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Research activities

Dynamics in RIXS of gases and liquids

The dynamics of fragmentation and vibration of molecular systems with a large number of coupled degrees of freedom are key aspects for understanding chemical reactivity and properties. A resonant inelastic X-ray scattering (RIXS) is shown to be a wery powerfull tool, which allows in principle to break down a complex problem into elementary transition steps [1-5]. Moreover, local multi-mode nuclear wave packets created by X-ray excitation to different core-excited potential energy surfaces (PESs) will act as spatial gates to selectively probe the particular ground state vibrational modes and, hence, the PES along these modes. We demonstrate this principle in our recent work by combining ultra-high resolution RIXS measurements for gas-phase water with state-of-the-art simulations [3,5]. Our findings demonstrate how the core-excitation to an appropriate gating intermediate state leads to spatially selective wave packets which enables a disentanglement of the bending and the symmetric and anti-symmetric stretching vibrations in the water molecule. These methods were also succesfully applied to study of the structure and dynamics in the liquide phase of water and acetic acid [2-4].

1. F. Gel'mukhanov et al."Dynamics of resonant x-ray and Auger scattering," Reviews of Modern Physics, vol. 93, no. 3, 2021.
2. V. Savchenko et al."Hydrogen bond effects in multimode nuclear dynamics of acetic acid observed via resonant x-ray scattering," Journal of Chemical Physics, vol. 154, no. 21, 2021.
3. J. Niskanen et al."Compatibility of quantitative X-ray spectroscopy with continuous distribution models of water at ambient conditions," Proceedings of the National Academy of Sciences of the United States of America, vol. 116, no. 10, s. 4058-4063, 2019.
4. V. V. da Cruz et al."Probing hydrogen bond strength in liquid water by resonant inelastic X-ray scattering," Nature Communications, vol. 10, 2019.
5. R. C. Couto et al., "Selective gating to vibrational modes through resonant X-ray scattering," Nature Communications, vol. 8, 14165, 2017.

Strong field x-ray physics and x-ray pump-probe techniques 

The project is motivated by the progress in free-electron x-ray lasers (XFELs) around the world. Our aim is to develop a theory and simulation techniques relevant for the interpretation of the new experimental data in order to stimulate the process of scientific discovery in the strong field x-ray science. A strong focus will be put on developing and extending computational techniques and programs which target physical processes of key importance in future experimental activities in the area, such as stimulated x-ray emission and lasing [1], stimulated resonant inelastic x-ray scattering (SRIXS) [2,3], as well as strong x-ray induced wave packet dynamics [4,5]. Several new x-ray pump-probe techniques were proposed as new tools for advanced study of the induced vibrational and rotational dynamics in real time [5-9]. These techniques will then be applied to analyze and interpret data from experimental studies of new materials, biomolecules in relevant environments, the kinetics of phase transitions, heterogeneous catalysis as well as the properties of liquids and solutions. The project will help to enrich the expected scientific output from the new XFEL facilities. The project was supported by the Knut and Alice Wallenberg foundation, Swedish Research Council (VR), and Carl Tryggers Foundation. 

