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XUV/X-ray Multidimensional Spectroscopy of Fundamental Electron Dynamics and Impulsive Control of X-ray Light

Final Report Summary - X-MUSIC (XUV/X-ray Multidimensional Spectroscopy of Fundamental Electron Dynamics and Impulsive Control of X-ray Light)

Within the ERC-funded X-MuSiC project we develop and scientifically apply novel methods for extreme ultraviolet (XUV) and x-ray light in order to observe, understand, and control few-and multi-electron motion in atoms and molecules. On the one hand, this is done by performing time-resolved measurements with (intense) short XUV and near-visible (VIS) laser pulses. On the other hand, we are pushing x-ray spectroscopy to its ultimate limits of resolution, to test fundamental theories by precision measurements. The project is structured into two subprojects:
i) Soft-x-ray multi-dimensional spectroscopy with attosecond and Free-Electron Laser (FEL) pulses applied to fundamental time-dependent electronic quantum processes in atoms
ii) Impulsive control of gain/absorption and dispersion of soft- and hard-x-ray light for resonant gain without inversion and x-ray frequency combs
Results already achieved in these two directions:
a) We performed the first XUV four-wave mixing experiment on atomic inner-valence states. In that experiment, we observed laser-induced coupling between even and odd inner-valence hole states after the absorption of an XUV photon. This approach holds promise for element-specific multidimensional spectroscopy of larger polyatomic systems and site-selective laser control when transferred to molecules.
Furthermore, in a successful FEL beamtime campaign at FLASH (DESY, Hamburg), we were able to measure and quantify the influence of a strong XUV light field on the correlated doubly-excited states of helium. Applying two copies of such strong-field XUV bursts to gas-phase molecules (O2 and organic iodine derivates), we temporally resolved molecular ionization dynamics in an XUV-pump–XUV-probe transient-absorption experiment.

b) In the intense XUV fields produced by Free-Electron-Lasers, we observed resonant Stark shifts for the first time in this high-frequency range. This observation could open new avenues to explore atomic structure and dynamics, and to test quantum-dynamics theories.

a) By applying our recently developed impulsive-control concept (the dipole control model) to the temporal overlap between an attosecond-pulsed XUV burst and a strong near-VIS laser pulse, we managed to reconstruct the strong-field pulse duration and shape. This measurement provides an in-situ characterization of the strong-field pulses at work in attosecond-transient and time-resolved absorption spectroscopy experiments, including their intensity. It will be extremely helpful in future precision tests of quantum-dynamics theories. Towards generalization of the concept, the impulsive-control mechanism was demonstrated in the liquid phase, on complex molecules in solution. This massively expands its applicability towards new routes of intense-laser control of reaction chemistry, including science fields such as (molecular) biology as well as technological applications in the chemical industry.

b) Regarding the impulsive control of hard x-rays, in beamtime campaigns in cooperation with the groups of Jörg Evers (MPIK) and Ralf Röhlsberger (DESY) at the synchrotron sources PETRAIII (DESY, Hamburg) and ESRF (Grenoble), we achieved the first demonstration of the enhancement of resonant hard-x-ray light by time-domain control of a Mössbauer absorber. We were able to use impulsive control to achieve resonant enhancement of hard x-ray light at 14.4 keV. The resonance employed in this case was a nuclear transition in 57Fe, which is of crucial importance for Mössbauer spectroscopy. This result and developed approach may benefit the entire multidisciplinary Mössbauer community by effectively creating a light source with enhanced brilliance/brightness on the Mössbauer transition. This result also further proves the generality of the method for arbitrary frequencies and interactions (here: nuclear instead of electronic resonance).

As a major unexpected outcome of the project, at the interface of XUV multidimensional spectroscopy and impulsive control, we found a general conceptual approach to temporally resolve resonant processes. It is based on the controlled and ultrafast switching, which we could implement with strong laser pulses and apply experimentally for atomic resonance. By doing so, we observed the buildup of the Fano resonance in Helium atoms (Science 2016). Along with the generalizing dipole-control model (J. Phys. B 2016) these concepts are universal and, in their applicability, go beyond lasers and atoms. In the future, they can be used for understanding and controlling dynamics in complex systems such as solid-state electronics/spintronics or dynamics in large (bio-)molecules.

In summary, these emerging scientific directions and their herein developed enabling technologies allow for tracking and interacting with the temporal evolution of multi-electron systems on the femtosecond and attosecond time scale in both atomic, molecular and condensed-phase environments. In particular, they hold the key to novel (e.g. element-selective) concepts of strong-field control of chemical reactions in polyatomic molecules. The synergy of both projects thus pave the road to future experiments in complex systems, that are also benefiting from the here developed concepts for combined (sub-)femtosecond temporal and (sub-)nm spatial resolution.