Time-resolved EXAFS

Katsuya Oguri, Yasuaki Okano, Tadashi Nishikawa and Hidetoshi Nakano
Optical Science Laboratory

@The recent development of various ultrashort x-ray-pulse sources based on high-power femtosecond lasers has stimulated progress on ultrafast time-resolved x-ray probing techniques with femtosecond or picosecond resolution. In particular, the time-resolved extended x-ray absorption fine structure (EXAFS) approach is expected to become a powerful technique for probing ultrafast structural dynamics, because EXAFS provides such structural properties as bond distance and coordination number for various materials including amorphous materials and liquids [1]. Here, we present a time-resolved EXAFS technique with picosecond resolution that employs a soft x-ray pulse emitted from femtosecond laser-produced plasma. By employing this technique, we successfully observed the time evolution of an ultrafast melted Si L-edge EXAFS induced by femtosecond laser irradiation [2].
@We constructed an experimental laser-pump and x-ray probe setup based on a 100-fs Ti:sapphire laser system. Figure 1 shows an example of the absorption spectrum of Si foil. We clearly observed the LII,III edge at 99 eV, the LI edge at 150 eV, and the damped oscillation, which corresponds to EXAFS, in the region of 150 - 270 eV. Figure 2 shows the time evolution of the EXAFS spectrum at various time delays, which was extracted from each absorption spectrum. We can clearly see that the oscillation amplitude decreases with a small peak shift to a lower wave number at a time delay of 0 ps while there is no great difference between the EXAFS spectra of the pumped and unpumped samples at a time delay of -330 ps. The small peak shift indicates that the oscillation period becomes shorter than that of the unpumped Si and this strongly suggests that the Si-Si atomic distance was broadened slightly by the laser excitation. We obtained a Si-Si atomic distance of 2.43 Å, which is clearly larger than the value of 2.32 Å for solid Si obtained by Fourier transformation of the data. This expansion of the atomic distance can be explained in terms of the ultrafast production of liquid Si. At a time delay of +1670 ps, the EXAFS becomes too weak for us to analyze its oscillation structure. The disappearance of the oscillation indicates the further structural disordering of Si due to the onset of evaporation from the liquid phase to the gas-like phase. This result is the first step towards establishing ultrafast time-resolved EXAFS technique.
[1] T. Lee et al., Chem. Phys. 299 (2004) 233.
[2] K. Oguri et al., Appl. Phys. Lett.87(2005)011503

Fig. 1. Example of Si-absorption spectrum.   Fig. 2. Transient EXAFS spectra with (thin black line) and without (thick gray line) laser irradiation.