an inherent problem – electrons in the system experience spurious self-interaction. In many applications, this is not critical, but in some cases, it does matter.
A way to overcome it is to calculate exchange energy exactly. However, the perspective strategy is to add this “exact” correction only at long distances. The smooth function switching between short- and long-range behavior is density-dependent and thus varies for different systems. We use a purely first principles procedure on how to determine the parameters of the switching function for the particular system. As an end effect, we correct the orbital energies such that they become better estimates of ionization potentials which has immediate implication for the accuracy of the computed photoelectron spectra.
We have applied this procedure to the prediction of properties of charge-transfer states of photosensitizers and also to photoelectron spectroscopy before. In the recent publication
T. Möhle, O.S. Bokareva, G. Grell, O. Kühn, S.I. Bokarev Tuned Range-Separated Density Functional Theory and Dyson Orbital Formalism for Photoelectron Spectra J. Chem. Theory Comput. 14 (2018), 5870–5880.
we systematically analyze the performance of a combination of optimally-tuned density functionals with Dyson orbital approach. Thus, this approach relies on two cornerstones: reliable prediction of ionization energies and more accurate treatment of intensities using Dyson orbital formalism together with TDDFT.
We critically discuss the advantages and disadvantages of this procedure. It should be advisable in cases when the system studied with photoelectron spectroscopy has a non-singlet ground state. In this case, one has two non-equivalent ionization spin-channels which are otherwise unsatisfactorily reproduced. Moreover, TDDFT with the Dyson approach even levels the error of conventional functionals, and the results are not much different from more accurate range-sepated functional. Another two issues which might be problematic for our approach are the stability of the ground state with respect to orbital variations and spin-contamination. The latter one is unavoidable as either a non-ionized or ionized system has open electronic shells and needs to be treated by the unrestricted variant of DFT which introduces this undesirable spin mixing.
The recommendations formulated in this publication should facilitate the practical application of the protocol. Some of the unsolved issues warrant further research.
As a result of collaboration between our group and the group of Michael Odelius from Stockholm University an open-access article has been recently published on pages of Physical Chemistry Chemical Physics journal:
Jesper Norell, Gilbert Grell, Oliver Kühn, Michael Odelius, Sergey I. Bokarev Photoelectron shake-ups as a probe of molecular symmetry: 4d XPS analysis of I3– in solution Phys. Chem. Chem. Phys., 20 (2018) 19916.
The previous work from Michaels group dealt with the influence of the polarity and proticity of the solvent on the photoelectron spectrum of this model system. For instance, in water I3– represents a quite asymmetric moiety which can be described as I2—I–, whereas in less protic solvents the charge is fairly delocalized [I–I–I]–. In present work, we joined our forces and applied a protocol derived by us to the analysis of photoelectron intensities. The main focus was on assignment and relative intensities of different transitions. We have considered the so-called main transitions, when one electron is kicked out of a system, and the shake-ups, when in addition to removal of an electron another one is excited to a bound state. Relative intensities of these two types of bands appeared to be a convenient measure of the asymmetry of the structure. Understanding the interaction of I3– with the solvent may serve for better design of redox systems, e.g., dye-sensitized solar cells.
Christoph Merschjann, a colleague, who has been working in Rostock in the group of our experimental collaborator Stefan Lochbrunner and then moved to the group of our collaborator Emad Aziz in Berlin, has created a new personal website. Please enjoy!
An interesting fact is that we have not done any joint study while working nearby, but our collaboration was quite fruitful when Christoph changed for Helmholtz-Zentrum and resulted in a recently published paper.
