Search

The journal of physical chemistry / A 125, 10138���10143 (2021). doi:10.1021/acs.jpca.1c06033, We performed a time-resolved spectroscopy experixment on the dissociation of oxygen molecules after the interactionwith intense extreme-ultraviolet (XUV) light from the free-electronlaser in Hamburg at Deutsches Elektronen-Synchrotron. Using anXUV-pump/XUV-probe transient-absorption geometry with asplit-and-delay unit, we observe the onset of electronic transitionsin the O$^{2+}$ cation near 50 eV photon energy, marking the end ofthe progression from a molecule to two isolated atoms. We observetwo different time scales of 290 �� 53 and 180 �� 76 fs for theemergence of different ionic transitions, indicating differentdissociation pathways taken by the departing oxygen atoms.With regard to the emerging opportunities of tuning the centralfrequencies of pump and probe pulses and of increasing the probe���pulse bandwidth, future pump���probe transient-absorptionexperiments are expected to provide a detailed view of the coupled nuclear and electronic dynamics during molecular dissociation, Published by Soc., Washington, DC

Nature Communications 12(1), 643 (1-7) (2021). doi:10.1038/s41467-020-20846-1, High-intensity ultrashort pulses at extreme ultraviolet (XUV) and x-ray photon energies, delivered by state-of-the-art free-electron lasers (FELs), are revolutionizing the field of ultrafast spectroscopy. For crossing the next frontiers of research, precise, reliable and practical photonic tools for the spectro-temporal characterization of the pulses are becoming steadily more important. Here, we experimentally demonstrate a technique for the direct measurement of the frequency chirp of extreme-ultraviolet free-electron laser pulses based on fundamental nonlinear optics. It is implemented in XUV-only pump-probe transient-absorption geometry and provides in-situ information on the time-energy structure of FEL pulses. Using a rate-equation model for the time-dependent absorbance changes of anionized neon target, we show how the frequency chirp can be directly extracted and quantified from measured data. Since the method does not rely on an additional external field, we expect a widespread implementation at FELs benefiting multiple science fields by in-situ on-target measurement and optimization of FEL-pulse properties., Published by Nature Publishing Group UK, [London]

Rhabdomyolysis can present as a clinical challenge due to broad etiological spectrum, non-specific symptomatology and numerous systemic complications including acute kidney injury. Emerging evidence suggests that rhabdomyolysis may be an associated early or late complication of coronavirus disease 2019 (COVID-19) infection. We present a case of a 55-year-old woman with recent COVID-19 pneumonia who was later found to have non-traumatic rhabdomyolysis incidentally captured on COVID-19 screening labs.

Physical review / X 11(3), 031001 (1-9) (2021). doi:10.1103/PhysRevX.11.031001, In this work, we use an extreme-ultraviolet (XUV) free-electron laser (FEL) to resonantly excite the I: 4$d_{5/2}–σ^∗$ transition of a gas-phase di-iodomethane (CH$_2$I$_2$) target. This site-specific excitation generates a 4$d$ core hole located at an iodine site, which leaves the molecule in a well-defined excited state. We subsequently measure the time-dependent absorption change of the molecule with the FEL probe spectrum centered on the same I: 4$_d$ resonance. Using ab initio calculations of absorption spectra of a transient isomerization pathway observed in earlier studies, our time-resolved measurements allow us to assign the timescales of the previously reported direct and indirect dissociation pathways. The presented method is thus sensitive to excited-state molecular geometries in a time-resolved manner, following a core-resonant site-specific trigger., Published by APS, College Park, Md.

