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18 May 2009 Optical emission spectroscopy of various materials irradiated by soft x-ray free-electron laser
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The beam of Free-Electron Laser in Hamburg (FLASH) tuned at either 32.5 nm or 13.7 nm was focused by a grazing incidence elliptical mirror and an off-axis parabolic mirror coated by Si/Mo multilayer on 20-micron and 1-micron spot, respectively. The grazing incidence and normal incidence focusing of ~10-fs pulses carrying an energy of 10 μJ lead at the surface of various solids (Si, Al, Ti, Ta, Si3N4, BN, a-C/Si, Ni/Si, Cr/Si, Rh/Si, Ce:YAG, poly(methyl methacrylate) - PMMA, stainless steel, etc.) to an irradiance of 1013 W/cm2 and 1016 W/cm2, respectively. The optical emission of the plasmas produced under these conditions was registered by grating (1200 lines/mm and/or 150 lines/mm) spectrometer MS257 (Oriel) equipped with iCCD head (iStar 720, Andor). Surprisingly, only lines belonging to the neutral atoms were observed at intensities around 1013 W/cm2. No lines of atomic ions have been identified in UV-vis spectra emitted from the plasmas formed by the FLASH beam focused in a 20-micron spot. At intensities around 1016 W/cm2, the OE spectra are again dominated by the atomic lines. However, a weak emission of Al+ and Al2+ was registered as well. The abundance ratio of Al/Al+ should be at least 100. The plasma is really cold, an excitation temperature equivalent to 0.8 eV was found by a computer simulation of the aluminum plasma OE spectrum. A broadband emission was also registered, both from the plasmas (typical is for carbon; there were no spectral lines) and the scintillators (on Ce:YAG crystal, both the luminescence bands and the line plasma emission were recorded by the spectrometer).
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