Strong field x-ray physics
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 [1-3], stimulated resonant inelastic x-ray scattering (SRIXS) [4-5], as well as strong x-ray induced wave packet dynamics . 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 is 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, S. B. Zhang and N. Rohringer, "X-ray lasing in the CO molecule," Journal of Physics B : Atomic, Molecular and Optical Physics, vol. 46, no. 16, 2013.
3. V. Kimberg, S. B. Zhang and N. Rohringer, "X-ray lasing in diatomic molecules," Journal of Physics, Conference Series, vol. 488, no. 1, 2014.
4. V. Kimberg and N. Rohringer, "Stochastic stimulated electronic x-ray Raman spectroscopy," Structural Dynamics, vol. 3, no. 3, 2016.
5. 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.
6. 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.
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 in RIXS .
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.
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, 2]. 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 [3,4] by combining ultra-high resolution RIXS measurements for gas-phase water with state-of-the-art simulations. 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.
 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.
 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.
 R. C. Couto et al., "Selective gating to vibrational modes through resonant X-ray scattering," Nature Communications, vol. 8, 14165, 2017.
 KTH News, "Closer look at atomic motion in molecules may benefit biotech researchers," by David Callahan, Feb 15, 2017.