The beamline monochromator is based on a Variable Line Space (VLS) plane grating. With a plane VLS grating, a large energy range can be coverd with a relatively small change in the resolving power. The VLS architecture allowed using only two gratings for the range from 50 eV to 1000 eV. This is achieved by coupling the rotation of the grating with preceeding plain mirror. The first grating has 200 l/mm and covers the energy range from 50 to 250 eV, while the second grating (400 l/mm) covers the range from 200 to 1000 eV. The calculated resolving power for the two VLS plane gratings is reported on the left hand side of the figure below. The full line is for the 200 l/mm grating and the dotted one for the 400 l/mm grating. The graph on the right shows the measured spectrum of the nitrogen 1s to 1π* absorption from which we deduce a resolving power of 4000 at 400.8 eV, in agreement with the calculations.
Resolving power of the monochromator with VLS 200 and VLS 400 gratings.
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