| 1. |
- Wituschek, Andreas, et al.
(author)
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High-gain harmonic generation with temporally overlapping seed pulses and application to ultrafast spectroscopy
- 2020
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In: Optics Express. - 1094-4087. ; 28, s. 29976-90
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Journal article (peer-reviewed)abstract
- Collinear double-pulse seeding of the High-Gain Harmonic Generation (HGHG) process in a free-electron laser (FEL) is a promising approach to facilitate various coherent nonlinear spectroscopy schemes in the extreme ultraviolet (XUV) spectral range. However, in collinear arrangements using a single nonlinear medium, temporally overlapping seed pulses may introduce nonlinear mixing signals that compromise the experiment at short time delays. Here, we investigate these effects in detail by extending the analysis described in a recent publication (Wituschek et al., Nat. Commun., 11, 883, 2020). High-order fringe-resolved autocorrelation and wave packet interferometry experiments at photon energies > 23 eV are performed, accompanied by numerical simulations. It turns out that both the autocorrelation and the wave-packet interferometry data are very sensitive to saturation effects and can thus be used to characterize saturation in the HGHG process. Our results further imply that time-resolved spectroscopy experiments are feasible even for time delays smaller than the seed pulse duration.
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| 2. |
- Wituschek, Andreas, et al.
(author)
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Tracking attosecond electronic coherences using phase-manipulated extreme ultraviolet pulses
- 2020
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In: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 11
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Journal article (peer-reviewed)abstract
- The recent development of ultrafast extreme ultraviolet (XUV) coherent light sources bears great potential for a better understanding of the structure and dynamics of matter. Promising routes are advanced coherent control and nonlinear spectroscopy schemes in the XUV energy range, yielding unprecedented spatial and temporal resolution. However, their implementation has been hampered by the experimental challenge of generating XUV pulse sequences with precisely controlled timing and phase properties. In particular, direct control and manipulation of the phase of individual pulses within an XUV pulse sequence opens exciting possibilities for coherent control and multidimensional spectroscopy, but has not been accomplished. Here, we overcome these constraints in a highly time-stabilized and phase-modulated XUV-pump, XUV-probe experiment, which directly probes the evolution and dephasing of an inner subshell electronic coherence. This approach, avoiding any XUV optics for direct pulse manipulation, opens up extensive applications of advanced nonlinear optics and spectroscopy at XUV wavelengths.
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| 3. |
- Callegari, Carlo, et al.
(author)
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Application of matched-filter concepts to unbiased selection of data in pump-probe experiments with free electron lasers
- 2017
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In: Applied Sciences (Switzerland). - : MDPI AG. - 2076-3417. ; 7:6
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Journal article (peer-reviewed)abstract
- Pump-probe experiments are commonly used at Free Electron Lasers (FEL) to elucidate the femtosecond dynamics of atoms, molecules, clusters, liquids and solids. Maximizing the signal-to-noise ratio of the measurements is often a primary need of the experiment, and the aggregation of repeated, rapid, scans of the pump-probe delay is preferable to a single long-lasting scan. The limited availability of beamtime makes it impractical to repeat measurements indiscriminately, and the large, rapid flow of single-shot data that need to be processed and aggregated into a dataset, makes it difficult to assess the quality of a measurement in real time. In post-analysis it is then necessary to devise unbiased criteria to select or reject datasets, and to assign the weight with which they enter the analysis. One such case was the measurement of the lifetime of Intermolecular Coulombic Decay in the weakly-bound neon dimer. We report on the method we used to accomplish this goal for the pump-probe delay scans that constitute the core of the measurement; namely we report on the use of simple auto- and cross-correlation techniques based on the general concept of "matched filter". We are able to unambiguously assess the signal-to-noise ratio (SNR) of each scan, which then becomes the weight with which a scan enters the average of multiple scans. We also observe a clear gap in the values of SNR, and we discard all the scans below a SNR of 0.45. We are able to generate an average delay scan profile, suitable for further analysis: in our previous work we used it for comparison with theory. Here we argue that the method is sufficiently simple and devoid of human action to be applicable not only in post-analysis, but also for the real-time assessment of the quality of a dataset.
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| 4. |
- Andersson, Tomas, 1984-
(author)
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Free Metal Clusters Studied by Photoelectron Spectroscopy
- 2012
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Doctoral thesis (other academic/artistic)abstract
- Clusters are aggregates of a finite number of atoms or molecules. In the present work, free clusters out of metallic parent materials have been created and studied by synchrotron radiation-based photoelectron spectroscopy. The clusters have been formed and studied in a beam and the electronic structure of the clusters has been investigated. Conclusions have been drawn about the spatial distribution of atoms of different elements in bi-component clusters, about the development of metallicity in small clusters, and about the excitation of plasmons.Bi-component alloy clusters of sodium and potassium and of copper and silver have been produced. The site-sensitivity of the photoelectron spectroscopy technique has allowed us to probe the geometric distribution of the atoms of the constituent elements by comparing the responses from the bulk and surface of the clusters. In both cases, we have found evidence for a surface-segregated structure, with the element with the largest atoms and lowest cohesive energy (potassium and silver, correspondingly) dominating the surface and with a mixed bulk.Small clusters of tin and lead have been probed to investigate the development of metallicity. The difference in screening efficiency between metals and non-metals has been utilized to determine in what size range an aggregate of atoms of these metallic parent materials stops to be metallic. For tin this has been found to occur below ~40 atoms while for lead it happened somewhere below 20-30 atoms.The excitation of bulk and surface plasmons has been studied in clusters of sodium, potassium, magnesium and aluminium, with radii in the nanometer range. The excitation energies have been found to be close to those of the corresponding macroscopic solids. We have also observed spectral features corresponding to multi-quantum plasmon excitation in clusters of Na and K. Such features have in macroscopic solids been interpreted as due to harmonic plasmon excitation. Our observations of features corresponding to the excitation of one bulk and one surface plasmon however suggest the presence of sequential excitation in clusters.
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