Comparison between classical and off-plane diffraction efficiency for the soft X-ray region

In this paper, we will present the measurements of diffraction efficiency of commercially available replica reflection gratings which are applied to the realization of monochromators for soft X-rays in the classical and off-plane geometry. The efficiency curves for different blaze wavelengths has been measured at the Circular Polarization beamline of the ELETTRA Synchrotron (Italy) and simulated with GSolver, which is a full vector implementation of a class of algorithms known as Rigorous Coupled Wave Analysis (RCWA). Aim of the measurements is the comparison between the two diffraction geometries when used for the realization of instruments with ultrafast response. The advantages and drawbacks of both configurations when applied to the spectral selection of ultrafast pulses below 10 nm are discussed.