(Color online) The schematic layout of the SAXES spectrometer, which is based on a VLS spherical grating and a two dimensional position sensitive CCD detector. Panel (A): the main optical parameter symbols are defined. Panel (B): the main mechanical components are sketched together with their degrees of freedom. The entrance slit width can be regulated with a accuracy. The grating can be moved along the entrance arm and can be rotated to change and , respectively. The CCD detector can be displaced to follow the focusing curve and can be rotated to change the incidence angle . A mask with a set of apertures of different vertical widths is placed just before the grating to allow a regulation of the illuminated footprint on the grating, in order to have a full control of optical aberrations.
(Color online) The set of optical parameters minimizing the coma aberration contribution over the whole SAXES energy range. Lower panel: incidence angle on the grating and entrance arm length . The MIN and MAX lines indicate the limits of allowed by the actual SAXES mechanics. Top panel: the corresponding values of the total horizontal length of SAXES.
(Color online) The mechanical simplified construction draft of SAXES. The sample is at the right side. The big grating chamber ( internal diameter) houses the motorized in vacuum mechanics with four degrees of freedom. The whole spectrometer is supported by a single steel girder allowing fine regulation of the vertical position. The girder can rotate around a vertical axis passing on the sample, thanks to an air cushion device; this will allow the study of transferred photon momentum dependent RIXS spectra.
(Color online) The schematic entrance slit mechanical layout. The two blades open symmetrically, thanks to the choice of the lever arms. One single high precision actuator acts, from the air side, in the leverage. The rotation is provided by two metallic flexible hinges. The long arms are needed to place the actuator along the entrance arm, away from the sample vacuum chamber. Panel (A): The beam proceeds, inside the slit mechanics, from the sample towards the grating after having been selected by the slit. A couple of in vacuum springs are used to counterbalance the actuator. Panel (B): The simple leverage ratios that bring to a 1:2 opening of the slit with respect to the actuator movement .
(Color online) The histogram of the charge cloud distribution on the CCD detector in the event of a single photon detection. The pixel lateral size is . In both directions of the CCD array (full black squares perpendicular to energy dispersion direction, open red circles along the dispersion direction), the full width at half maximum of the curve is around , i.e., almost twice the pixel size. See text for details on the procedure used to construct this histogram.
(Color online) The expected resolution performances of SAXES. Top panel: The expected linewidth (LW) vs photon energy for various openings of the entrance slit (small symbols). For the minimum slit , the slit contributions compared to the expected contributions from the grating slope error and CCD spatial resolution (with ). Bottom panel: The corresponding values of the resolving power . The big red circles indicate the resolving power estimated by ray tracing; all the other values are estimated by analytical expressions. In the inset the LW dependence on the slit opening at the important case of .
The Mg and emission spectra measured with SAXES. The satellites are used to calibrate in a precise way the energy scale by comparison with published spectra. The calibration is in good agreement with the expected one. The main peak in both cases is asymmetric due to the level spin-orbit splitting. The spectra were measured with a entrance slit and an accumulation time of about . See text for details.
To determine the actual resolving power of SAXES in the case of the Al spectrum of Fig. 6, we compare the experimental data to the simulated spectra obtained by a Gaussian broadening of the ideal spectrum measured with infinite resolving power. It must be noted that the Lorentzian lifetime broadening is unavoidable in the fluorescence emission lines. In the top panel the comparison is made using the derivatives of the measured and simulated spectra. At SAXES can clearly work with a resolving power better than 7000.
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