Beamline Description

Insertion device

The 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.
The monochromator is provided with baffles to reduce stray light and filters will be installed to reduce higher order radiation. Measurements of the higher order contribution are in progress and indicate that the contamination is a few %.

BCS

The 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.  

Monocromator

The 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.

A schematic picture of the BaDElPh end station is reported in figure 1. It consists of three independent ultra-high vacuum (UHV) chambers and a load-lock. There are valves between the analysis chamber and the beamline, between the analysis chamber and the preparation chamber, between the preparation chamber and the heater chamber, and between the heater chamber and load lock. All these chambers are pumped by turbo pumps.

The Analysis Chamber

Since April 2024, a new mu-metal experimental chamber has been installed. It houses a 4^th-generation high-resolution (1 meV / 0.1°) and -transmission (200 mm mean radius) hemispherical electron energy analyzer MBS A-1 with lens scanning and wide acceptance angle, a fast 2D-CMOS detector for ARPES, and the MBS Spin Manipulator and a VLEED (FOCUS Ferrum) detector for 3D Spin-ARPES (under commissioning). The analyzer is mounted on a fixed geometry with an angle of 50° relative to the synchrotron radiation direction. The angular dispersive plane of the analyzer coincides with the polarization plane of the synchrotron light in first harmonic (horizontal plane). The use of a 2D-CCD detector combined with the lens scanning offers the possibility of simultaneous acquisition of the energy as well as the in-plane angular distributions of the photoelectrons. The span of (k_x,k_y) vectors simultaneously probed by the analyzer is defined by the angular acceptance of the particular lens mode and by the photon energy. Three different angular resolved modes of the L4 lens operation have been designed for ARPES measurements reaching a maximum angular acceptance of more than 30°; moreover, an ultimate angular resolution of about 0.1° can be achieved. On-site measurements have so far demonstrated an ultimate energy resolution of less than 3 meV from the gas phase (Xe line at about 9 eV).
The chamber also hosts a low-energy electron diffraction (LEED) optics and a residual gas analyzer (RGA). The base pressure in the analysis chamber is in 10^-11 mbar range.

The Prepartion and Heater Chambers

The preparation chamber is normally equipped with an ion sputter gun and a silver evaporator while in the heater chamber is present a 7-slot sample flag parking device and an electron bombardment heater stage for high-temperature (up to about 2400 K) annealing of the sample. Both these chambers have several free flanges to mount the needed tools for the required sample preparation (cleavage, scraping, gas treatment) and for UHV in-situ growth of thin films. The base pressure in the preparation chamber is in 10^-10-10^-11 mbar range while in the heater chamber the base pressure is in the 10^-10 mbar range.
The heater chamber is equipped with a load-lock that allows to transfer samples in about 60 minutes from air to a vacuum of better than 10^-7 mbar.

                Figure 1: BaDElPh end station

The temperature can be measured by a C-type thermocouple and by a standard Lake Shore silicon diode installed next to the sample and on the cold finger, respectively. To improve the reliability of the readings at low temperatures, the thermocouple has been additionally calibrated with the help of the silicon diode. The head of the cryostat includes also a cartridge heater allowing a remote control of the temperature in the range 5-400 K.

Samples must be resistant to radiation damage and conductive for photoemission measurements. Using the transfer system, samples must be mounted on a suitable sample flag (see Figure 2) and the maximum sample size is of about 10 mm in diameter and thickness. If special sample mountings are required, please contact the beamline responsible well in advace in order to plan suitable solutions. Sample annealing at high temperature (> 400 K) can be performed transfering the sample in a dedicated heating stage (see Figure 3) where temperature up to 2400 K can be reached by electron bombardment.

Insulating samples are practically impossible to measure due to charging effects. Therefore, insulating thick (bulk) crystals are not welcome. Metallic, two-dimensional thin films or layered crystals with small Brillouine zone are the sample of choice for this beamline.

A new commercial six-axis cryo-manipulator was recently acquired and is under commissioning.
 

Fig. 1: Sample holder with motorized azimuthal angle


Fig. 2: Base sample flag


Fig. 3: Heating stage