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Towards applications of nonlinear X-ray spectroscopy

English title Towards application of nonlinear X-ray spectroscopy
Applicant Knopp Gregor
Number 165550
Funding scheme Project funding (Div. I-III)
Research institution Allgemeine Energieforschung Paul Scherrer Institut
Institution of higher education Paul Scherrer Institute - PSI
Main discipline Other disciplines of Physics
Start/End 01.02.2017 - 31.01.2020
Approved amount 268'907.00
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All Disciplines (2)

Discipline
Other disciplines of Physics
Physical Chemistry

Keywords (14)

nonlinear X-ray spectroscopy; X-ray absorption; two-photon-absortion; Free electron lasers; multiphoton ; Resonances ; X-ray emsision; cross-sections; four-wave-mixing; transient grating; stimulated Raman; X-ray Raman; TPA; HEROS

Lay Summary (German)

Lead
Die Röntgenspektroskopie ist eine der wesentlichen Methoden, um die Dynamik atomarer und elektronischer Strukturen in Materie zu untersuchen. Die Verwendung von ultrakurzen-Röntgenpulsen erlaubt hierbei lokal um den Kern gelegene Rumpf-Elektronen in unbesetzte Zustände anzuregen, während die Detektion der inelastisch emittierten Röntgenphotonen Information über die besetzten elektronischen Zustände der Materie liefert. Mit Hilfe freier Elektronen Laser (FEL), wie dem SwissFEL, können nun Röntgenpulse mit extrem hohen Intensitäten und ultrakurzen Pulsdauern erzeugt werden. Dieser bahnbrechende technische Fortschritt in der Röntgenpulserzeugung ermöglicht es neue spektroskopische Methoden im Bereich der nichtlinearen Spektroskopie und der Quantenoptik bei sehr kurzen Wellenlängen zu entwickeln.
Lay summary
Das Ziel des Projektes ist zu einem verbesserten Verständnis der nichtlinearen Wechselwirkungen von Licht und Materie im Röntgenbereich beizutragen. Hierbei sollen auch Wirkungsquerschnitte der zu untersuchenden Prozesse (z.B. der zwei Photonen Absorption) in Abhängigkeit der Röntgenenergie bestimmt werden. Im optischen Bereich lassen sich Prozesse wie Zwei-Photonen-Absorption, Stimulierte Raman Streuung oder Vier-Wellen-Mischen durch verschiedene Anteile der induzierten nichtlinearen Polarisation der 3’ten Ordnung beschreiben. Zusammenhänge unterschiedlicher nichtlinearer Prozesse im Röntgen Bereich und die Gültigkeit bestehender und gegebenenfalls die Entwicklung neuer Theorien ist ebenfalls Gegenstand der Untersuchung. Von besonderem Interesse sind hierbei Anregungen in die Nähe der Absorptionskanten. Unterschiede im Emissionsspektrum zwischen ein- und mehr-Photonen Anregungen über und unter der Kantenenergie erlauben die Quantifizierung der Zwei-Photonenabsorptionswirkungsquerschnitte. Diese sollen im Rahmen des Projekts für verschiedene Materialien mittels der Röntgenfluoreszenz und Röntgen Raman-Spekroskopie bestimmt werden.
Direct link to Lay Summary Last update: 30.01.2017

Responsible applicant and co-applicants

Employees

Abstract

Nonlinear Phenomena in X-ray absorption and emission spectroscopy will be investigated. Multiphoton processes induced at short X-ray wavelengths are experimentally challenging. Two-photon absorption (TPA) was originally predicted in 1931 by Maria Göppert-Mayer [1] and was experimentally demonstrated 30 years later in the optical regime [2]. The equivalent process in the hard X-ray regime has only recently been realized [3]. Even though nonlinear X-ray spectroscopy is considered as one of the most promising spectroscopies for unraveling quantum state correlations on atomic time and length scales, quantitative work on nonlinear X-ray scattering amplitudes and thus the values of cross sections of the involved processes is currently not available. The aim of this project is to better understand and investigate nonlinear effects, with a focus on the TPA process, caused by interaction of high intensity X-ray pulses with solid matter. Our intention is triggered by previous measurements of spectrally dispersed linear ‘high energy resolution off-resonant X-ray spectroscopy’ (HEROS), in which we observed spectral features that clearly show TPA processes at high X-ray fluences. In order to establish the general principles behind these results further experiments are required to investigate off-resonant one-photon and two-photon processes in other materials and for other core-level electronic excitations. To generate a more complete picture of X-ray TPA, we will explore the amplitudes for OPA (one-photon absorption) processes by means of off-resonant scattering measurements in the vicinity of an absorption edge. These experiments will be performed at the Swiss Light Source (SLS). In this context different oxidation states and elements (metals) will be addressed. The experimental synchrotron data will be combined with existing FEL results, to determine cross-sections for X-TPA processes from the OPA results. With a focus on particular resonance conditions, the energy dependence and the relation between OPA and TPA cross-sections in the X-ray regime will be investigated in detail. A further understanding of the underlying and competing processes in X-TPA will be investigated by performing combined optical-pump X-ray-probe experiments. In these measurements, both, the optical and the X-ray photon energy will be tuned around the involved resonances. The dispersed X-ray emission will be detected, using a von Hamos spectrometer setup, which enables us to derive the signal strength as function of the frequencies in a three-dimensional energy map. In accompanying experiments we will also draw also the attention to the subject of photoionization of multi-electron atoms with visible light. Additional information can be derived by optical multiphoton ionization (MPI) processes at high intensities. However, the transfer of results to the X-ray regime is not always straight forward, because competing processes can be different. Investigation on this limitation is achieved by comparative experiments that can be performed in the laser lab (PSI). The relation between TPA and the third order nonlinear susceptibility enables to determine the strength of other nonlinear X-ray spectroscopies, including X-ray stimulated Raman (X-SR), transient gratings (X-TG) or four wave mixing (X-FWM), once the TPA cross-sections are known. The generality and limitations of this relation in the X-ray regime will be explored. All findings will be compared to theoretical models describing X-ray nonlinear phenomena. The strengths and profiles of cross-sections in the X-ray regime and the understanding of the underlying processes is essential for the realization of advanced nonlinear experiments involving multiple X-ray photons [4] at FELs. We expect this project to contribute significantly in preparation of this new area of nonlinear X-ray spectroscopy. PSI provides the unique situation to employ a compact high resolution X-ray von Hamos emission spectrometer with guaranteed beam-time slots at the Swiss light source synchrotron (SLS) in addition to permanent access to our optical laser labs. Thus PSI is an excellent environment for realizing this project, which is quite congruent with intention to explore and develop methods for nonlinear X-ray experiments with the ultimate aim of applying these at the SwissFEL. During the PHD period, further experiments, which we continuously propose to FELs, will be conducted during this project.
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