Spatiotemporally Resolved Soft X-Ray Absorption Spectroscopy of a Femtosecond Laser Ablation Plume


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

 Time-resolved x-ray measurement techniques have attracted much attention as a result of the recent progress on the ultrashort x-ray pulse generation technology based on laser-based x-ray sources and synchrotron radiation sources.  These techniques enable us to measure the ultrafast dynamics of crystal structures, atomic distances, coordination numbers, and electronic states in a highly nonequilibrium state, and they are expected to become a key technique for the new interdisciplinary field linking conventional x-ray science and ultrafast optical science, namely "ultrafast x-ray science" [1].
 We developed a spatiotemporally resolved XAS system by improving our previous time resolved XAS system [2].  Using the system, we measured the space and time evolution of a plume generated by the femtosecond-laser-ablation process, which is expected to become a new laser-processing technique [3].  We demonstrated that the system is a powerful tool for probing the complicated phenomenon that evolves temporally and spatially with the solid to liquid to gas phase transition, and the dissociation of chemical bonding [4].
 The most noteworthy characteristic of our system is that it combines the short pulse duration of femtosecond laser plasma soft x-rays and the high spatial resolution of the Kirkpatrick-Baez microscope, thus considerably improving the temporal and spatial resolution, and the soft x-ray flux on a sample.  The soft x-ray microscope and a transmission grating simultaneously provide us with spectral information and one-dimensional spatial information of an expanding laser ablation plume (Fig. 1).  Figure 2 shows the temporal evolution of a spatially resolved absorbance spectrum for an Al ablation plume induced by 100-fs laser irradiation with an intensity of 1.5×1014 W/cm2.  This figure clearly shows that the Al ablation plume expands from the Al tape surface toward the vacuum with time.  The most remarkable feature of the ablation plume is the significant shift of the LII,III absorption edge towards a shorter wavelength compared with that of solid Al.  Since the wavelength of the LII,III absorption edge of the ablation plume corresponds to the transition energy from the 2p state to continuum states, the edge shift depends on each particle constituting the ablation plume.  This result demonstrates that our system is a powerful tool for measuring the dynamics of laser ablation plumes.

[1] Bressler and Chergui, Chem. Rev. 104 (2004) 1781.
[2] Oguri, et al., Appl. Phys. Lett. 87 (2005) 011503.
[3] Okano, et al., Rev. Sci. Instrum. 77 (2006) 046105.
[4] Okano, et al., Appl. Phys. Lett. 89 (2006) 221502.

Fig. 1. Schematic illustration of the system.
Fig. 2. Temporal evolution of spatially resolved absorbance spectrum for Al ablation plume.

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