Multilayer reflectometry and quantitative analysis of higher order diffraction impurities of grating monochromator

Spectral impurity of the beam delivered by a soft X-ray synchrotron beamline is often a problem because higher order components of a grating monochromator cause some error in quantitative measurements. To achieve a correct data analysis, characterization of the higher order diffraction impurities is needed. This paper describes a simple and easy method, where a reflective multilayer is used to pick one out of a series of diffraction orders with the angle of incidence adjusted. The reflectance of the multilayer is first measured at harmonic energies 2E, 3E and 4E at an angle of incidence φn satisfying the Bragg condition in the first order, where E is the fundamental energy. Then, setting sequentially the angle of incidence of the multilayer at φn, reflectance is measured around E, where the measured value is reflectance for the energy nE multiplied by the proportion of the nth order component. Thus, impurity spectrum is obtained. The present method can be generally used for any beamline and any energy. Experiments were performed at BL-11D of the Photon Factory, KEK, at 282 eV using a Cr/C multilayer, and the proportion of the first order was found to be 0.915. The normal incidence reflectance of the multilayer was measured, and the intrinsic value for pure 282 eV light was estimated to be 20.4% by purity factor correction.