The WR-SGM beamline is based on the Dragon type, six-meter spherical grating monochromator (SGM) design, and supports two endstations with photons delivered over a wide spectral range from 15 to 1600 eV by using a total of six gratings. Two movable entrance slits in conjunction with a common movable exit slit are employed to provide two different grating included angles of 160° and 174° required for low energy and high energy branches, respectively.
The first optic element, a bendable HFM located at 5.5 m from the source, collects 11 mrad. of the horizontal fan of bending magnet radiation and focuses the photon beam horizontally at either after the exit slit or after re-focusing mirror (RFM). Two cylindrical VFMs, one for low-energy branch and the other for high-energy branch, are employed to provide two different optical paths. The photon beam of the low-energy branch is focused vertically and deflected upwards by the VFML and then reflected downwards by a plane mirror. A common movable exit slit is used for both branches. A single toroidal RFM is used to focus the beams onto the sample (22.6 m from the source). The final focused image at the sample position is about 0.7 × 0.3 mm (FWHM).
The switching of the optical path between high and low energy branches is straightforward, thanks to the precision pre-alignment of the mechanical and optics components. The switching requires only a simple vertical translation of the VFMs followed by a minute readjustment of the pitch angle of VFM done with a piezoelectric translator. The horizontal focus of the beamline can be positioned between two endstations by utilizing bendable feature of HFM; however, this bendable feature has rarely been used due to a concern of structural fatigue of the bender.
Two end-stations are positioned in the beamline. A traditional photoelectron spectroscopy (PES) endstation is located after the exit slit (S2), and the second endstation dedicated to novel ambient pressure X-ray photoelectron spectroscopy (APXPS) is located at the beamline focus after the RFM.
Fig.1 The optical layout of the wide-range, 6m-SGM beamline. A total of six gratings mounted in parallel and at two different grating included angles are employed to cover a rather wide photon energy range from 15 to 1600 eV.
Table 1. Covered energy range for six gratings
|Grating Ruling Density(l/mm)||Usable Photon Energy Range (eV)|
|Low Energy Branch ||300||15-40|
|High Energy Branch ||400||120-380|
Fig.2 Measured and calculated resolving power values for the wide-range 6 m-SGM beamline. The dashed curves are referred to a fixed entrance slit coupled to a movable exit slit, while the solid curves refer to a condition with both slits allowed to move to meet the Rowland-circle conditions. In these calculations, both slits are set at 10 mm. The discrete data shown in green were obtained from gas phase photoabsorption measurements performed with an ion chamber. The resolution is at least 20000 in the low-energy branch and 8000 in the high-energy branch.
Fig.3 Published photon flux data of the beamline measured with the entrance and exit slit openings kept at the same 100 µm. The photon flux was measured with a silicon photodiode operating without a voltage bias and corrected against the quantum efficiency of photodiode. Note that the measurements were done many years ago and the present flux should be higher by at least a factor of three due to a reduced ring emittance and twice higher ring current.