1. V. Kimberg and N. Rohringer, "Amplified X-Ray Emission from Core-Ionized Diatomic Molecules," Physical Review Letters,vol. 110, no. 4, 2013.
2. V. Kimberg et al."Stimulated X-ray Raman scattering : a critical assessment of the building block of nonlinear X-ray spectroscopy," Faraday discussions (Online), 2016.
3. V. Kimberg och N. Rohringer, "Stochastic stimulated electronic x-ray Raman spectroscopy," Structural Dynamics, vol. 3, no. 3, 2016.
4. S. B. Zhang, V. Kimberg and N. Rohringer, "Nonlinear resonant Auger spectroscopy in CO using an x-ray pump-control scheme," Physical Review A. Atomic, Molecular, and Optical Physics, vol. 94, no. 6, p. 063413, 2016.
5. Y. Zhu et al."Core-Excited Molecules by Resonant Intense X-Ray Pulses Involving Electron-Rotation Coupling," Chinese Physics Letters, vol. 38, no. 5, 2021.
6. V. Savchenko et al."Dynamical phase shift in x-ray absorption and ionization spectra by two delayed x-ray laser fields," Physical Review A, vol. 104, no. 1, 2021.
7. V. Savchenko et al."Photodissociation of water induced by a long UV pulse and probed by high-energy-resolution x-ray-absorption spectroscopy," Physical Review A, vol. 104, no. 3, 2021.
8. J.-C. Liu et al."Polarization-sensitive IR-pump-x-ray-probe spectroscopy," Physical Review A, vol. 103, no. 2, 2021.
9. N. Ignatova et al."Infrared-pump–x-ray-probe spectroscopy of vibrationally excited molecules," Physical Review A, vol. 95, no. 4, 2017.

Nuclear dynamics beyond the Born-Oppenheimer approximation

Nonadiabatic interaction between potential energy surfaces of different electronic states is a fundamental phenomenon to determine dynamical processes including the coupling of nuclear motion with electronic states (vibronic or electron-molecular-vibration coupling) in physics, chemistry, and biology. However, interaction even between two states has not yet been fully understood... To carry out such advanced studies of complicated excitation dynamics, we need to accumulate detailed information on the interaction of potential energy surfaces (PESs) theoretically and experimentally. We can now use not only lasers but also intense x rays from recently available high-brilliant synchrotron radiation and free electron lasers. The PES crossing between bound and dissociative states is one of the most fundamental interactions, which goes beyond the Born-Oppenheimer approximation. In order to study these processes in various x-ray spectroscopies we using quantum wave packet methods, taking into the vibronic coupling in intermediate core-excited [1-4] and final valence excited state [5-6]. Many interesting phenomena can be described in this case, e.g. recently observed anomalously strong two-electron one-photon transition [7] and valcence-Rydberg mixing [8] in RIXS and photoemission spectroscopy.

1. Y. Velkov et al."Origin of fine structures on the dissociative 1s ->sigma* resonance in X-ray absorption spectra of O-2,"Chemical Physics Letters, vol. 476, no. 4-6, pp. 147-150, 2009.
2. C. Miron et al."Vibrational Scattering Anisotropy Generated by Multichannel Quantum Interference," Physical Review Letters, vol. 105, no. 9, p. 093002, 2010.
3. V. Kimberg et al."Rydberg-valence mixing and interchannel coupling in resonant oxygen 1s inelastic x-ray scattering of O_2," Physical Review A. Atomic, Molecular, and Optical Physics, vol. 85, no. 3, 2012.
4. A. Lindblad et al."Vibrational scattering anisotropy in O2 -- €”dynamics beyond the Born–Oppenheimer approximation," New Journal of Physics, vol. 14, no. 11, 2012.
5. V. Kimberg et al."Single-Molecule X-Ray Interferometry : Controlling Coupled Electron-Nuclear Quantum Dynamics and Imaging Molecular Potentials by Ultrahigh-Resolution Resonant Photoemission and Ab Initio Calculations," Physical Review X, vol. 3, no. 1, 2013.
6. R. C. Couto et al."Coupled electron-nuclear dynamics in resonant 1 sigma -> 2 pi x-ray Raman scattering of CO molecules,"Physical Review A, vol. 93, no. 3, 2016.
7. R. C. Couto et al."Anomalously strong two-electron one-photon X-ray decay transitions in CO caused by avoided crossing,"Scientific Reports, vol. 6, 2016.
8. T. Gejo et al."Resonant inelastic x-ray scattering and photoemission measurement of O2 : Direct evidence for dependence of Rydberg-valence mixing on vibrational states in O 1 s → Rydberg states," Journal of Chemical Physics, vol. 147, no. 4, 2017.

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