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Strocov V.N.,Paul Scherrer Institute | Schmitt T.,Paul Scherrer Institute | Flechsig U.,Paul Scherrer Institute | Patthey L.,Paul Scherrer Institute | Chiuzbaian G.S.,CNRS Laboratory of Physical Chemistry - Matter and Radiation
Journal of Synchrotron Radiation | Year: 2011

Operation of an X-ray spectrometer based on a spherical variable-line-spacing (VLS) grating is analyzed using dedicated ray-tracing software allowing fast optimization of the grating parameters and spectrometer geometry. The analysis is illustrated with optical design of a model spectrometer to deliver a resolving power above 20400 at a photon energy of 930 eV (Cu L-edge). With this energy taken as reference, the VLS coefficients are optimized to cancel the lineshape asymmetry (mostly from the coma aberrations) as well as minimize the symmetric aberration broadening at large grating illuminations, dramatically increasing the aberration-limited vertical acceptance of the spectrometer. For any energy away from the reference, corrections to the entrance arm and light incidence angle on the grating are evaluated to maintain the exactly symmetric lineshape. Furthermore, operational modes when these corrections are coordinated are evaluated to maintain either energy-independent focal curve inclination or maximal aberration-limited spectrometer acceptance. The results are supported by analytical evaluation of the coma term of the optical path function. This analysis thus gives a recipe for designing a high-resolution spherical VLS grating spectrometer operating with negligible aberrations at large acceptance and over an extended energy range. © 2011 International Union of Crystallography. Source

Guillemin R.,University Pierre and Marie Curie | Simon M.,CNRS Laboratory of Physical Chemistry - Matter and Radiation | Shigemasa E.,Japan Institute for Molecular Science
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2010

Atomic autoionization following O 1s resonant excitation in O 2 is studied using high-resolution electron spectroscopy. The resonant Auger spectrum showing evidence of ultrafast dissociation as well as the autoionization of oxygen atoms is measured and discussed. Evidence of the electronic Doppler effect is observed in both electronic decay mechanisms. ©2010 The American Physical Society. Source

Rueff J.-P.,Synchrotron Soleil | Rueff J.-P.,CNRS Laboratory of Physical Chemistry - Matter and Radiation | Raymond S.,Joseph Fourier University | Taguchi M.,RIKEN | And 7 more authors.
Physical Review Letters | Year: 2011

Measurement of the Ce valence in the heavy fermion CeCu2Si 2 is reported for the first time under pressure and at low temperature (T=14K) in proximity of the superconducting region. CeCu 2Si2 is considered as a strong candidate for a new type of pairing mechanism related to critical valence fluctuations which could set in at high pressure in the vicinity of the second superconducting dome. A quantitative estimate of the valence in this pressure region was achieved from the measurements of the Ce L3 edge in the high-resolution partial-fluorescence yield mode and subsequent analysis of the spectra within the Anderson impurity model. While a clear increase of the Ce valence is found, the weak electron transfer and the continuous valence change under pressure suggests a crossover regime with the hypothetical valence line terminating at a critical end point Tcr close to zero. © 2011 American Physical Society. Source

Salieres P.,CEA Saclay Nuclear Research Center | Maquet A.,CNRS Laboratory of Physical Chemistry - Matter and Radiation | Haessler S.,Vienna University of Technology | Caillat J.,CNRS Laboratory of Physical Chemistry - Matter and Radiation | Taieb R.,CNRS Laboratory of Physical Chemistry - Matter and Radiation
Reports on Progress in Physics | Year: 2012

The recently developed attosecond light sources make the investigation of ultrafast processes in matter possible with unprecedented time resolution. It has been proposed that the very mechanism underlying the attosecond emission allows the imaging of valence orbitals with Ångström space resolution. This controversial idea together with the possibility of combining attosecond and Ångström resolutions in the same measurements has become a hot topic in strong-field science. Indeed, this could provide a new way to image the evolution of the molecular electron cloud during, e.g. a chemical reaction in real time. Here we review both experimental and theoretical challenges raised by the implementation of these prospects. In particular, we show how the valence orbital structure is encoded in the spectral phase of the recombination dipole moment calculated for Coulomb scattering states, which allows a tomographic reconstruction of the orbital using first-order corrections to the plane-wave approach. The possibility of disentangling multi-channel contributions to the attosecond emission is discussed as well as the necessary compromise between the temporal and spatial resolutions. © 2012 IOP Publishing Ltd. Source

Dahlstrom J.M.,Albanova University Center | L'Huillier A.,Lund University | Maquet A.,CNRS Laboratory of Physical Chemistry - Matter and Radiation
Journal of Physics B: Atomic, Molecular and Optical Physics | Year: 2012

This tutorial presents an introduction to the interaction of light and matter on the attosecond timescale. Our aim is to detail the theoretical description of ultra-short time delays and to relate these to the phase of extreme ultraviolet (XUV) light pulses and to the asymptotic phaseshifts of photoelectron wave packets. Special emphasis is laid on time-delay experiments, where attosecond XUV pulses are used to photoionize target atoms at well-defined times, followed by a probing process in real time by a phase-locked, infrared laser field. In this way, the laser field serves as a clock to monitor the ionization event, but the observable delays do not correspond directly to the delay associated with single-photon ionization. Instead, a significant part of the observed delay originates from a measurement induced process, which obscures the single-photon ionization dynamics. This artefact is traced back to a phaseshift of the above-threshold ionization transition matrix element, which we call the continuum-continuum phase. It arises due to the laser-stimulated transitions between Coulomb continuum states. As we shall show here, these measurement-induced effects can be separated from the single-photon ionization process, using analytical expressions of universal character, so that eventually the attosecond time delays in photoionization can be accessed. © 2012 IOP Publishing Ltd. Source

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