Optical emission spectroscopy of various materials irradiated by soft x-ray free-electron laser

The beam of the Free-Electron Laser in Hamburg (FLASH) was focused by a grazing incidence elliptical mirror (32.5 nm and 13.5 nm) and an off-axis parabolic mirror coated with a Si/Mo multilayer (13.5 nm) to 20-micron and 1-micron spot, respectively. The grazing incidence and normal incidence focusing of ∼25-fs pulses with an energy of 10 μJ resulted in an irradiance of 10¹³ W/cm² and 10¹⁶ W/cm², respectively, at the surface of various solids (Si, Al, Ti, Ta, Si ₃N₄, BN, a-C/Si, Ni/Si, Cr/Si, Rh/Si, Ce:YAG, poly(methyl methacrylate) - PMMA, stainless steel, etc.). The optical emission of the plasmas produced under these conditions was registered by a 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 spectral lines of 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 has an excitation temperature equivalent to 0.8 eV found by a computer simulation of the aluminum plasma OE spectrum. A broadband emission was also registered, both from the single-element plasmas (typical is for carbon; there were no spectral lines) and the scintillating slabs (on Ce:YAG crystal, both the luminescence bands and the line plasma emission were recorded by the spectrometer).