Deciphering the fingerprints of chemical bonds using X-ray spectroscopy

It is my pleasure to point your attention to our article on theoretical X-ray spectroscopy recently published in the 123th issue of the Research Features magazine. It describes some of our developments and case studies as well as our view on the future of the X-ray spectroscopy and its theoretical description on a popular level. Even if you are not expert in this field, you might find it curious to look inside. The article has open access.

RIXS

 

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A new twist on quasi-classical approaches to vibronic spectra

In continuation of our efforts for the description of nuclear vibrational effects in X-ray (or in general in vibronic) spectra, we have recently published an article:

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With this publication we tried to resolve the deficiency of the previously suggested method: while talking about transitions between several electronic states, the actual dynamics follows the ground state potential only. Here, making use of the expertise of Sven Karsten and Sergei Ivanov, we employ imaginary-time path integral technique to formulate a method accounting for the dynamics on multiple potential energy surfaces.
In principle, the quasi-classical approaches to the dynamics in the electronic ground state are well established. They employ the so-called Kubo-transformed time correlation functions. Kubo form is beloved by physicists due to its convenient symmetry properties making it the most classical-like quantum correlation function. In our article, we raised a question whether Kubo correlation function stays the most optimal choice when electronic transitions come into play? The answer is – not really.
If you criticize – suggest a better choice. That is why we introduce a generalized quantum time correlation function. It contains many well-established variants, including the Kubo one, as particular cases. But most importantly, it also provides a way to construct a family of new quantum correlation functions.  On the example of a 1D anharmonic model, we have shown that the new approaches may lead to superior results. This generalized strategy paves the way to seek for the even more optimal formulations.

Bachelor and Master: four at a time

Some time has already passed, and I realized that I have not written about the productive end of summer and beginning of autumn resulted in a two Bachelor and two Master defenses under my supervision. Although this was an interesting experience, the dense defenses together with a number of conferences and workshops kicked me out of balance, and I could barely return to my work during this period. Still trying to come up with the research schedule!

Another reason is that Julius Zimmermann has got on 7th of December the faculty price for his Master studies as the best student from Institute of Physics together with other three students from Biology, Chemistry, and Mathematics. He has given two honorable talks on his Master thesis: one at our institute and one at the annual colloquium of the Faculty of Mathematics and Natural Sciences of University of Rostock.  This is already the second student whom I supervised and who got this price. Lucky coincidence as my boss said 😉 This time I was not able to deliver my laudatory speech as I was in Paris on the X-ray workshop and Oliver Kühn did this job for me. Unfortunately, as the other honored student Marie Preuße, Julius did not stay in our group to get his Ph.D. and continued at Faculty of Engineering.

Überreichung der Fakultätspreise der MNF am 7.12.2017

The Master thesis of Julius entitled “Nuclear Dynamical Effects in Theoretical X-Ray Spectroscopy” goes along the lines of our recent publications on this topic in J. Phys. Chem. Lett. and J. Chem. Phys. He has introduced some improvements to the program written by Sven Karsten resulting in better performance. He has also implemented a protocol to allow for correlated electronic structure methods including the spin-orbit coupling which has appeared to be a major development. As a test system, we have chosen Fe(CO)5 as a prototypical catalytic species. I should acknowledge assistance from Dr. Dimitrios Manganas from the group of Prof. Frank Neese because he adjusted some features of RODFT-CIS module of ORCA package to our needs.

Another Master thesis “First principle trajectory based propagation of vibronic wavepackets in non-adiabatic systems” has been defended by Ludwig Ahrens-Iwers. Here, we have tried to apply direct dynamics variational multi-configurational Gaussian (DD-vMCG) method to describe X-ray time-resolved experimental studies of molecular photoionization in strong laser fields. This work was inspired by Prof. Hans Jakob Wörner from Zürich and his recent publication. We are also thankful to Prof. Gram Worth from Imperial College London for his advice in adjusting Quantics package to our needs. Ludwig has done a good job and suggested a couple of ways how to treat this complicated problem using diabatization of many potential energy surfaces on the fly.

Further, two Bachelor theses have been defended by Andy Kaiser and Otto Geburtig. In his work “Modeling Auger spectra for simple systems” cosupervised by Gilbert Grell, Andy has studied the effects of one- and two-center approximations on the Auger spectra of small molecules and respective speed up of calculations. Within his thesis “X-ray resonant inelastic scattering spectra of complex systems” Otto has implemented a method for calculation of RIXS spectra based on restricted excitation window time-dependent density functional theory.

This was a hard but still nice time. Thank you guys, it was a pleasure to work with you!

 

XTRAM 17

People living in central and northern Europe may have noticed that this summer has been way colder than the average. That is why I was happy to get an invitation from Nadja Doslic, one of the organizers of XTRAM17 summer school, to sunny Sicily at the end of July.

