VUV photoemission beamline description
The beamline
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Insertion deviceThe VUV has been designed primarily for surface and solid state experiments involving high resolution photoemission. The light source is an undulator with a range of 20 to 750 eV, while the minimum energy at a ring energy of 2.4 GeV is about 25 eV. The undulator for the VUV beamline consists of 36 periods, divided in three sections. With minimum gap (K=5.3) the high energy part of the spectrum is like that of a wiggler, i.e. continuous and fairly smooth. The light available here has reduced brightness and a lower degree of polarisation compared with an undulator. Flux
Between 100 and 900 eV, the measured flux on the sample varies from 1.4 x 1013 to 5 x 1010 photons/sec/0.1%bw/200mA, in a spot of maximum size 0.5 mm. Test experiments at a second generation light source and on this beamline indicated at least two orders of magnitude higher count rates at higher resolution. BCSThe beamline is controlled by the Beamline Control System (BCS) except for the monochromator (including the entrance optics and the exit slit position) which is under the experimental station software control. |
MonocromatorThe Kirkpatrick-Baez entrance optics focus the light after which it enters the entrance slit of the monochromator. The monochromator is a Spherical Grating Monochromator with five interchangeable gratings and pre- and post-focusing optics. The set of five gratings provides a resolving power of 10,000 over the whole energetic range. The light emerges from the movable exit slit and is then refocused by a post-focusing mirror onto the sample. For lower photon energies (< ca.130 eV), the angle of deflection of the light at the grating is increased from 7° to 20° by means of an additional pair of plane mirrors, one of which is fixed and the other removable. The resolution has been measured using gas phase absorption at a number of energies. The resolving power is estimated to be as follows:
All spectra were taken using the first order of diffraction and the appropriate harmonic of the undulator. The accuracy with which the resolution can be determined is limited by the natural widths of the absorption lines because, except for He, the lines are much broader than the resolution.
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Sources
Two of evaporation sources can be replaced without venting the preparation chamber as they are mounted on an independently pumped section. Another three are mounted directly to the chamber. The minimum distance to an independently pumped source is about 25 cm. Moreover an Omicron triple evaporator is mounted directly to the chamber, with the minimum distance from the source to the sample about 15 cm.
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Samples and sample holders
The samples must be conductive. We use sample mounting on Ta or Cu sample holder plates.
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Preparation chamber
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The base pressure of the preparation chambe is 2·10-10 mbar. The horizontal manipulator has five degrees of freedom (there is no azimuthal rotation anymore), allowing the sample preparation, LEED analysis and photoemission measurements. The sample can be cooled down to 77K and heated up to 500K. Sputtering facility as well as LEED optics are available in the preparation chamber; there are also gas dosing facilities and sources for overlayer deposition. The manipulator can host one sample only. Please note that we can not allow pressures higher than 5·10-6 mbar of Ar in the preparation chamber.
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Heating station
The load lock was modified in an independently baked chamber, destinated for the high temperature annealing.
Changing a sample which does not require high temperature annealing and/or low pressures can be done within 3-4 hours, in another case it will take about 12 hours, since the heating station need to be baked.
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Measurement chamber
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The main chamber operates in 10-11 mbar regime. It is equipped with vertical a 5 degree of freedom manipulator. The lowest temperature range is 9K for the fixed stage and 13K for rotatable stage. The azimuthal angle motion of rotatable stage is motorized and allows to acquire automatically Fermi surfaces with angle step less than 0.01° with angular range ±160°. The polar angle can span 100° and has 0.1° resolution. Read more...
Fermi surface measurements are performed by Labview 10.0 and analyzed by Wavemetrics Igor 6.2 procedures. Read more... The electron analyser is a Scienta R4000-WAL with variable acceptance angle (7-30 degrees). It is mounted at 45° to the beam direction. Read more... Please note that we can not allow pressures higher than 10-9 mbar while measuring. |
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New manipulator
In 2015 we installed and tested a new manipulator with liquid Helium closed-cycle cryostat.
The stage (Vab Vacuum) allows linear motion along the X, Y, and Z axes and polar rotation (360°) with 0.2° accuracy.
The closed-cycle cryostat, provided by ARSCryo, works in the temperature range 9-450K. There is no He consumption (the autonomy of the compressor is 12000 hours).
Azimuthal rotation (±160°) is performed with 0.01° accuracy through piezoelectric rotary stage (AttoCube ANR240 rotary stepper positioner).
Fermi surface measurements
The way to study Fermi surface which we use at the VUV beamline is the azimuthal rotation, when by changing the azimuthal angle one changes together kx and ky. It is illustrated below.