Lake Kivu, located on the border of Rwanda and the Democratic Republic of Congo, is a very peculiar lake in several aspects. The meromictic lake shows a vertical stratification dominated by high salt concentrations of up to 6 ‰ resulting in a very thick monimolimnion of 420 m (max depth ~492 m). This extremely large non mixing part of the lake functions as a reservoir for very high concentrations of volcanogenic gases like methane and carbon dioxide (up to 20 and 100 mmol/l respectively) resulting in a growing hazard for millions of local residents. Our aim of this study is to get insights into the hydrological dynamics, solute transport and the lakes mixing behavior utilizing radiometric dating with 39Ar.The noble gas isotope 39Ar (t1/2 = 269 a) covers a unique time span for studying the dynamics of aquatic and glacial systems of the last millennium. Although this tracer has been acknowledged for decades, studies so far are limited by its low abundance, little radioactivity and hence huge required sample sizes (~1000 L water). Until today environmental routine measurements are mainly confined to groundwater reservoirs, where nearly unlimited sampling is possible. The application of techniques from atomic physics using a magneto optical atom trap (MOT) solves the problem by reducing sample volume requirements by several orders of magnitude. The problem of the very low isotopic abundance of 10-16 is resolved by resonant multi-photon scattering of 39Ar in the MOT. This technique named Argon Trap Trace Analysis with its very low minimal sample size of 0.5 cm³STP pure argon enables easy sample handling in the field as well as common sampling procedures like Niskin bottles for aquatic systems, drill core sampling for glacial systems or as in the case of Lake Kivu spray chamber gas sampling in remote places. It is thus a door opener for new geophysical research fields that were excluded from radio-argon dating so far.Here we present our most recent results of sampling campaigns in 2018 and 2019 using samples of about 25 – 40 L gas-water mixtures corresponding to 0.5 – 10 cm³STP pure argon showing surprisingly high ages for the lake water.

Groundwater dating and travel time distributions are important tools and data for assessment of the vulnerability of water supply wells towards pollution from the surface. Here we present selected results from more than 30 water supply and monitoring wells from major Danish water companies. The wells were recently sampled and investigated using multiple environmental tracers including 85Kr, 39Ar, 3H/3He, 14C, SF6, CFCs and noble gases and different groundwater modeling techniques. The results demonstrate the value of groundwater dating and travel time estimations for the assessment of the history and fate of contaminants in the subsurface. This information is crucial for the assessment of the efficiency of measures to mitigate pollution of groundwater by harmful substances such as pesticides, nitrate and a large range of emerging contaminants. We demonstrate how groundwater ages and travel time distributions can be used to assess the vulnerability or susceptibility of water supply wells towards pollution, and how level specific sampling in long well screens can provide additional important information for assessment of the vulnerability of deep and shallow parts of a water supply well. Potential applications of the estimated travel time distributions include 1) improved management of well fields 2) development of pumping strategies and well screens minimizing the risk of pollution of drinking water wells, and 3) assessment of the adequacy of regulations established by authorities to protect valuable groundwater resources against pollution.

We present a technique for the direct measurement of the frequency chirp of high-frequency free-electron-laser pulses based on fundamental nonlinear optics. It is implemented in transient absorption geometry.

We report on the experimental observation of strong-field dressing of an autoionizing two-electron state in helium with intense extreme-ultraviolet laser pulses from a free-electron laser. The asymmetric Fano line shape of this transition is spectrally resolved, and we observe modifications of the resonance asymmetry structure for increasing free-electron-laser pulse energy on the order of few tens of $\mu$J. A quantum-mechanical calculation of the time-dependent dipole response of this autoionizing state, driven by classical extreme-ultraviolet (XUV) electric fields, reveals a direct link between strong-field-induced energy and phase shifts of the doubly excited state and the Fano line-shape asymmetry. The experimental results obtained at the Free-Electron Laser in Hamburg (FLASH) thus correspond to transient energy shifts on the order of few meV, induced by strong XUV fields. These results open up a new way of performing non-perturbative XUV nonlinear optics for the light-matter interaction of resonant electronic transitions in atoms at short wavelengths., Comment: 16 pages, 3 figures

Physical review letters 123(10), 103001 (2019). doi:10.1103/PhysRevLett.123.103001, We demonstrate time-resolved nonlinear extreme-ultraviolet absorption spectroscopy on multiply charged ions, here applied to the doubly charged neon ion, driven by a phase-locked sequence of two intense free-electron laser pulses. Absorption signatures of resonance lines due to $2p-3d$ bound–bound transitions between the spin-orbit multiplets $^3P_{0,1,2}$ and $^3D_{1,2,3}$ of the transiently produced doubly charged Ne$^{2+}$ ion are revealed, with time-dependent spectral changes over a time-delay range of (2.4±0.3) fs. Furthermore, we observe 10-meV-scale spectral shifts of these resonances owing to the ac Stark effect. We use a time-dependent quantum model to explain the observations by an enhanced coupling of the ionic quantum states with the partially coherent free-electron laser radiation when the phase-locked pump and probe pulses precisely overlap in time., Published by APS, College Park, Md.