Design and characterization of the XUV monochromator for ultrashort pulses at the ARTEMIS facility

ARTEMIS, a multi-partner and multidisciplinary project, will be a users-dedicated facility that will provide ultrashort XUV pulses through high harmonic generation in a gas target. This high repetition rate synchronized source will allow new science in research fields spanning from the material science to the molecular physics and chemistry. The XUV radiation is expected to cover the range 10 - 100 nm with an estimated photons flux up to 10 (11) photons/s per harmonic. In this work we present the design and characterization of the monochromator that will be used in the beamline for the experiments requiring wavelength and bandwidth selection. The working principle is based on a plane grating operated at grazing incidence in the off-plane mount. This geometry has been selected because of the high diffraction efficiency, expected to be about 30%. To cover the entire spectral range four gratings can be selected which span over different regions and with different spectral resolution. When the appropriate grating is chosen, the wavelength scanning is performed by rotating the grating around an axis passing through the grating center and parallel to the grooves direction. The off-plane mount requires the grating to be used in collimated light, consequently the optical scheme is completed by two toroidal mirrors, the first in front of the source that collimates the XUV radiation before the grating and the second after the grating to focalize the spectrally dispersed photons on the exit slit. Using a single grating, the configuration is not time-delay-compensated, nevertheless the time broadening (depending on the source divergence, the wavelength, and the grating) is less than 50 fs.