Higher order contributions in the synchrotron radiation spectrum of a toroidal grating monochromator determined by the use of a transmission grating

The synchrotron radiation spectra of toroidal grating monochromators (TGM) working in the short wavelength VUV are often contaminated by significant amounts of higher orders. Even by the use of suitable filters they cannot completely be suppressed. Using a transmission grating behind the exit slit of the monochromator the exit beam can be dispersed and the contributions of the different orders analyzed. For the 162 degree TGM at the radiometric laboratory of PTB at BESSY the higher-order distributions have been quantitatively determined for two gratings of line densities of 200 and 600 l/mm at electron beam energies of 337 and 756 MeV. It is shown that at the energy of 337 MeV and the wavelength region between 5 and 45 nm the contributions of the higher orders to the detector signal are restricted to less than 10% even without the use of filters. At the energy of 756 MeV the same result can be achieved by inserting suitable filters