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The place of the conference was absolutely fantastic – it was a small ancient town Erice on top of the mountain overlooking the mostly champaign landscape of western Sicily. Despite its distal location, Erice is a scholarly center, where many of the scientific schools and conferences take place at the same time. The whole infrastructure of the town is adapted not only for crowds of tourists but the constant presence of conference attendees as well. The center of the scientific life in Erice is Ettore Majorana Center. Many of famous scientists such as Feynman, Wigner, Dirac, and many others have been reading lectures within the walls of this center. According to center’s website “Every year since 1963, authors of new discoveries and inventions come to Erice; 85 of them were awarded the Nobel Prize after their participation to the EMFCSC activities and 49 were already Nobel laureates.” Remarkably, the walls of the lecture hall are decorated with artistic depictions of Feynman diagrams. I hope you can imagine the supportive atmosphere of this meeting.

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Enough said about the place, but the meeting itself deserves positive comments as well. The abbreviation XTRAM is short for “School on X-UV time-resolved advanced methods” and is supported by the EU. Frankly speaking, this was a rare event when the scientific program was that relevant and interesting to me that I could not even decide which talks to skip to carve out time to prepare for my own lecture. Moreover, this was a nice opportunity to meet with people whom I did not know before. I think it is not a secret, that scientific community clusters around some prevailing topics and some cores of affluential researchers, who determine the agenda in each subgroup. Often, communities dedicated to the same subject overlap very little, and one can see publications where people cite works of their peers belonging to the same cluster, without knowing (or just ignoring) the others. Luckily, I have learned a lot about research going on in the community with which I am not that actively interacting but which deals with problems very related to my research. And of course, it was nice to talk and exchange ideas with people whom I know.

It is tough to select my favorites among the talks because they all were exciting. I was very interested to see how scientist working with non-equilibrium Green’s functions, Andrea Marini, Claudio Verdozzi, Enrico Perfetto, are treating electron dynamics of photoionization and Auger decay. There were also some talks on spintronics by Peter Eliot, Marco Battiato, Flavio Capotondi, as well as on electron dynamics treated with real-time TDDFT. For me, the talk by Martin Beye on recent progress in stimulated resonant inelastic X-ray scattering was very encouraging, since I consider this method as the primary experimental tool to examine ultrafast spin-dynamical effects, which are the main subject of my recently accepted project. This workshop provided me with an excellent overview of activities in the field and supplied me with a vast amount of information and literature which I would need to digest for months.

I should also mention lectures by Jesper Norell, Michael Odelius, and Philippe Wernet reporting the highlights of their recent and very prominent work representing a nice combination of novel experimental and theoretical tools in the field of time-resolved X-ray spectroscopy (see, e.g., nice this article). My own lecture was mainly devoted to ultrafast spin-dynamics in the core-excited states as it was explicitly requested by the organizers.

I would also like to thank Nina Ignatova, Jesper Norell, Vlad Kochetov, and Michael Odelius for nice time spent together in Erice. It was not only fun, but I hope should also promote our joint research.

DFG project accepted!

I would like to invite you for the piece of virtual cake!

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What’s the reason for that? Two days ago I have got an approval of my DFG project entitled “Soft X-ray spectroscopy and correlated many-electron dynamics of molecular systems from first principles theory”. (For those who don’t work in science in Germany, DFG (Deutsche Forschungsgemeinschaft) stands for German Research Foundation.) This project goes along the lines of our recent publications in Physical Review Letters and Molecular Physics which served as a preliminary work basis for the project.  It includes my own research position and a Ph.D. student for 3 years as well as money for the midterm workshop.

Remarkably, I was pleased to get excellent reviews, and what is surprising, referees even give me encouraging pieces of advice how to promote my scientific career and use the financial support from DFG in the most efficient way.

The chocolate cake was made by my wife Olga and decorated with the basic working expression for this project. It was successfully annihilated by my colleagues, that is why I can suggest you only its virtual counterpart. Nice offer, zero calories!

Transient photoelectrons and linkage isomerism

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:

A.A. Raheem, M. Wilke, M. Borgwardt, N. Engel, S.I. Bokarev*, G. Grell, S.G. Aziz, O. Kühn, I.Yu. Kiyan, Ch. Merschjann, E.F. Aziz Ultrafast kinetics of linkage isomerism in Na2[Fe(CN)5NO] aqueous solution revealed by time-resolved photoelectron spectroscopy Structural Dynamics 4, 044031 (2017)

Nuclear vibrations in X-ray spectra with a fine-tooth comb

One might remember the post where I have written about nuclear correlation effects showing up in absorption and resonant inelastic X-ray scattering spectra. A few days ago we have published a follow-up article, where this effect is scrutinously dissected:

Sven Karsten,  Sergey I. Bokarev,  Saadullah G. Aziz,  Sergei D. Ivanov, Oliver Kühn A time-correlation function approach to nuclear dynamical effects in X-ray spectroscopy J. Chem. Phys. 146, 224203 (2017).

X-ray nuclear dynamics

In the article, you can find an explicit derivation of the time-domain working expressions, a detailed description of our protocol, loads of formulas and graphs – the whole nine yards. Fans of math should do appreciate Sven’s efforts. Even more important, it represents a critical view of the method and suggests the route how to improve the main pitfalls of classical approximation with moderate effort.