Study of warm dense matter using time-resolved x-ray absorption spectroscopy

Abstract

Warm dense matter (WDM) is a highly transient state of matter with thermal energy comparable to the Fermi energy, and ions are strongly coupled to each other. Typically, WDM exhibits a temperature of 1 – 100 eV and a density close to that of a solid. This extreme condition can be produced by isochoric heating of nano-foil with the femtosecond optical laser pulse. WDM has garnered significant research interest in various fields as an intermediate state between condensed matter and plasma. Although it is challenging to study these extreme conditions owing to the complex interplay of physical processes in WDM, understanding the properties of WDM provides insights into non-equilibrium phase transitions, thermophysical properties, and energy relaxation processes under extreme conditions. This dissertation investigates WDM using the time-resolved x-ray absorption spectroscopy (tr-XAS) technique. The tr-XAS is implemented with XFEL and laser-plasma x-ray sources. First, the non-equilibrium dynamics and electron-phonon coupling parameters in WD-Cu are investigated using femtosecond tr-XAS with XFEL. Second, the characteristics of a grazing incidence x-ray spectrometer are introduced. The Ta laser-plasma x-ray source is characterized using this x-ray spectrometer, and the tr-XAS experimental station is constructed in the 150 TW laser facility, CoReLS. Finally, the evolution of bonding structures of carbon allotropes (amorphous carbon and diamond) using picosecond tr-XAS is presented.