More on spin state dynamics

Recently, I wrote about our ultrafast spin dynamics project. The first publication in Phys. Rev. Lett. was a proof of concept article where we have shown the possibility of soft X-ray light to trigger an unprecedentedly fast change of a spin state. A follow-up article presenting the theoretical method used in this investigation in detail also appeared recently in the issue of Molecular Physics devoted to the anniversary of Andre D. Bandrauk:

Huihui Wang, Sergey I. Bokarev, Saadullah G. Aziz, Oliver Kühn Density matrix-based time-dependent configuration interaction approach to ultrafast spin-flip dynamics Mol. Phys. (2017) 1-10.

In this article, we reformulate the problem in the form of a density matrix which allows one to treat general open quantum systems with energy dissipation. In addition to the more thorough study of the influence of different parameters of an excitation pulse on the dynamics, we also discuss a regime where the strong electron correlation plays a decisive role. It was shown that core-excited electronic states may demonstrate entangled dynamics both due to the strong spin-orbit coupling and electron correlation. This makes them interesting objects for the future studies of the ultimate limits of the ultrafast electron motion in atoms, molecules, and extended systems.

Photosensitizers for photocatalytic water splitting

I suppose that you have noticed the hype around new energy sources during last decades even if you are not working in this field. As a continuation of the Light2Hydrogen project, we have also addressed the properties of quite popular iridium-based photosensitizers – molecules which absorb light and transfer energy or electrons further to catalysts. The idea behind this project is to use sunlight to split water into hydrogen and oxygen. The former can be then used as an ecologically clean fuel since upon combustion it produces only water.

inorganics-05-00023-ag-550

In collaboration with the group of Stefan Lochbrunner, we have studied the influence of chemical derivation onto the absorption spectra as well as the nature of the most important electronic states.  The “bright” absorbing states are important to acquire as much solar energy as possible. For this purpose, it is of interest to shift them to lower energies to increase the overlap with the sun spectrum. In turn, the character of the lowest “dark” state is essential for the efficiency of further reactions, for instance, electron transfer to water reduction agents. We have looked at a series of organic ligands bound to iridium ions and formulated recommendations which can help for the rational design of the photocatalysts.

The article is published in a themed issue under the editorship of Matthias Bauer:

O.S. Bokareva, T. Möhle, A. Neubauer, S.I. Bokarev, S. Lochbrunner, O. Kühn Chemical Tuning and Absorption Properties of Iridium Photosensitizers for Photocatalytic  Applications Inorganics 2017, 5(2), 23.


 

Direct or sequential? Actually both! – Spin crossover in iron compounds

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.

Febpy3_ToC_55x50_1

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.

Nuclear correlation effects viewed by X-ray spectroscopy

Continuing the developments of theoretical approaches to X-ray spectroscopy in our group we have recently published an article on the interplay (correlation) of nuclear motions and its implications for absorption (XAS) and resonant inelastic scattering spectra (RIXS):

S. Karsten, S.D. Ivanov, S.G. Aziz, S.I. Bokarev, O. Kühn Nuclear Dynamical Correlation Effects in X‑ray Spectroscopy from a Theoretical Time-Domain Perspective J. Phys. Chem. Lett., 2017, 8 (5), pp 992–996.

Despite working with high-energy electronic transitions and very short lifetimes, X-ray spectroscopy demonstrates remarkable sensitivity to nuclear motions which are characterized by much smaller energies and larger timescales. Given the prominent place of X-rays in material science, it is of importance since it broadens the scope of the effects which can be studied.

jz-2017-00037m_0005

Until now the coupling between electronic and nuclear degrees of freedom in core-level spectra has been analyzed following two strategies: performing numerically exact wave-packet quantum dynamics and applying analytic Franck-Condon model. The former being actively promoted by F. Gel’mukhanov’s group from Stockholm is in general too complicated for large molecules and needs reduction of complexity which could be a non-trivial task. The latter one, in turn, is too simplistic to recover nuclear effects beyond the harmonic approximation for nuclear vibrations.

In our article, we suggest a trajectory-based approach of intermediate complexity, where a system “decides” itself which regions of the phase space to explore and, thus, saving substantial computational effort for large molecules in comparison to exact quantum dynamics. Moreover, our protocol allows disentangling correlated and uncorrelated nuclear dynamics that opens new perspectives in the analysis of vibrational motion with the help of X-rays. Remarkably, we have demonstrated that second-order RIXS spectroscopy should be much more sensitive to nuclear correlation than the first-order XAS.

However, this is only the first proof-of-the-concept step to establishing a robust and versatile tool. In the process of derivation, implementantion, and discussion with colleagues, we have realized the key points, where the protocol needs to be improved.  Now we have a roadmap how to systematically approach the exact dynamics and the development is to be continued.

Ultrafast spin-flip dynamics in focus

Nowadays physics and chemistry are intensively developing within the ‘ultrafast paradigm’ addressing processes occurring on the femto- (10-15 s) and attosecond timescales (10-18 s). An outstanding progress has been achieved in understanding molecules in motion with femtosecond resolution including movies of chemical reactions. Going further down to hundreds and even tens of attoseconds, one can explore the fundamental limits of electronic motion in atoms and molecules.

tumblr

Apart from being of fundamental interest, the peculiarities of intricate electron dynamics in molecules have their practical implications, for instance, for molecular electronics limiting the speed of the signal transmission. That is why it has been extensively studied, although mostly theoretically since experimental observations represent a very non-trivial task and stay scarce.

During last decades, the devices where the spin of the electrons plays a decisive role has been suggested that gave birth to the new field of physics called spintronics. With this respect, the materials which undergo ultrafast spin-flip upon absorption of light attracted much attention. Systems, where such an effect was observed, are mostly transition metal complexes which can exist in low- and high-spin forms.

Recently, an article from the subgroup headed by me appeared on the pages of Physics Reviews Letters suggesting a new mechanism for the ultrafast spin-flip in transition metal complexes. It was demonstrated on the example of the Fe(II) aqua complex where the excitation with the soft X-ray light created a hole in the 2p level of iron. Due to the strong spin-orbit coupling in the core-excited electronic state the spin transition takes only about 2 fs what is about 100 times faster than rates reported for valence excitations before. Moreover, with a modest variation of the excitation pulse length and its carrier frequency one can potentially govern the efficiency of such spin transition. This makes a basis for future manipulations of the spin states using short wavelength light.

The effect in focus, of course, needs experimental verification. However, such an experiment requires intense isolated attosecond X-ray pulses and is very difficult to realize. Nevertheless, we expect the experimental evidence to appear due to the upcoming X-ray free electron lasers and future developments of high-harmonic generation setups.