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.
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:
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.
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)
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:
A time-correlation function approach to nuclear dynamical effects in X-ray spectroscopy J. Chem. Phys. 146, 224203 (2017).
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.
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.
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.