FEL multilayer optics damaged by multiple shot laser beam: experimental results and discussion

The irradiation effects of multiple femtosecond shots of a 400 nm laser beam with estimated fluences of some tens of mJ/cm2 on a EUV Mo/Si multilayer have been studied. The study has been motivated by the need of multilayer Mo/Si optics for the delay lines of the FEL source FERMI@Elettra, where these mirrors will be used to reflect 100 fs pulses at 13 nm with a fluence of some mJ/cm2. The analysis was carried on by means of different techniques as EUV and soft X-ray reflectivity, XPS, Total Electron Yield and Standing wave enhanced XPS [1]. Measurements have been performed at the BEAR synchrotron beamline @ Elettra. Simulations have been carried on by means of a new home made software 'OPAL' for the calculation of the intensity of the electric field inside the stratified medium [2]. Preliminary measurements have been performed also with a new fluorescence detector. A simple thermodynamic model have been also used to investigate the propagation of heat during the laser irradiation. AFM and SEM surface images have been also acquired. We observed a significant change in the multilayer performance at fluences of 100 mJ/cm2 and above with a significant reduction of reflectivity. Spectroscopic analysis allowed to correlate the decrease of reflectivity with the degradation of the multilayer stacking, ascribed to Mo-Si intermixing at the Mo/Si interfaces of the first layers, close to the surface of the mirror.