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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - ATTOLIQ (Attosecond X-ray spectroscopy of liquids)

Teaser

Summary

The goal of this project is the development of attosecond soft-X-ray spectroscopy in the gas and liquid phases 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 gase 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.

Work performed

In this first phase, several experimental setups have been built. A complete beamline for attosecond transient absorption reaching up to the oxygen K-edge (540 eV) has been completed. First attosecond time-resolved experiments have successfullly been carried out on ethylene, methane and ethane. The interpretation of the results on ethylene are the subject of a theory collaboration, which turned out to be very successful, leading to complete and quantitative agreement between theory and experiment. Strong-field ionization has been found to populate the first-excited (D1) state of the ethylene cation and its lifetime has been measured to be less than 15 fs. The associated electronic and structural dynamics, as well as vibrational dynamics in the D0 state have been fully resolved and explained. Very promising results have also been obtained on CH4+, revealing few-femtosecond structural dynamics induced by a very strong Jahn-Teller effect. These results are currently being analyzed. In parallel, a second soft-X-ray absorption beamline has been built and equipped with a flat microjet setup. This setup has been used to observe high-harmonic generation in liquids. This observation represents a change of paradigm since the previous consensus in the literature was that liquids are too dense for coherent high-harmonic emission to take place. Our experiments have proven the opposite and have resulted in a detailed characterization of HHG in liquids. We have found that the high-harmonic cut-off scales close to linearly with the electric field, similar to HHG in solids, and contrasting with HHG in gases. All observed high-harmonic orders (in the extreme-ultraviolet) were found to scale non perturbatively with the driving field. The ellipticity dependence of HHG was found to be broader in liquids than in gases, which is attributed to the delocalization of the created valence hole in the liquid phase. Finally, comparing HHG spectra from different liquids, a characteristic spectral shape was observed for liquid water as compared to alcohols, which was explained in terms of the different electronic structure of the liquids with the help of the semi-conductor Bloch equations. This work has been published in Nature Communications. After this work, the flat microjet has been used for X-ray absorption spectroscopy. The electronic and structural dynamics induced by strong-field ionization of liquid methanol, ethanol and pyridine have been studied. In all pure liquids, the formation of a pre-edge absorption feature was observed and assigned to the creation of a valence hole. The results on methanol and ethanol have been summarized in a manuscript submitted to the Journal of Physical Chemistry Letters. The results on pyridine are currently being analyzed in collaboration with an external theory group. This analysis has been very successful resulting in a near-quantitative agreement between experiment and theory and the identification of the dissociation products of the pyridine cation. The most significant results that were obtained within the present reporting period concern the understanding of attosecond time delays in ionization of liquid water. This work had a strong theory and modelling aspect, which was complemented by the development of an apparatus for the measurement of attosecond time delays on water clusters. We have been able to show that the delays of 50-70 attoseconds that we had measured between gaseous and liquid water almost exclusively originate from the immediate environment of the water molecule in the liquid phase. This has been established by showing on one hand that the contribution of electron scattering during transport is extremely small (<2 as) and, on the other hand, that the first two solvation shells can explain the observed delays. A manuscript on these results is ready to be submitted. These conclusions have been further validated by measuring attosecond photoionization delays for small water clusters with full size r

Final results

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 already been achieved. 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. Finally, 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. The progress realized within this first reporting period is well in line, and even slightly ahead of the anticipated time line. This makes us confident that the next important steps, i.e. the observation of attosecond charge migration in gases and the realization of attosecond time-resolved X-ray absorption spectroscopy in liquids will be successful within the next reporting period.