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Beamline Description


The BaDElPh beamline is an undulator-based normal incidence monochromator (NIM) instrument which provides photons in the energy range 4.6-40 eV with high flux, high energy resolution, and horizontal-vertical linear polarization. The beamline serves an end station to perform primarily high-resolution angle-resolved photoemission spectroscopy (ARPES) experiments from solids in the low photon energy regime. Low photon energies (5-15 eV ) provide enhanced bulk sensitiviry, allow for the highest momentum and energy resolution, and are useful for tuning matrix elements which vary rapidly at low energy. The availability of photon energies below 15 eV makes BaDElPh unique among the other ARPES beamlines at Elettra and around the world.  For more details see also L. Petaccia et al., Nucl. Instr. and Meth. A 606, 780 (2009).

Insertion device

The radiation source of the BaDElPh beamline is the same Figure-8 undulator of the IUVS beamline. This undulator has two sections for a total of 32 periods of NdFeB magnets with a 140 mm period lenght and it provides a maximum photon flux of about 1015 photons/s/0.1%BW in the range 5-10 eV. The radiation spectrum (see for example the figure on the right) is composed of two sets of harmonics, conventionally defined by integer (i=1,2,3,...) and half-integer (i=1/2, 3/2,5/2,...) indices having alternatively horizontal (i=1,2,3,...) and vertical (i=1/2,3/2,5/2,...) polarization direction.   More details can be found here.

Monochromator and optics

The BaDElPh beamline is based on a 4-m-long normal incidence monochromator (NIM) with a constant included angle of 5°.
The radiation from the Fgure-8 undulator is deflected toward the BaDElPh branch by a silicon plane-switching mirror. The spherical prefocusing mirror upstream of the monochromator focuses the beam into the entrance slit. The bare silicon portion of the surface of the mirror is used for photon energies up to about 13 eV while the reflectivity of the Pt-coated part is suitable for higher photon energies.
The monochromator is equipped with three (Al/MgF2, SiC, Pt) interchangeable spherical gratings to cover the photon energy range 4.6-40 eV at 2.0 GeV of electron ring energy. When Elettra operates at 2.4 GeV the lowest photon energy available is 6.7 eV. The Al/MgF2 grating has 1500 l/mm, a laminar profile, and is used for the photon energy range of 4.6-13 eV. The SiC grating has 3000 l/mm with a laminar profile and, since March 2009 when the Pt grating was installed, can be used for the photon energy range 13-19 eV. The Pt grating has 3000 l/mm, a blaze profile, and is optimized for the photon energy range of about 19-40 eV.
The monochromatic beam is focalized into the moveable exit slit. Both the entrance and exit slit apertures can be set in the 10-400 µm range. The last optical element of the beamline, namely the gold-coated toroidal refocusing mirror, focuses the light onto the sample inside the experimental chamber.
The measured size of the synchrotron radiation beam spot at the sample position is in the order of the exit slit aperture in the vertical direction and less than 300 µm in the horizontal direction.

Layout of the BaDElPh beamline


Photon flux and Resolving power

The experimental photon flux through the beamline has been measured using a removable AXUV-100 photodiode, located between the refocusing mirror and the end station (see Figure 1). The typical quantum efficiency of the photodiode has been taken into account. With slits of 300 µm width and with 200 mA electron current accumulated in the storage ring, the maximum photon flux, reached at 9 eV in first harmonic, is more than 5x1012 photons/s, while at 6.5 eV and 11 eV it decreases to about 1x1012 photons/s. For the SiC grating, the maximum photon flux of more than 3x1011 photons/s is reached at about 20 eV. For the Pt grating, available since March 2009, the maximum photon flux, reached at 23 eV, is more than 1x1012 photons/s, while at 19 and 35 eV it decreases to about 3x1011 photons/s.

In the above conditions the calculated total resolving power (E/DE) of the beamline is 2850 at 9 eV, 2570 at 20 eV, and 1670 at 31 eV of photon energy but it can be improved, by closing the slits, up to about 50000. The calculated resolving power (E/DE) of the monochromator is shown in Figure 2 for a complete open entrance slit and exit slit widths of 10 (top curves), 20, 50, 100, and 300 µm (bottom curves).

                              Fig.1: Photon flux of the BaDElPh beamline                                      Fig.2: Calculated resolving power (E/DE) of the BaDElPh NIM

End Station

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 March 2008, a new mu-metal experimental chamber has been installed. It houses the new electron energy analyzer, a SPECS Phoibos 150 with a 2D-CCD detector system, a high-intensity VUV source (He), a conventional X-ray source (Al & Mg), a low-energy electron diffraction (LEED) optics, a gas-cell, and a residual gas analyzer (RGA). The base pressure in the analysis chamber is in 10-11-10-12 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 30 minutes from air to a vacuum of better than 10-7 mbar.

                Figure 1: BaDElPh end station


The Electron Analyzer

Since March 2008, a new electron energy analyzer, a SPECS Phoibos 150 with a 2D-CCD detector system, is operative in the BaDElPh end station. 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 offers the possibility of simultaneous acquisition of the energy as well as the angular distributions of the photoelectrons. The span of  k|| vectors simultaneously probed by the analyzer is defined by the angular acceptance of the particular lens mode and by the photon energy. Four different angular resolved modes of the lens operation have been specifically designed for ARPES measurements reaching a maximum angular acceptance of about 26°; moreover, an ultimate angular resolution of about 0.1° could be achieved. From gas phase photoemission, an ultimate energy resolution of less than 4 meV has been certified by the supplier. The energy resolution in photoemission experiments from solids can be conveniently determined by measuring the Fermi edges of polycrystalline noble metals at low temperature (see Fig. 1). The fitting function was the product of a linear background with the Fermi–Dirac distribution (FDD) at the experimental temperature convoluted with a Gaussian. As a result of the fit the full width at half maximum (FWHM) of the Gaussian, i.e., the combined experimental resolution of the beamline and the analyzer, was found to be 5.4 meV. Under the above conditions, the calculated energy resolution of the BaDElPh NIM is 2.4 meV. Therefore, an energy resolution of 4.8 meV can be estimated for the electron analyzer.

Figure 1: Photoemission spectrum of the Fermi edge of polycrystalline silver at 10 K acquired at the photon energy of 7.8 eV together with the fit of the data and its components. The exit slit of the beamline was set to 0.3 mm, while the entrance slit and the pass energy of the analyzer were set to 0.5 mm and 1 eV, respectively.
Under the above conditions, an energy resolution of 4.8 meV can be estimated for the electron analyzer.

Manipulator and Sample holder

The sample manipulator has four degree of freedom (xyz translations and polar rotational axes). Since October 2009, an additional angular degree of freedom (azimuthal rotational axes) is available using a suitable sample holder. Different sample holders, capable of accomodating transferable samples, can be mounted on a cryostat that reaches with liquid helium a temperature lower than 5 K. Since April 2012, a new sample holder with motorized azimuthal angle, based on an attocube rotator (ANR200/RES), has been successfully assembled in our liquid-helium cryostat manipulator and tested. It allows to acquire computer-controlled Fermi surface maps with high angular accuracy (about 0.02°) and it is now available for Users beamtimes (see Figure 1). Since December 2017, the manipulator movement is fully motorized, by stepper motors for the polar rotation and xyz translations and by the piezo motor for the azimuthal rotation, and it can be remotely controlled.

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 2500 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.

Fig. 1: Sample holder with motorized azimuthal angle

Fig. 2: Base sample flag

Fig. 3: Heating stage

Ultima modifica il Lunedì, 15 Gennaio 2018 18:05