Study of a soft x-ray (1-5 nm) laser plasma source for femtosecond time resolved XAS

The dynamics of ultrafast phase transition and reaction mechanisms can be deduced from ultrafast x-ray diffraction or absorption measurements. Femtosecond lasers have been used recently to study matter dynamics with optical-pump and x-ray probe spectroscopy, using monochromatic K alpha x-ray radiation. We present here our most recent progress in the development of a femtosecond time- resolved x-ray absorption spectroscopy (XAS) system based on a broadband soft x-ray source in the 1-5 nm range. The femtosecond XAS system is designed to probe the electronic dynamics occurring during the vanadium dioxide (VO 2 ) semiconductor to metal phase transition following excitation by a femtosecond laser pulse. In the present experiments, broadband spectra near the vanadium L edge (511 eV) and oxygen K edge (525 eV) of VO 2 have been generated and measured with simultaneously high signal to noise ratio (100), high spectral resolution (ΔE/E=4x10 -3 ) and a 1.2 ps temporal resolution.