Ultrafast X-ray & EUV Absorption Spectroscopy of Non-Equilibrium Warm Dense Matter

Abstract

Experimental setups for time-resolved extreme ultraviolet (EUV) and X-ray absorption spectroscopy were developed and a series of femtosecond and picosecond measurements have been performed to study the ultrafast electron dynamics in non-equilibrium warm dense matter (WDM). First, using high-harmonic generation (HHG) of a Ti:sapphire laser, femtosecond EUV absorption measurement setup for warm dense aluminum was developed. An aluminum nano foil was highly excited by intense femtosecond laser pulses, and the ultrafast evolution of EUV absorption around the L-edge was measured. The results indicate the existence of non-equilibrium electron distribution right after optical excitation, and the Fermi–Dirac distribution with a finite electron temperature is established in about 300 fs. This measurement can visualize the equilibration process of highly excited electronic system in aluminum, and the thermalization time-scale is an order of magnitude longer than the time scales of known electron-electron scattering processes. Second, using femtosecond X-ray pulses from an X-ray free electron laser (XFEL), an experimental setup for X-ray absorption spectroscopy was developed at the soft X-ray beamline of PAL-XFEL. As a representative of noble metal, a copper nano foil is excited by a femtosecond laser pulse. The femtosecond changes in the X-ray absorption around the L3-edge of warm dense copper show the highly excited d-band which thermalizes with sp-electrons over the duration of a picosecond. This result showed the widely used two-temperature model is non-applicable and questioned the fast thermalization concept to describe the nascent stage of intensively photoinduced material responses. Finally, by using picosecond X-ray Absorption Near Edge Structure (XANES) measurement setup at the Advanced Light Source, the electron mean-free-path in warm dense gold was investigated. A gold/copper nano foil is used as a target. The front Au layer is heated with a femtosecond laser pulse. The real Cu layer is heated via hot electrons transferred from Au at delayed time. The evolution of temperature at the real side of Au is determined by tr-XANES measurement of Cu L-edge. We investigated the electron motion is either ballistic or diffusive. In Au with a few eV temperatures, the electron mean-free-path seems to be less than 50 nm, which is a half of the known value (100 nm) in the ambient gold. This result infers the thermal conductivity of warm dense matter should be re-evaluated, and even for WDM experiments using nano-structured targets, the effects by thermal gradient should be considered more carefully.