Spectrométrie d'absorption des rayons X

Résumé

La spectrométrie d'absorption des rayons X est une spectrométrie d'absorption qui aide à déterminer la structure d'un matériau. Elle a l'avantage d'être sélective quant à l'espèce atomique observée. Histoire Au début du siècle, la technique de spectroscopie d’absorption des rayons X (XAS) (SAX) n’était pas assez avancée pour représenter le futur de la science. Son avancement est surtout dû à des chercheurs avec énormément de motivation, mais n’ayant pas les capacités d’expliquer la théorie derrière un phénomène. Les débuts de la spectroscopie d’absorption des rayons X ont commencé en l’an 1913. Cette découverte est attribuée à Maurice de Broglie grâce à son expérience. Celle-ci se compose d’un cristal sur un cylindre de baromètre enregistreur. Le cristal peut tourner à vitesse angulaire constante autour de l’axe vertical grâce à un mécanisme d’horloge intégré. L’observation de Broglie est que l’énergie du rayon X résultant varie en fonction de l’angle du cristal entre l’a

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Liquid phase in situ spectroscopic investigations of heterogeneous catalysts

Thibault Xavier Florian Fovanna

In this work, we have studied various aspects of catalysts and catalytic processes at the solid-liquid interface, e.g. the reduction of supported PdO, the formation of Pd hydrides, their quantification, the their reactivity in a transient in time-resolved manner, Ru and P-modified Ru based catalysts and zeolites. In order to provide structural information mainly X-ray absorption spectroscopy, infrared spectroscopy and X-ray diffraction were used, while the measurements were made possible by the realization of a dedicated setup including reactor cells. The cells are designed to allow for monitoring structural changes of oxidation state and nature of adsorbates in a time-resolved manner. This is of particular interest to study the reactivity of species, such as the Pd hydrides, formed on the catalyst in the liquid environment exploiting tools that are already widely applied to study gas phase reactions.

EPFL2020

Chargement

Over the last decade, ultrafast time-resolved X-ray absorption spectroscopy (XAS) has evolved to be now a mature and well-established experimental technique, giving extremely detailed information about the local geometrical and electronic structure in the early stages of a chemical reaction or biological process. Electronic structure changes are the fundamental driving forces triggering structural modifications in many chemical and biological reactions. Ultrafast time-resolved XAS is an ideal experimental technique to probe these changes in "real time" during the course of a chemical reaction, a biological function or a physical process. Ideally, the observation of ultrafast processes is made in conditions as close as possible as the natural ones, i.e., biological processes in physiological media (instead of crystals or frozen films). In this sense XAS offers unique capabilities, since it can be applied to any kind of system. In this thesis we successfully implemented a new scheme for measuring time-resolved XAS spectra with picosecond temporal resolution at MHz repetition rate. The increase in the data acquisition repetition rate provided an increase in the signal-to-noise (S/N) of a factor of > 20 compared to previous experiments in the kHz regime. To assess the improvements in this new data acquisition methodology, we used the light-induced spin transition in aqueous solutions of [Fe(bpy)3]2+ to benchmark our experiments. This system has been well-characterized by both ultrafast laser and x-ray time-resolved spectroscopies. The expected gain in data quality was confirmed, which allowed the recording of subtle changes in the pre-edge region of the XAS spectrum, reflecting the different electronic structure of [Fe(bpy)3]2+ upon the spin transition. We also present the investigation of the electronic and geometric structures of a series of metalloproteins (Myoglobin) in physiological solutions by means of X-ray absorption spectroscopy. To our knowledge, this is the first study of the structure of the different forms of Myoglobin in physiological media using XAS. The analysis of the XANES region of the spectrum using full-multiple scattering (FMS) formalism revealed that the ironnitrogen bond length in the porphyrins ring converged to a common value of about 2 Å, in contrast to the wide variation found in the crystallographic structures. Porphyrins are known to be very rigid structures due to the big number of carbon-carbon double bounds, supporting our conclusion. The relative geometry of the ligands with respect to the heme is reported for the whole series of Myoglobins investigated. In addition, time-resolved XAS has been measured for two types of Myoglobin, Carboxy-Myoglobin (MbCO) and (Nitrosyl-Myoglobin) MbNO. It has been shown that, as expected from previous studies, the transient structure of photo-excited MbCO resembles that of the deligated ferrous Myoglobin (deoxyMb). On the other hand, the transient structure of photo-excited MbNO at 50 ps deviates slightly from that of deoxyMb. The analysis of the transient XANES indicates that the NO molecule moves 2.88 Å away from the heme, staying closer to the iron atom than in the case of photo-excited MbCO. The NO geminate recombination time with the heme was also measured in "real time", and it has been found to occur in 216 ± 24 ps. This is the first direct measurement, i.e. sensitive to structural changes, of this geminate recombination time.

EPFL2011

Chargement

Beyond structure: ultrafast X-ray absorption spectroscopy as a probe of non-adiabatic wavepacket dynamics

Majed Chergui, Simon Neville

The excited state non-adiabatic dynamics of polyatomic molecules, leading to the coupling of structural and electronic dynamics, is a fundamentally important yet challenging problem for both experiment and theory. Ongoing developments in ultrafast extreme vacuum ultraviolet (XUV) and soft X-ray sources present new probes of coupled electronic-structural dynamics because of their novel and desirable characteristics. As one example, inner-shell spectroscopy offers localized, atom-specific probes of evolving electronic structure and bonding (via chemical shifts). In this work, we present the first on-the-fly ultrafast X-ray time-resolved absorption spectrum simulations of excited state wavepacket dynamics: photo-excited ethylene. This was achieved by coupling the ab initio multiple spawning (AIMS) method, employing on-the-fly dynamics simulations, with high-level algebraic diagrammatic construction (ADC) X-ray absorption cross-section calculations. Using the excited state dynamics of ethylene as a test case, we assessed the ability of X-ray absorption spectroscopy to project out the electronic character of complex wavepacket dynamics, and evaluated the sensitivity of the calculated spectra to large amplitude nuclear motion. In particular, we demonstrate the pronounced sensitivity of the pre-edge region of the X-ray absorption spectrum to the electronic and structural evolution of the excited-state wavepacket. We conclude that ultrafast time-resolved X-ray absorption spectroscopy may become a powerful tool in the interrogation of excited state non-adiabatic molecular dynamics.

Royal Soc Chemistry2016

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