Beamline

A drawing of the ESCA microscopy beamline after the switching mirrors section is shown in Figure. These sections contain the following components: (i) prefocusing mirror chamber; (ii) monochromator block consisting of entrance and exit slit subchambers and a main chamber, which hosts two gratings and a plane mirror; (iii) connecting elements, water cooling, automatic electropneumatic gate values, and pumping units (ion pumps).

The SGM prefocusing system consists of a toroidal mirror performing a sagittal focusing of the beam into the entrance slit and tangential focusing into the exit slit. The plane mirror in combination with the grating allows the deflection angle at the grating to be changed in order to operate with minimal defocus aberration. The monochromator has two gratings with 600 and 1200 grooves/mm, which operate in outside diffraction order configuration with a deflection angle of 174⁰ and perform sagital focusing to the exit slit (see figure for the optical scheme of the beamline). A variable pinhole at the exit allows the beam to be tailored to the right conditions for illumination of the zone plates. According to the specification, the instrumental transmission should be of the order of 1% providing a resolution of 0.1 eV at the C K edge in a 30 µm exit slit. The tuning range covers photon energies from 250 to 1200 eV.

Energy calibration and evaluation of the resolving power of the monochromator were performed measuring photoabsorption spectra of well characterized gases (N₂ and Ar). The performance of the monochromator is illustrated by the data in Figure. The theoretical resolving power reported in the figure was calculated considering different contributions like exit slit dimension, coma aberration, diffraction limit, and measured grating slope error. Experimental measurements at two photon energy points for Ar (244eV) and N (400eV) edges show good quantitative agreement with theoretical values.

The brightness and coherent flux of the undulator source at 0.1% of the bandwidth are reported in the following table. One should keep in mind that the transmission of the beamline is of the order of 1% which was also calculated theoretically and confirmed experimentally. The transmission is maximised at around 500eV for the 600l/mm grating and at 700eV for 1200l/mm grating. Relative transmissions at different photon energies are shown in the figure where the 2.0 GeV spectra for different undulator gaps and harmonics and differnt monocromator gratings are plotted.