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| Home > Publications database > Propagation and Scattering of High-Intensity X-Ray Pulses in Dense Atomic Gases and Plasmas |
| Book/Dissertation / PhD Thesis/Internal Report | PUBDB-2015-04152 |
2015
Verlag Deutsches Elektronen-Synchrotron
Hamburg
Report No.: DESY-THESIS-2015-041
Abstract: Nonlinear spectroscopy in the x-ray domain is a promising technique to explore the dynamicsof elementary excitations in matter. X-rays provide an element specificity that allows themto 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-linearprocesses in the x-ray domain for the first time. XFELs provide short femtosecond x-ray pulseswith peak powers that exceed previous generation synchrotron x-ray sources by more than nineorders of magnitude. This thesis focuses on the theoretical description of stimulated emissionprocesses in the x-ray regime in atomic gases. These processes form the basis for more complexschemes in molecules and provide a proof of principle for nonlinear x-ray spectroscopy. Thethesis also includes results from two experimental campaigns at the Linac Coherent LightSource 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 anintense stimulated x-ray emission beam in forward direction. If the incoming x-rays havea photon energy above the neon K edge, they can efficiently photo-ionize 1s electrons andgenerate short-lived core excited states. The core-excited states decay mostly via Auger decaybut have a small probability to emit a spontaneous x-ray photon. The spontaneous emissionemitted in forward direction can stimulate x-ray emission along the medium and generate ahighly directional and intense x-ray laser pulse.If the photon energy of the incoming x-rays however is below the ionization edge in the regionof the pre-edge resonance the incoming x-rays can be inelastically scattered. This spontaneousx-ray Raman scattering process has a very low probability, but the spontaneously scatteredphotons 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 isdeveloped. The radiation is propagated through the medium with the help of the Maxwellequations and the radiation is coupled to the atomic system via the polarization. The atomicsystem is treated in the density matrix formalism and the time evolution of the coherencesdetermine the polarization of the medium.
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