Propagation und Streuung von hochintensiver Röntgenstrahlung in dichten atomaren Gasen und Plasmen

Nonlinear spectroscopy in the x-ray domain is a promising technique to explore the dynamics of elementary excitations in matter. X-rays provide an element specificity that allows them to target individual chemical elements, making them a great tool to study complex molecules. The recent advancement of x-ray free electron lasers (XFELs) allows to investigate non-linear processes in the x-ray domain for the first time. XFELs provide short femtosecond x-ray pulses with peak powers that exceed previous generation synchrotron x-ray sources by more than nine orders of magnitude. This thesis focuses on the theoretical description of stimulated emission processes in the x-ray regime in atomic gases. These processes form the basis for more complex schemes in molecules and provide a proof of principle for nonlinear x-ray spectroscopy. The thesis also includes results from two experimental campaigns at the Linac Coherent Light Source and presents the first experimental demonstration of stimulated x-ray Raman scattering. Focusing an x-ray free electron laser beam into an elongated neon gas target generates an intense stimulated x-ray emission beam in forward direction. If the incoming x-rays have a photon energy above the neon K edge, they can efficiently photo-ionize 1s electrons and generate short-lived core excited states. The core-excited states decay mostly via Auger decay but have a small probability to emit a spontaneous x-ray photon. The spontaneous emission emitted in forward direction can stimulate x-ray emission along the medium and generate a highly directional and intense x-ray laser pulse. If the photon energy of the incoming x-rays however is below the ionization edge in the region of the pre-edge resonance the incoming x-rays can be inelastically scattered. This spontaneous x-ray Raman scattering process has a very low probability, but the spontaneously scattered photons in the beginning of the medium can stimulate Raman scattering along the medium. The scattering signal can thus be amplified by several orders of magnitude. To study stimulated x-ray emission a generalized one-dimensional Maxwell-Bloch model is developed. The radiation is propagated through the medium with the help of the Maxwell equations and the radiation is coupled to the atomic system via the polarization. The atomic system is treated in the density matrix formalism and the time evolution of the coherences determine the polarization of the medium.Nichtlineare Spektroskopie im Röntgenbereich ist eine vielversprechende Technik, um die Dynamik von elementaren Anregungen in Materie zu erforschen. Die Wechselwirkung von Röntgenstrahlen mit Materie ist elementspezifisch und ermöglicht es individuelle chemische Elemente anzusprechen. Diese Eigenschaft ist besonders hilfreich, um komplexe Moleküle zu untersuchen. Der jüngste Fortschritt bei Freie-Elektronen-Lasern ermöglicht es zum ersten Mal nichtlineare Prozesse im Röntgenbereich zu untersuchen. Freie-Elektronen-Laser erzeugen Femtosekunden Röntgenpulse mit einer Maximalleistung, die vorherige Synchrotron Strahlungsquellen um neun Größenordnungen übertrifft. Diese Arbeit behandelt die theoretische Beschreibung von stimulierter Emission im Röntgenbereich von atomaren Gasen. Diese Prozesse bilden die Basis für komplexere Methoden in Molekülen und dienen als Machbarkeitsbeweis für nichtlineare Spektroskopie im Röntgenbereich. Die Arbeit enthält außerdem Resultate von zwei experimentellen Kampagnen an der Linac Coherent Light Source and präsentiert die erste experimentelle Demonstration von stimulierter Raman-Streuung im Röntgenbereich. Ein Freie-Elektronen-Laser Puls, der in ein langgezogenes Gas Medium fokussiert wird, erzeugt einen intensiven Strahl aus stimulierter Emission im Röntgenbereich in Vorwärtsrichtung. Wenn die Photonenenergie der einkommenden Röntgenstrahlung über der K-Absorptionskante in Neon liegt, können die Röntgenstrahlen kurzlebige hochangeregte Zustände durch effiziente Photoionisation von 1s Elektronen erzeugen. Diese angeregten Zustände zerfallen hauptsächlich durch Auger-Zerfall, aber haben eine geringe Wahrscheinlichkeit, ein spontanes Röntgenphoton zu emittieren. Die spontane Emission in Vorwärtsrichtung kann die Emission von weiteren Röntgenphotonen entlang des Mediums stimulieren and einen intensiven Röntgenlaser Puls erzeugen. Wenn die Photonenenergie des einkommenden Pulses allerdings knapp unterhalb der Ionisationskante in der Gegend der Resonanzen liegt, können die Röntgenstrahlen inelastisch gestreut werden. Diese spontane Raman-Streuung hat nur eine geringe Wahrscheinlichkeit, aber die spontan gestreuten Photonen am Anfang des Mediums können den Streuprozess im weiteren Medium stimulieren. Durch die stimulierte Raman-Streuung kann das Streusignal um mehrere Größenordnungen verstärkt werden. Um die stimulierte Emission im Röntgenbereich zu untersuchen, wird ein generalisiertes eindimensionales Maxwell-Bloch Model entwickelt. Die Strahlung wird dabei mit Hilfe der Maxwell-Gleichungen durch das Medium propagiert und über die Polarisation an das atomare System gekoppelt. Das atomare System wird als Dichtematrix behandelt and die Zeitentwicklung der Kohärenzen bestimmt die Polarisation des Mediums

Mapping Out Unequal Englishes in English-medium Classrooms

The link between globalization and the spread of English is well established in the literature, resulting in the emergence and burgeoning of studies on the pluralization and localization of English. However, Englishes are also valued unequally and, thus, impact the lives and identities of their speakers differently as well. This paper aims to discuss the politics of Unequal Englishes by mapping out the specific ways inequalities of Englishes are realized in classrooms in Singapore. This requires mapping out accurately both the dynamics of locally produced but globally shaped teaching of English, as well as concrete instantiations of culturally responsive pedagogies which aim to make learning and teaching more nondiscriminatory and equitable

