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Attosecond X-ray spectroscopy of liquids

Periodic Reporting for period 2 - ATTOLIQ (Attosecond X-ray spectroscopy of liquids)

Reporting period: 2019-10-01 to 2021-03-31

The goal of this project is the development of attosecond soft-X-ray spectroscopy to resolve the primary quantum-mechanical processes underlying charge and energy transfer. Such processes are involved in all chemical reactions, biological processes and in many technological applications. Their understanding on the most fundamental level, i.e. on the time and length scales of electronic motion in the microcosm, may lead to important scientific and technological advances based on a rational design of molecular function and reactivity. The overall objective consists in demonstrating soft-X-ray absorption spectroscopy with attosecond temporal resolution, first in the gas phase and then in the liquid phase using the recently introduced flat-microjet technology. In the gas phase, the goal is to observe attosecond charge migration and its dephasing through nuclear motion with element and site-sensitivity. In the liquid phase, the goal is to observe electronic coherence induced by strong-field ionization and its influence on subsequent structural dynamics of the ionized liquid. These techniques will then be extended to probe electronic dynamics of solvated molecules, nanoparticles and transition-metal complexes.
A beamline for attosecond time-resolved soft-X-ray spectroscopy up to the oxygen K-edge (~540 eV) has been developed. This beamline has been applied to realize the first attosecond transient-absorption experiments at the carbon K-edge. We have studied the dynamics following strong-field ionization of ethylene in the gas phase. This has led to the observation of the D1D0 electronic relaxation of the ethylene cation in less than 7 fs, which is the fastest electronic relaxation dynamics observed to date. Comparison with detailed quantum-dynamics simulations has revealed ballistic motion across a conical intersection to be responsible for this extremely fast and efficient process. Promising results have also been obtained on the methane and ethane cations. A second beamline for X-ray absorption spectroscopy equipped with a flat microjet has also been developed. This setup has been used to demonstrate extreme-ultraviolet high-harmonic generation for the first time. The properties of high-harmonic generation in liquids have been characterized in detail, including the scaling of the cut-off and yield with driving intensity, the ellipticity dependence of the harmonic yield and the coherence of the high-harmonic emission. More recently, the polarization and the wavelength dependence of HHG in liquids have also been studied. Moving on from high-harmonic spectroscopy to X-ray absorption spectroscopy, we have obtained the first femtosecond time-resolved spectra in the soft-X-ray domain from a liquid target. Specifically, we have studied the electronic and nuclear dynamics following the ionization of liquid methanol and pyridine. Turning from absorption to photoelectron spectroscopy, we have completed the first attosecond time-resolved measurements in liquids by measuring photoemission delays of 50-70 attoseconds between liquid and gaseous water. The interpretation of this measurements has shown that the delays are dominated by solvation effects, i.e. they probe the influence of the liquid environment on the photoionization delays of water molecules. In a complementary experiment based on electron-ion-coincidence spectroscopy, we have measured photoionization delays as a function of the size of water clusters. We have found that the delays correlate linearly with the spatial extension of the electron hole quantified as the first moment of the electron-hole density. This result confirmed the interpretation of the liquid-phase results and provided a molecular-level understanding of attosecond photoionization dynamics of liquid water. Using electron-electron coincidence spectroscopy, we have realized the first observation of intermolecular Coulombic decay in liquid water and have compared the corresponding electron spectra with those of water clusters. Finally, returning to the gas phase, we have observed attosecond charge migration in a neutral molecule, its de- and recoherence caused by nuclear wave-packet motion and the transfer of electronic coherence through a conical intersection.
The main achievements extending beyond the state of the art are the following. We have scaled the time resolution of soft-X-ray absorption spectroscopy from the femtosecond into the attosecond domain. This was the main goal of the present project and has been achieved. This progress has allowed us to resolve the fastest electronic relaxation ever measured and to show that it can take place on a time scale much shorter than any vibrational period. We have further observed the first emission of non-perturbative HHG from liquids. This represents a change of paradigm compared to previous knowledge. We have realized the first time-resolved soft-X-ray absorption measurements in liquids using a HHG source. These experiments have been done with ~30 fs time resolution, but it is clear that attosecond temporal resolution is within reach without considerable additional efforts. The measurements of time delays of liquid water represent the first attosecond time-resolved experiment on a liquid. Most importantly, we have been able to quantitatively explain these delays, which is perhaps the most significant scientific achievement of our group since its foundation. The validity of our interpretation has been experimentally confirmed in a completely independent experiment, i.e. the measurement of photoionization delays of water clusters as a function of cluster size. These measurements have revealed a novel, surprisingly direct relationship between photoionization time delays and the spatial extension of the created electron hole. Most importantly, we have been able to observe attosecond charge migration in a neutral molecule, its de- and recoherence caused by vibrational dynamics and the transfer of electronic coherence from the initially prepared pair of states to a different lower-lying electronic state. The progress realized within this reporting period is well in line with the anticipated time line. This makes us confident that the next important steps, i.e. the time-resolved measurement of ICD in liquid water and the realization of attosecond time-resolved X-ray absorption spectroscopy in liquids will be successful within the next reporting period.
High-harmonic generation in a liquid ethanol flat jet using a 1500-nm, 30-fs laser pulse