Lebensdauermessungen metastabiler atomarer Zustände in heliumähnlichen schweren Ionen

Die Messung von atomaren Lebensdauern eröffnet die Möglichkeit, diverse Aspekte der atomaren Struktur aber auch der Kernstruktur zu untersuchen. Im Fall von He-ähnlichem Gold läßt sich die relativistische Feinstrukturaufspaltung 2${}^3$P${}_0$-2${}^3$P${}_1$ untersuchen, da die Lebensdauer des metastabilen 2${}^3$P${}_0$-Zustands durch Hyperfein-Mischen aufgrund des Kernspins $I=3/2^+$ von ${}^{197}$Au von dieser Feinstrukturaufspaltung abhängt. In He-ähnlichem Uran läßt sich hingegen die 2s-Lamb-Verschiebung untersuchen, da aufgrund des verschwindenden Kernspins in ${}^{238}$U kein Hyperfein-Mischen auftritt. Basierend auf der Methode der Beam-Foil-Spektroskopie, wobei mithilfe eines Magnetspektrometers der Vorteil von Teilchen-Röntgen-Koinzidenzen ausgenutzt werden konnte, ist an der Beschleunigeranlage der GSI in Darmstadt die Lebensdauer des 2${}^3$P${}_0$-Zustands in heliumählichem Gold zu $au=22,08pm0,96$ ps und in heliumähnlichem Uran zu $au=58,2pm9,5$ ps bestimmt worden. Die experimentellen Resultate stimmen mit den theoretischen Vorhersagen überein. Das Hyperfein-Mischen hängt auch vom magnetischen Moment des Kerns ab. Unter der Voraussetzung, daß die Feinstrukturaufspaltung genau genug bekannt ist, können durch Messungen der hyperfein-gemischten Lebensdauer des 2${}^3$P${}_0$-Zustands umgekehrt Kern-g-Faktoren bestimmt werden

Interplay of thermal and non-thermal effects in x-ray-induced ultrafast melting

X-ray laser-induced structural changes in silicon undergoing femtosecond melting have been investigated by using an x-ray pump-x-ray probe technique. The experimental results for different initial sample temperatures reveal that the onset time and the speed of the atomic disordering are independent of the initial temperature, suggesting that equilibrium atomic motion in the initial state does not play a pivotal role in the x-ray-induced ultrafast melting. By comparing the observed time-dependence of the atomic disordering and the dedicated theoretical simulations, we interpret that the energy transfer from the excited electrons to ions via electron-ion coupling (thermal effect) as well as a strong modification of the interatomic potential due to electron excitations (non-thermal effect) trigger the ultrafast atomic disordering. Our finding of the interplay of thermal and non-thermal effects in the x-ray-induced melting demonstrates that accurate modeling of intense x-ray interactions with matter is essential to ensure a correct interpretation of experiments using intense x-ray laser pulses

Electronic Quantum Coherence in Glycine Molecules Probed with Ultrashort X-ray Pulses in Real Time

Structural changes in nature and technology are driven by charge carrier motion. A process such as charge-directed reactivity that can be operational in radiobiology is more efficient, if energy transfer and charge motion proceeds along well-defined quantum mechanical pathways keeping the coherence and minimizing dissipation. The open question is: do long-lived electronic quantum coherences exist in complex molecules? Here, we use x-rays to create and monitor electronic wave packets in the amino acid glycine. The outgoing photoelectron wave leaves behind a positive charge formed by a superposition of quantum mechanical eigenstates. Delayed x-ray pulses track the induced electronic coherence through the photoelectron emission from the sequential double photoionization processes. The observed sinusoidal modulation of the detected electron yield as a function of time clearly demonstrates that electronic quantum coherence is preserved for at least 25 femtoseconds in this molecule of biological relevance. The surviving coherence is detected via the dominant sequential double ionization channel, which is found to exhibit a phase shift as a function of the photoelectron energy. The experimental results agree with advanced ab-initio simulations.Comment: 54 pages, 11 figure

Alignment, orientation, and Coulomb explosion of difluoroiodobenzene studied with the pixel imaging mass spectrometry (PImMS) camera

