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Weninger C.,Max Planck Institute for the Physics of Complex Systems | Rohringer N.,Center for Free Electronic Laser Science
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2013

We present a theoretical study on stimulated electronic Raman scattering in neon by resonant excitation with an x-ray free electron laser (XFEL). This study is in support of the recent experimental demonstration [C. Weninger, Phys. Rev. Lett. (to be published)] of stimulated x-ray Raman scattering. Focusing the broadband XFEL pulses into a cell of neon gas at atmospheric pressure a strong inelastic x-ray scattering signal in the forward direction was observed, as the x-ray energy was varied across the region of core-excited Rydberg states and the K edge. The broadband and intrinsically incoherent x-ray pulses from the XFEL lead to a rich, structured line shape of the scattered radiation. We present a generalized Maxwell-Liouville-von Neumann approach to self-consistently solve for the amplification of the scattered radiation along with the time evolution of the density matrix of the atomic and residual ionic system. An in-depth analysis of the evolution of the emission spectra as a function of the Raman gain is presented. Furthermore, we propose the use of statistical methods to obtain high-resolution scattering data beyond the lifetime broadening despite pumping with incoherent x-ray pulses. © 2013 American Physical Society.

Patterson D.,Harvard University | Schnell M.,Max Planck Institute For Struktur Und Dynamik Der Materie | Schnell M.,Center for Free Electronic Laser Science
Physical Chemistry Chemical Physics | Year: 2014

Chirality plays a fundamental role in the activity of biological molecules and broad classes of chemical reactions. The chemistry of life is built almost exclusively on left-handed amino acids and right-handed sugars, a phenomenon known as "homochirality of life". Furthermore, most drugs developed in the last decade are of specified chirality. Thus, fast and reliable methods that can differentiate molecules of different handedness, determine the enantiomeric excess of even molecular mixtures, and allow for an unambiguous determination of molecular handedness are of great interest, in particular with respect to complex mixtures. In this perspective article, we discuss the recent developments, with an emphasis on modern spectroscopic methods using gas-phase samples, such as photoelectron circular dichroism, Coulomb explosion imaging, and microwave three-wave mixing. © 2014 the Partner Organisations.

Schuster J.,Ludwig Maximilians University of Munich | Kohn R.,Center for Free Electronic Laser Science | Doblinger M.,Ludwig Maximilians University of Munich | Keilbach A.,Ludwig Maximilians University of Munich | And 2 more authors.
Journal of the American Chemical Society | Year: 2012

A new mechanism for mesostructure formation of ordered mesoporous carbons (OMCs) was investigated with in situ small-angle X-ray scattering (SAXS) measurements: thermally induced self-assembly. Unlike the well-established evaporation-induced self-assembly (EISA), the structure formation for organic-organic self-assembly of an oligomeric resol precursor and the block-copolymer templates Pluronic P123 and F127 does not occur during evaporation but only by following a thermopolymerization step at temperatures above 100 °C. The systems investigated here were cubic (Im3̄m), orthorhombic Fmmm) and 2D-hexagonal (plane group p6mm) mesoporous carbon phases in confined environments, as thin films and within the pores of anodic alumina membranes (AAMs), respectively. The thin films were prepared by spin-coating mixtures of the resol precursor and the surfactants in ethanol followed by thermopolymerization of the precursor oligomers. The carbon phases within the pores of AAMs were made by imbibition of the latter solutions followed by solvent evaporation and thermopolymerization within the solid template. This thermopolymerization step was investigated in detail with in situ grazing incidence small-angle X-ray scattering (GISAXS, for films) and in situ SAXS (for AAMs). It was found that the structural evolution strongly depends on the chosen temperature, which controls both the rate of the mesostructure formation and the spatial dimensions of the resulting mesophase. Therefore the process of structure formation differs significantly from the known EISA process and may rather be viewed as thermally induced self-assembly. The complete process of structure formation, template removal, and shrinkage during carbonization up to 1100 °C was monitored in this in situ SAXS study. © 2012 American Chemical Society.

Rohringer N.,Max Planck Institute for the Physics of Complex Systems | Rohringer N.,Center for Free Electronic Laser Science | Santra R.,German Electron Synchrotron | Santra R.,University of Hamburg
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2012

We present theoretical studies of a two-step resonant Auger process at high x-ray intensity. Tuning a short x-ray pulse to the initially closed resonant channel of the 1s-2p transition in singly ionized neon, the initially neutral neon target is valence ionized. Subsequently, the strong resonant x-ray field transfers an inner-shell electron to the created outer valence vacancy, thereby creating a core-excited state. The strong resonant coupling, giving rise to Rabi oscillations involving a core transition, results in a modification of the resonant Auger-electron spectral line profile. If the valence photoelectron remains unobserved, the system of the residual ion undergoing the resonant Auger decay can be treated by an open quantum system approach. The resonant Auger-electron spectral line shape is shown to be determined by an analog of the reduced density matrix that depends on two time arguments. The equations of motion of this reduced density matrix are derived and numerical results are presented, in support of the recent experimental verification [E. Kanter, Phys. Rev. Lett.PRLTAO0031-900710.1103/PhysRevLett.107.233001 107, 233001 (2011)] of this nonlinear x-ray optical effect. © 2012 American Physical Society.

Shubert V.A.,Max Planck Institute For Struktur Und Dynamik Der Materie | Shubert V.A.,Center for Free Electronic Laser Science | Schmitz D.,Max Planck Institute For Struktur Und Dynamik Der Materie | Schmitz D.,Center for Free Electronic Laser Science | And 5 more authors.
Angewandte Chemie - International Edition | Year: 2014

Chirality-sensitive broadband microwave spectroscopy was performed on mixtures of carvone enantiomers and conformers to distinguish enantiomers, measure enantiomeric excesses, and determine the absolute configurations of the enantiomers. This method uses microwave three-wave mixing and is inherently well-suited to the analysis of mixtures - a unique advantage over other techniques. In contrast to conventional microwave spectroscopy, the phase of the received signal is also exploited. This phase depends upon the signs of the molecules' dipole-moment components and is used to identify the excess enantiomer. The measured signal amplitude determines the size of the excess. The broadband capabilities of the spectrometer were used to simultaneously excite and measure two conformers of carvone, demonstrating the analysis of a sample with multiple chiral species. Employing quantum chemical calculations and the measured phases, the absolute configurations of the enantiomers are determined. Enantiomer differentiation, enantiomeric excess measurement, and absolute configuration determination within a mixture of gas-phase chiral molecules are demonstrated. In these experiments, microwave three-wave mixing within supersonic jets is combined with chirped-pulse broadband microwave spectroscopy. This new technique is now a significant step closer to broader application. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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