Linkage isomerism is a well-known phenomenon in coordination chemistry. Multiatomic ligands can bind to a central metal atom with either of their ends. In some compounds, ligands can undergo a change of their orientation upon absorption of light. This effect can be used to, e.g., store information and energy. The prominent example is nitroprusside anion [Fe(CN)5NO]2-, where NO+ moiety changes from Fe-NO orientation to a side-on one with both N and O bound to iron. In the recent work, we have applied both transient photoelectron spectroscopy and theoretical modeling to reveal the ultrafast kinetics of this process:
Ultrafast kinetics of linkage isomerism in Na2[Fe(CN)5NO] aqueous solution revealed by time-resolved photoelectron spectroscopy Structural Dynamics 4, 044031 (2017)
In continuation of our collaboration with Prof. Emad F. Aziz and Dr. Igor Yu. Kiyan from Helmholtz-Zentrum in Berlin, a new investigation has been recently published. In this study, we address the early photodynamics of ferricyanide ion in solution applying transient XUV photoelectron spectroscopy in tandem with theoretical modeling.
Light-induced relaxation dynamics of the ferricyanide ion revisited by ultrafast XUV photoelectron spectroscopy Phys. Chem. Chem. Phys., 2017,19, 14248-14255
This combination has been already applied by us to unravel peculiarities of spin crossover in [Fe(bpy)3]2+ complex. Here, we have addressed the problem of charge localization and symmetry-breaking in the simple prototypical coordination compound – ferricyanide. Upon absorption of UV light, it is excited to the charge-transfer state, which can undergo non-radiative relaxation to the ground state or be involved in further chemical reactions. This is a usual trait of coordination and organometallic compounds, which is often used by nature and chemists in, e.g., photosynthesis or photocatalytic retrieval of ecologic fuels.
In previous UV pump – IR probe spectroscopic study of the photochemical fate of ferricyanide, it was concluded that the initially populated charge-delocalized state relaxes to the localized one and the process is driven by the reorganization of the polar solvent. However, we obtained strong evidence for the spin crossover followed by geometrical distortions due to Jahn–Teller effect, rather than localization/delocalization dynamics, as suggested previously. Remarkably, our interpretation also consistently explains the transient features observed in UV-IR pump-probe experiments along with transient XUV PES.
With the immense growth of the amount of information, the development of new principles in high-density data storage comes into the foreground. Exploiting spin magnetic moment for the storage of data is one of the perspective directions. Remarkably, the orientation of spin can be changed by absorption of light with high selectivity and this new magnetic state can live relatively long. Thus, studies of transition metal materials undergoing so-called spin crossover are growing in number. We have also put our two cents in the discussion closing down the debates whether the mechanism of spin transition is direct or sequential.
Usual tools to study spin flips comprise transient optical spectroscopic methods such as pump-probe spectroscopies. Our colleagues from Free University in Berlin have addressed spin crossover in classical material iron(II) tris-bipyridine with a new type of spectroscopy where photoionization is used as a probe and we have provided theoretical support for the interpretation of their entangled data. This study has been recently published on pages of ChemPhysChem journal:
A. Moguilevski, M. Wilke, G. Grell, S.I. Bokarev, S.G. Aziz, N. Engel, A.A. Raheem, O. Kühn, I.Yu. Kiyan, E.F. Aziz Ultrafast Spin Crossover in [FeII(bpy)3]2+: Revealing Two Competing Mechanisms by Extreme Ultraviolet Photoemission Spectroscopy ChemPhysChem 18 (2017) 465-469.
There has been formulated no unambiguous opinion in the community on the mechanism of light-driven spin flip in iron(II) tris-bipyridine. In this process, the singlet spin configuration (no unpaired electrons) is changing to the long-living quintet (four unpaired electrons with parallel spins) within sub-100 fs time upon excitation with 400-580 nm light. After a long discussion, it has been concluded that direct spin flip of two electrons is highly improbable and a manifold of triplet intermediate states needs to be involved in a sequential process. However, recently an indication of direct mechanism has been found.
In our study, we have observed both pathways and even determined the branching ratio between both direct and sequential channels. To our opinion, our study resolves the cognitive dissonance involving mutually exclusive assumptions and adds to the profound mechanistic understanding of the spin crossover.