BeamLine description

GasPhase Photoemission is a high resolution (resolving power 10000 or better) and high flux versatile beam line which covers a wide energy range (13-900 eV) with a stable and almost circular spot size (around 200micron) at the target.

The line is fed by an undulator (U12.5 at exit U6.2, tab. 1), which delivers light in the range from 13.5 eV to well above 900 eV. The monochromator is a Variable Angle Spherical Grating Monochromator, with fixed positions of the slits. The optical design was developed by Sincrotrone Trieste, including a toroidal prefocusing and two refocusing mirrors (spherical and plane-elliptical). The monochromator was commissioned and not only met its specifications (E/ΔE >= 10000) over all the planned working range (13-900 eV) but even exceeded them considerably. A resolving power of 60000 has been measured at 48 eV. It remains higher than 10000 at 540 eV and a value of 8000 is obtained at 860 eV. This resolving power is the highest among the monochromators at ELETTRA. The use of two refocusing mirrors provides a roughly circular spot at the sample (diameter »350micron for a 20 micron exit slit, decreasing to less than 150micron as the photon energy increases above 200 eV). The beamline is connected to the experimental station through a differential pumping section (5 orders of magnitude difference in pressure), to decouple the UHV section of the mirror chambers from the high vacuumin the experimental region.

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.

The BranchLine

After the exit slit it is possible to insert  a plane mirror and to deflect the monochromatized radiation to a branch line. The deflection angle (6°) results in an energy cut-off of about 200 eV.
The low-energy branch line of the beamline is equipped with a tunable Ti:Sapphire oscillator (Tsunami, SpectraPhysics). The main laser specifications are summarized here. 

GasPhase Beamline - BL instrumentations & infos

Differently from most other beamlines at ELETTRA, there is not a permanent end station at GasPhase. Our beamline is presently equipped with many distinct interchangeable apparatuses, listed below. Any apparatus is available to Users for their proposals, the choice of the end station depending on experimental requirements.

Moreover Users can bring their own apparatus to perform experiments, provided that it is compatible with beamline requirements (please contact beamline staff to check).

photoemission experiments on aggressive molecular species (radicals, transients, reactive species)

1. VG-220i set up for photoemission experiments on condensable vapours, also equipped for TOF mass spectrometry
2. Multi-coincidence experiments set up, for angle resolved and electron-electron coincidence experiments in a versatile multichannel configuration.
3. Molecular beam set up, for PEPICo mass spectroscopy
4. Velocity Map Imaging (VMI) 
5. PIFS, Photon Induced Fluorescence Spectroscopy (UV-Vis dispersed fluorescence with Minuteman 305V Czerny-Turner 0,5 m Spectrometer) 
6. ARPES, angle-resolved photoemission experiments on aggressive molecular species (radicals, transients, reactive species)
7. Two colors experiments SR + IR laser beam.
8. Absolute absorption cross section cell 

Two colors experiments SR + IR laser beam can be performed with set up 1 to 5 on the low energy branch line.

1.  SES200-ULLA, for high resolution photoemission of molecular vapours (LTP 20115157, prof. Carla Puglia, University of Uppsala, Sweden) [doi: 10.1063/1.4907723];
2. PIFS+ "XUV-PIK”, for dispersed emission experiments with a compact XUV spectrometer (LTP 20130233, dr. Luca Poletto, IFN-CNR, Padua, Italy) [doi: 10.1063/1.4898315];
3. “CESyRa”, for photoionization  of clusters of refractory materials, via electron-ion multicoincidence detection. (LTP 20170487; prof. Paolo Piseri, CIMAINA, University of Milan, Italy [on going upgrade; doi: 10.1016/j.elspec.2008.05.003]

Moreover each user can bring his own apparatus to perform experiments, provided that it is compatible with the beamline requirements (please contact beamline staff to check).

Experimental Facilities

• He lamp.
• Gas line with precision leak valves.
• UV-Vis monochromator
• Absolute absorption cross section cell