BeamLine description

The Monochromator and its performances

Details of the VASGM (Variable Angle Spherical Grating Monochromator) of Gas Phase BL , that is an improvement of that developed for the SpectroMicroscopy BL [1, 2] and of the beamline performances have been described in the literature [3]. Here the optical layout is shown schematically in fig. 1.

The optical layout of the beamline allows to achieve:

1. wide energy range (about 13 ÷ 900 eV)
2. high energy resolution (E/ΔE >10000) all over the entire energy range
3. fixed exit slit
4. small and almost circular spot light  (200 μm x 200 μm) with a small angular divergence (about 3 mrad) at the sample position (that is 2.5 m after last optical component).

Due to these costrains, the overall monochromator lenght is around 6 m, with two refocussing mirros the second one with a short exit arm and moving the focus at 1.762 m from ground floor with an angle of 4°, moreover the monochromator must use entrance and exit slits, with a variable width between 5 and 200 μm.

Fig. 1 Schematic side view of the beamline.

The monochromator consists of a plane mirror and five spherical gratings (see tab. 2). To cover the required wide energy range we use 5 gratings, with different radius or/and ruling (tab. 3).
The maximum achievable energy resolution with VASGM is limited by the mechanics that moves the plane mirror and the spherical grating. To improve the resolution,  even if just in  short range, it is possible to move only the spherical grating around the best focus position for the plane mirror. Moreover the resolution is function of the exit slits width, obviously at expenses of the photon flux. Here we give the theoretical curves of energy resolution of  4 of the available gratings as function of photon energy and , parametrically, of the exit slits. There is not a curve for the low energy grating (#6).
G1, G1@2^ndorder =G2, G3, G4, G5. 
Here we report the contributions to resolution deriving from the different optical elements vs hv for the same gratings: R1, R2, R3, R4, R5.

The sensibility of resolution at the entrance slits width is lower. However we have to say that  too open exit slits cannot control instabilities in beam position, with not reproducible effetcs also on energy calibration.