Ultrafast x-ray absorption and emission spectroscopy using broad band x-ray pulse sources

Abstract

This thesis describes the ultrafast x-ray absorption and emission spectroscopy using broad band x-ray pulse sources. X-rays played very vital role in all field of science specially physics since after Wilhelm Conrad Roentgen's discovers of "Roentgenstrahlen" or X-rays in 1895. The biggest contribution of x-ray towards research is the information about the atomic structure of matter. The electronic configurations changes during chemical reactions, molecular or lattice vibrations, and structural phase transitions occur on a time scale of femtoseconds therefore in order to study and understand these process we required a radiation source which have pulse duration of femtosecond. During recent years x-ray science enters a new era of scientific research due to the emerging of novel type of pulsed x-ray sources which are equipped with very short pulse duration up-to few tens of femtoseconds. These ultra-short pulses are generated by the electron accelerators. In synchrotron femtosecond laser are used to cut out a femtosecond slice from the picosecond electron bunches and these sources are known as slicing source. Ultrashort x-ray pulses are also obtained from the laboratory based laser plasma x ray sources. These ultra-short x-ray pulse sources are very effective for the time resolved studies in pump probe scheme. We used laser plasma x ray source to perform these studies in which x rays from the laser plasma x ray source are used as the probe to observe the changes in the electronic system made by the pump laser pulse. Ultrafast time-resolved x-ray absorption near edge spectroscopy (XANES) experiment was performed on a magnetite Fe3O4 film using a femtosecond laser plasma x-ray source delivering Bremsstrahlung radiation. Temporal evolution of the XANES of Fe3O4 following an excitation by an infra-red (IR) laser pulse was observed in a pump-probe scheme. The charge transfer also causes an ultrafast increase in the IR transmission. We developed a hard x-ray spectrometer for the purpose of measuring x-ray emission spectrum excited by a single x-ray free electron laser (XFEL) pulse. A highly oriented pyrolytic graphite (HOPG) crystal was placed in von Hamos geometry to focus x-rays of the equal energy on a specific point in the detector plane. The spectrometer was tested at PAL-XFEL using Ni and NiO films. This spectrometer can be utilized to conduct various XFEL x-ray emission and absorption measurements using XFELs.