Comment on Mie Scattering from a Sonoluminescing Bubble with High Spatial and Temporal Resolution [Physical Review E 61, 5253 (2000)]

A key parameter underlying the existence of sonoluminescence (SL)is the time relative to SL at which acoustic energy is radiated from the collapsed bubble. Light scattering is one route to this quantity. We disagree with the statement of Gompf and Pecha that -highly compressed water causes the minimum in scattered light to occur 700ps before SL- and that this effect leads to an overestimate of the bubble wall velocity. We discuss potential artifacts in their experimental arrangement and correct their description of previous experiments on Mie scattering.Comment: 10 pages, 2 figure

Sonoluminescence: Nature's Smallest BlackBody

The Spectrum of the light emitted by a sonoluminescing bubble is extremely well fit by the spectrum of a blackbody. Furthermore the radius of emission can be smaller than the wavelength of the light. Consequences, for theories of sonoluminescence are discussed.Comment: 8 pages, 3 Figure

Two-component radiation model of the sonoluminescing bubble

Based on the experimental data from Weninger, Putterman & Barber, Phys. Rev. (E), 54, R2205 (1996), we offer an alternative interpretation of their experimetal results. A model of sonoluminescing bubble which proposes that the electromagnetic radiation originates from two sources: the isotropic black body or bramsstrahlung emitting core and dipole radiation-emitting shell of accelerated electrons driven by the liquid-bubble interface is outlined.Comment: 5 pages Revtex, submitted to Phys. Rev.

‘Wiggle Matching’ Radiocarbon Dates

This paper covers three different methods of matching radiocarbon dates to the ‘wiggles’ of the calibration curve in those situations where the age difference between the 14C dates is known. These methods are most often applied to tree-ring sequences. The simplest approach is to use a classical Chi-squared fit of the 14C data to the 14C curve. This gives the calendar date where the data fit best and allows tests of how good the fit is. The only drawback of this method is that it is difficult to ascertain the uncertainty in the date found in this way. An extension of this technique uses a Monte-Carlo simulation to sample possible 14C concentrations consistent with the measurement made and for each of these possibilities performs a Chi-squared fit. This method yields a distribution of values in the calendrical time-scale, from which the overall dating uncertainty can be derived. A third, rather different approach, based on Bayesian statistics, calculates the relative likelihood of each possible calendar year fit. This can then be used to calculate a range of most likely dates in a similar way to the probability method of 14C calibration. The theories underlying all three methods are discussed in this paper and a comparison made for the fitting of specific model sequences. All three methods are found to give consistent results and the application of any one of them depends on the nature of the scientific question being addressed

Non-ergodicity of a globular protein extending beyond its functional timescale

Internal motions of folded proteins have been assumed to be ergodic, i.e., that the dynamics of a single protein molecule averaged over a very long time resembles that of an ensemble. Here, by performing single-molecule fluorescence resonance energy transfer (smFRET) experiments and molecular dynamics (MD) simulations of a multi-domain globular protein, cytoplasmic protein-tyrosine phosphatase (SHP2), we demonstrate that the functional inter-domain motion is observationally non-ergodic over the time spans 1012 to 107 s and 101 to 102 s. The difference between observational non-ergodicity and simple non-convergence is discussed. In comparison, a single-strand DNA of similar size behaves ergodically with an energy landscape resembling a one-dimensional linear chain. The observed non-ergodicity results from the hierarchical connectivity of the high-dimensional energy landscape of the protein molecule. As the characteristic time for the protein to conduct its dephosphorylation function is 10 s, our findings suggest that, due to the non-ergodicity, individual, seemingly identical protein molecules can be dynamically and functionally different

Differential criterion of a bubble collapse in viscous liquids

The present work is devoted to a model of bubble collapse in a Newtonian viscous liquid caused by an initial bubble wall motion. The obtained bubble dynamics described by an analytic solution significantly depends on the liquid and bubble parameters. The theory gives two types of bubble behavior: collapse and viscous damping. This results in a general collapse condition proposed as the sufficient differential criterion. The suggested criterion is discussed and successfully applied to the analysis of the void and gas bubble collapses.Comment: 5 pages, 3 figure

Calpain cleavage of Junctophilin-2 generates a spectrum of calcium-dependent cleavage products and DNA-rich NT1-fragment domains in cardiomyocytes

Calpains are calcium-activated neutral proteases involved in the regulation of key signaling pathways. Junctophilin-2 (JP2) is a Calpain-specific proteolytic target and essential structural protein inside Ca2+ release units required for excitation-contraction coupling in cardiomyocytes. While downregulation of JP2 by Calpain cleavage in heart failure has been reported, the precise molecular identity of the Calpain cleavage sites and the (patho-)physiological roles of the JP2 proteolytic products remain controversial. We systematically analyzed the JP2 cleavage fragments as function of Calpain-1 versus Calpain-2 proteolytic activities, revealing that both Calpain isoforms preferentially cleave mouse JP2 at R565, but subsequently at three additional secondary Calpain cleavage sites. Moreover, we identified the Calpain-specific primary cleavage products for the first time in human iPSC-derived cardiomyocytes. Knockout of RyR2 in hiPSC-cardiomyocytes destabilized JP2 resulting in an increase of the Calpain-specific cleavage fragments. The primary N-terminal cleavage product NT1 accumulated in the nucleus of mouse and human cardiomyocytes in a Ca2+-dependent manner, closely associated with euchromatic chromosomal regions, where NT1 is proposed to function as a cardio-protective transcriptional regulator in heart failure. Taken together, our data suggest that stabilizing NT1 by preventing secondary cleavage events by Calpain and other proteases could be an important therapeutic target for future studies