We investigate the capability of attosecond transient absorption spectroscopy to characterize the dynamics of inner-shell-excited systems. In the transient absorption spectroscopy setup considered, wave packets are prepared by an attosecond XUV pulse and probed by a femtosecond NIR pulse. By using this, we study coherent electron dynamics in core-excited xenon atoms. In particular, we clarify which aspects of the dynamics can be revealed when the wave packets are probed using an NIR pulse and analyze why the inner-shell hole dynamics is more difficult to probe than the dynamics of the excited electron. We perform a theoretical analysis of the transient absorption signal as a function of the time delay between the XUV pump and NIR probe pulses, treating the excitation pulse perturbatively and the probe pulse nonperturbatively. We also demonstrate that an additional NIR dressing field can dramatically influence the transient absorption spectrum. Our theoretical predictions are compared with experimental results, suggesting that a precise characterization of the NIR pulse is necessary for a qualitative and quantitative comparison.
Abstract We study the interaction of intense extreme ultraviolet (XUV) light with the 2s2p doubly excited state in helium. In addition to previously understood energy-level and phase shifts, high XUV intensities may lead to other absorption-line-shape distortions. Here, we report on experimental transient-absorption spectroscopy results on the 2s2p line-width modification in helium in intense stochastic XUV fields. A few-level-model simulation is realized to investigate the origins of this effect. We find that the line-shape broadening is connected to the strong coupling of the ground state to the 2s2p doubly excited state which is embedded in the ionization continuum. As the broadening takes place for intensities lower than for other strong-coupling processes, e.g. observed asymmetry changes of the absorption profile, this signature can be identified already in an intermediate intensity regime. These findings are in general relevant for resonant inner-shell transitions in nonlinear experiments with XUV and x-ray photon energies at high intensity.
A setup for an all-XUV transient absorption spectroscopy at free-electron lasers, was developed and employed to explore XUV-excited dynamics and XUV-driven nonlinear phenomena.
Abstract The Fano absorption line shape of an autoionizing state encodes information on its internal atomic structure and dynamics. When driven near-resonantly with intense extreme ultraviolet (XUV) electric fields, the absorption profile can be deliberately modified, including observable changes of both the line-shape asymmetry and strength of the resonance, revealing information on the underlying dynamics of the system in response to such external driving. We report on the influence of the XUV pulse parameters at high intensity that can be achieved with a free-electron laser (FEL) with statistically broadened spectra based on self-amplified spontaneous emission (SASE). More specifically, the impact of the FEL pulse duration is studied for the example of the doubly excited 2s2p resonance in helium, where line-shape modifications have been measured with XUV transient absorption spectroscopy in Fraunhofer-type transmission geometry. A computational few-level-model provides insight into the impact of different average pulse durations of the stochastic FEL pulses. These findings are supported by measurements performed at the Free-Electron Laser in Hamburg (FLASH) and provide further insight into XUV strong-coupling dynamics of resonant transitions driven by intense high-frequency FEL sources.
Ion mobility spectrometry is known to be a fast and sensitive technique for the detection of trace substances, and it is increasingly in demand not only for protection against explosives and chemical warfare agents, but also for new applications in medical diagnosis or process control. Generally, a gas phase sample is ionized by help of ultraviolet light, ss-radiation or partial discharges. The ions move in a weak electrical field towards a detector. During their drift they collide with a drift gas flowing in the opposite direction and, therefore, are slowed down depending on their size, shape and charge. As a result, different ions reach the detector at different drift times, which are characteristic for the ions considered. The number of ions reaching the detector are a measure of the concentration of the analyte. The method enables the identification and quantification of analytes with high sensitivity (ng l(-1) range). The selectivity can even be increased - as necessary for the analyses of complex mixtures - using pre-separation techniques such as gas chromatography or multi-capillary columns. No pre-concentration of the sample is necessary. Those characteristics of the method are preserved even in air with up to a 100% relative humidity rate. The suitability of the method for application in the field of food quality and safety - including storage, process and quality control as well as the characterization of food stuffs - was investigated in recent years for a number of representative examples, which are summarized in the following, including new studies as well: (1) the detection of metabolites from bacteria for the identification and control of their growth; (2) process control in food production - beer fermentation being an example; (3) the detection of the metabolites of mould for process control during cheese production, for quality control of raw materials or for the control of storage conditions; (4) the quality control of packaging materials during the production of polymeric materials; and (5) the characterization of products - wine being an example. The challenges of such applications were operation in humid air, fast on-line analyses of complex mixtures, high sensitivity - detection limits have to be, for example, in the range of the odour limits - and, in some cases, the necessity of mobile instrumentation. It can be shown that ion mobility spectrometry is optimally capable of fulfilling those challenges for many applications.
Abstract Using attosecond transient absorption spectroscopy for time delays where the near-infrared pump and the extreme ultraviolet (XUV) probe pulses overlap, sub-cycle structures in the build-up of absorption lines in xenon ions are investigated as a function of the pump intensity during strong-field ionization. We observe a half-cycle-periodic change in the line-shape asymmetry of the ionic 4d–5p resonances. Analyzing the line shapes, we find that in particular the phase of the induced dipole emission is modified, and the magnitude of this phase modulation decreases with increasing laser intensity. We discuss the influence of ground state depletion on interfering pathways involved in XUV-assisted strong-field ionization.
Abstract Contracted by the company h2e Power Systems Pvt. Ltd. based in Pune, India, Fraunhofer IKTS has developed a 1 kW(el) solid oxide fuel cell (SOFC) power generator during a three year system engineering and technology transfer project. The fuel cell system is based on the CFY stack technology by Plansee SE and IKTS, which incorporates state‐of‐the‐art ESC with Scandia‐doped Zirconia electrolytes. For the SOFC power generator, a chromium based interconnects (CFY) stack was integrated with a pre‐reformer, a tail‐gas oxidizer and heat exchangers into a HotBox‐module following a novel concept for least‐space‐demanding reactor integration and flow distribution. The applied system concept aimed at a very compact and robust, yet highly efficient power generator with optional heat extraction. Two major design decisions have been introduced in the process layout, i.e., a rated fuel utilization in the stack of 85% as well as a POX‐air pre‐heater for reducing the reformer air flow to lowest possible values. This approach lead to a waterless SOFC system with a net electrical efficiency above 40%. Two proof‐of‐concept (PoC) prototype systems were commissioned and tested for system concept validation. Based on the operating experience, three improved prototype units were built at IKTS and shipped to India for initial demonstration projects and field trials at the customer's site. At the same time, the technology transfer to the customer was initiated, in order to enable for a local manufacturing and deployment of SOFC systems in India. This paper outlines the system development approach and major technical achievements demonstrated on PoC prototype level.
Abstract The laser-field-modified dipole response of the first ionization threshold of helium is studied by means of attosecond transient absorption spectroscopy. We resolve light-induced time-dependent structures in the photoabsorption spectrum both below and above the ionization threshold. By comparing the measured results to a quantum-dynamical model, we isolate the contributions of the unbound electron to these structures. They originate from light-induced couplings of near-threshold bound and continuum states and light-induced energy shifts of the free electron. The ponderomotive energy, at low laser intensities, is identified as a good approximation for the perturbed continuum response.
The authors study the ultrafast buildup of a doubly excited Rydberg series in helium. They observe how individual resonances emerge out of the continuous background absorption and quantify the time for individual spectral lines to be separated.