Citation: Amini, K., Boll, R., Lauer, A., Burt, M., Lee, J. W. L., Christensen, L., . . . Rolles, D. (2017). Alignment, orientation, and Coulomb explosion of difluoroiodobenzene studied with the pixel imaging mass spectrometry (PImMS) camera. Journal of Chemical Physics, 147(1). doi:10.1063/1.4982220Laser-induced adiabatic alignment and mixed-field orientation of 2,6-difluoroiodobenzene (C6H3F2I) molecules are probed by Coulomb explosion imaging following either near-infrared strong-field ionization or extreme-ultraviolet multi-photon inner-shell ionization using free-electron laser pulses. The resulting photoelectrons and fragment ions are captured by a double-sided velocity map imaging spectrometer and projected onto two position-sensitive detectors. The ion side of the spectrometer is equipped with a pixel imaging mass spectrometry camera, a time-stamping pixelated detector that can record the hit positions and arrival times of up to four ions per pixel per acquisition cycle. Thus, the time-of-flight trace and ion momentum distributions for all fragments can be recorded simultaneously. We show that we can obtain a high degree of one-and three-dimensional alignment and mixed-field orientation and compare the Coulomb explosion process induced at both wavelengths. © 2017 Author(s)

Jitter-correction for IR/UV-XUV pump-probe experiments at the FLASH free-electron laser

In pump-probe experiments employing a free-electron laser (FEL) in combination with a synchronized optical femtosecond laser, the arrival-time jitter between the FEL pulse and the optical laser pulse often severely limits the temporal resolution that can be achieved. Here, we present a pump-probe experiment on the UV-induced dissociation of 2,6-difluoroiodobenzene (C6H3F2I) molecules performed at the FLASH FEL that takes advantage of recent upgrades of the FLASH timing and synchronization system to obtain high-quality data that are not limited by the FEL arrival-time jitter. We discuss in detail the necessary data analysis steps and describe the origin of the time-dependent effects in the yields and kinetic energies of the fragment ions that we observe in the experiment

Role of heat accumulation in the multi-shot damage of silicon irradiated with femtosecond XUV pulses at a 1 MHz repetition rate

The role played by heat accumulation in multi-shot damage of silicon was studied. Bulk silicon samples were exposed to intense XUV monochromatic radiation of a 13.5 nm wavelength in a series of 400 femtosecond pulses, repeated with a 1 MHz rate (pulse trains) at the FLASH facility in Hamburg. The observed surface morphological and structural modifications are formed as a result of sample surface melting. Modifications are threshold dependent on the mean fluence of the incident pulse train, with all threshold values in the range of approximately 36-40 mJ/cm². Experimental data is supported by a theoretical model described by the heat diffusion equation. The threshold for reaching the melting temperature (45 mJ/cm²) and liquid state (54 mJ/cm²), estimated from this model, is in accordance with experimental values within measurement error. The model indicates a significant role of heat accumulation in surface modification processes

Time-resolved investigation of the optical phase change as a potential diagnostics tool for extreme-ultraviolet free-electron-laser pump and optical probe experiments

Measurement of transient optical properties (reflectivity and transmissivity) is performed widely in extreme-ultraviolet (XUV) pump–optical probe experiments to study the transient state of irradiated materials. In order to extend the material diagnostics, here we propose an additional measurement of the transient phase change of the optical probe pulse. It can be recorded in parallel to other transient optical properties, enabling access to full information on the complex refractive index and thickness of the radiation-modified material layer. The latter is essential for investigations of phase transitions progressing in XUV (and x-ray) irradiated materials. We perform a computational study that clearly shows that the measurement of the optical phase from a probe pulse at correctly tuned pulse parameters can provide a signal strong enough to extract information on transient material properties. The calculations suggest that in some cases, it is even more preferable to measure the transient phase change than other optical parameters. Such phase measurement, feasible with modern experimental setups, can then be a basis for an improved diagnostics tool for the temporal characteristics of an ultrashort XUV pulse

Photoelectron spectroscopy method to reveal ionization potential lowering in nanoplasmas

Here we propose a scheme for probing the outer-shell atomic energy levels within a laser-created nanoplasma, using photoelectron spectroscopy data obtained from the irradiation of the nanoplasma with an ultraintense 'probing' pulse from a soft x-ray free-electron laser. The proposed method can then detect shifts of outer-shell energy levels of an atom or an ion within a plasma, due to the effect of charged plasma environment on atomic potentials, known as the 'plasma screening effect'. Various theoretical models exist that estimate the magnitude of the screening effect. However, the first experimental data that can verify theoretical models have become available only recently (Vinko et al 2012 Nature 482 59). They were obtained with a hard x-ray-based experimental method which uses the information encoded in fluorescence spectra and is, therefore, restricted to deep atomic shells. Below, we show that our photoelectron spectroscopy method of probing a nanoplasma with a destructive, high-intensity soft x-ray pulse that brings the irradiated system to the regime of 'massively parallel' ionization (Gnodtke et al 2012 Phys. Rev. Lett. 108 175003) enables access to the information on the energy level (and ultimately energy level shift) of the valence orbital. In particular, the result of such a photoelectron spectroscopy experiment could help to clarify the discrepancy between the ion abundances in nanoplasmas observed during the recent high-harmonic-generation and free-electron-laser experiments with intense soft x-ray pulses