Damage in Mo/Si multilayer optics irradiated by intense short-wavelength FELs

Multilayer-coated optics for the new generation of intense radiation sources - short-wavelength Free Electron Lasers (FELs) - is investigated on radiation damage resistivity. We exposed Mo/Si multilayers to intense pulses from the FLASH (Hamburg) and LCLS (Stanford) FEL sources and compared the single-shot damage for two spectral regimes (XUV and soft X-ray, resp.). Although the damage threshold is wavelength dependent, a similar final state of damage occurs. We conclude that in spite of the difference in the radiation energy deposition, the leading mechanism of damage is the same: melting of the amorphous silicon layer is followed by Mo atoms diffusion into Si, what leads to Molybdenum-Silicide formation and compaction of the multilayer.