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.

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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.

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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.

WATOC 2017

I would like to pay your attention to the 11th triennial congress on theoretical chemistry taking place in Münich this year.  The early bird registration is closed already but you can go for the normal one. I have attended the previous WATOC in Santiago de Chile and my wife has taken part in the congress before that in Santiago de Compostela. This is definitely an event worth visiting. It will give you an opportunity not only to listen to the cutting edge developments in quantum chemistry in multithreaded regime (several sessions are running simultaneously)  but also learn about the research of those people who win medals and prizes. For instance, in Chile Arieh Warshel (Nobel Prize in Chemistry 2013) read a lecture. Hope to see you in Münich this summer!

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Photon school

Last Saturday I have had a pleasure to read a 2-hour lecture at the Photon School organized by the Helmholtz-Zentrum Berlin for Materials and Energy.  It was intended for chemistry and physics graduate students who are interested in the application of the spectroscopic methods to study the structure of matter. The program was quite tough and included one week of lectures and another one of practical training. The scope of topics comprised linear and non-linear spectroscopies in the range of photon energies from microwave to X-ray. It was a nice event and I could imagine that I would enjoy it if I were an attending student.

My lecture entitled “Introduction to electronic-structure theory for spectroscopy” focused on the hierarchy of the frequency- and time-domain methods as well as the basics of self-consistent field theory, configuration interaction, perturbation theory, and coupled-clusters techniques lying in the basement of the modern quantum chemistry. As far as I have understood talking to students most of them were experimentalists. I hope I did not overload them with theoretical details that much 🙂

This was my first time when I prepared a presentation with Prezi. It was a nice experience and I got quite some feedback concerning the spectacularity of the slides. For an illustration, the general overview of my lecture.

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An interested reader can find presentation itself on the repository of Prezi.

I would like to thank Annika Bande, Igor Kiyan, Kaan Atak, and Jean-Christophe Tremblay for discussions and nice time spent during lunch and dinner!

 

 

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.

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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.

A certified theoretical physicist gives birth to hopefully new brilliant scientist

The beginning of 2017 has been marked by two prominent events in our group. The long lasting struggle with the free electron functions finally resulted in the nice Master thesis of Tobias Möhle which was defended on 20th of January. Entitled “Finite Element Approach to Photoemission of Complex Molecules” it addresses the problem of obtaining wave functions of the electrons kicked out of a molecule by an absorbed light. These functions can be used further within the so-called Dyson orbital approach to predict intensities of photoelectron spectra. The novel idea in his work is to unite the power of optimally-tuned range-separated hybrid density functionals to obtain Dyson orbitals and the infinite/finite element method to get the free electron function accounting for the mean-field potential of the remainder ion. Despite numerical obstacles, Tobias has managed to formulate practical recipes and suggested ways of further improvement of the computational protocol. I must admit that supervising and working with such enthusiastic and hardworking people like Tobias gives me a permanent feeling of an explorer lifting the veil on the molecular world.

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The second pleasant event which has forced Tobias to speed up with his research is the birth of his son. Now in the family of two physicists who are both working on photoelectron spectroscopy from different sides, theory and experiment, appeared a new member which hopefully also will become a brilliant scientist. We all wish them a happy and healthy journey ahead of their family!

And here is some artwork illustrating both events which we have made with my wife Olga. The background is composed of the figures and formulae from Tobias’s thesis and the foreground is self-explanatory.

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Molecular Science blog

Here comes my first post in the Molecular Science blog. I am going to review the most prominent achievements in the molecular physics and quantum chemistry. In addition, some theoretical chemistry package howtos, overviews of quantum chemistry conferences, press releases of my own work, and news from my workgroup and our collaborators will appear here.