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SuperESCA beamline description

Overview

SuperESCA has been the first beamline operating at Elettra since 1993. The beamline has been designed primarily for soft X-ray photoemission experiments on surfaces. In order to achieve high energy resolution keeping at the same time a high photon flux, the light produced by the insertion device is collected, monochromatized and brought to the experimental station following a prefoucusing-monochromator-refocusing scheme.

Beamline layout



The SuperESCA light source is a 2-section undulator which provides horizontally polarized photons. At 10 m from the source a pinhole selects the desired region of the undulator emission cone. Since the SuperESCA beamline shares the insertion device with ESCA Microscopy, a plane mirror is used to switch the light towards the operating beamline. The radiation is pre-focused into the monochromator entrance slit by a vertically oriented cylindrical mirror (2° angle of incidence). The dispersed light coming out from the monochromator exit slit is finally re-focused by an ellipsoidal mirror on the sample in the experimental chamber.

Insertion device

SuperESCA and ESCA Microscopy beamlines share a recently (December 2010)  installed insertion device: a linear planar undulator (LPU) from Kyma s.r.l.
The new insertion device is a 46-mm-period undulator with 98 periods and consists of two equal modules which produce horizontally polarised light. By setting the gap value from a minimum of 13.5 mm up to 40 mm the photon energy can be varied in the range 90÷1500 or 130÷1800 eV when the Elettra storage ring works at 2.0 or 2.4 GeV, respectively.
The possibility of tapering the undulator gap (up to 1 mm) allows a controlled enlargement of the harmonic spectrum at particular energies.
The insertion device is characterized by a maximum brilliance of 4.36x1018 ph/s/100 mA/0.1% BW/mm2/mrad2.


Monochromator

The monochromator of the SuperESCA beamline is a modification of the standard version of the plane grating SX700 monochromator.
The new design, represented in the figure, has been developed in order to make the SX700 work stigmatically: the vertically-oriented cylindrical pre-focusing mirror sagittally focuses the radiation into the monochromator's horizontal entrance slit. Inside the monochromator the light is directed towards the plane grating (1200 l/mm) by a plane mirror with variable angle of incidence.

The dispersed radiation is finally focused into the exit slit by an ellipsoidal mirror, oriented vertically in order to reduce the slope error problem arising from its aspherical shape. The exit slit aperture can be varied from 5 to 100 um, giving the possibility to find the optimum compromise between energy resolution and flux for each kind of experiment.
The number of elements between the slits is the same as in the original scheme of the SX700, and the performance is mainly determined by the quality of the ellipsoidal mirror. The energy resolution can be further improved by acting on the baffles placed along the optical path inside the monochromator in order to reduce stray light; the monochromator resolving power (E/ΔE) is of the order of 104.
 


Experimental station

The SuperESCA end-station is represented in the figure. It basically consists of two Ultra High Vacumm (UHV) chambers separated by a gate valve.
 

Preparation chamber

The top, smaller chamber is designed for sample preparation. A VG sputter gun (ion beam energy 0.1÷3 keV) and a gas line allow for standard cleaning procedures. The chamber is also equipped with a plasma source for, e.g., sample functionalization. Moreover, a number of CF ports is availble for the installation of evaporation systems, giving the possibility to prepare samples as thin-films or supported nanoparticles.
The valve between the chambers allows venting the top chamber without breaking the vacuum in the bottom one. In this way, the time required for changing the manipulator as well as fitting the instrumentation needed for each particular experiment is strongly reduced.

Main chamber

The bottom chamber is made of μ-metal for optimum shielding, avoiding in this way the influence of magnetic fields that could distort the experimental results. It is equipped with a 150 mm hemispherical electron energy analyser with variable entrance slit, and a delay line detector developed at Elettra.
A Low Energy Electron Diffraction (LEED) system, monochromatized electron gun, gas inlet system and mass spectrometer are also available.



 


Sample Environment


Manipulators

Two different sample manipulators are available.

A modified CTPO (from VG) manipulator with 5 degrees of freedom (xyzΘΦ). Since the rotational movements are motorized this manipulator is generally used for angle resolved measuremetns (XPD). It is equipped with a liquid N2 crysostat and an electron bombardment heating system which allow for a sample temperature range of 120÷1500 K. A transfer system, connected to the top chamber by means of a


fast entry lock, can be used with this manipulator to insert new samples without breaking the vacuum.

The second manipulator is a liquid He cryostat with xyz translations and 1 rotational axes (see figure aside). The sample is heated by electron bombardment and, using a PID system, its temperature can be stabilized in the range between 15 and 1500 K. In this case all the manipulator movements are manually controlled.




Acquisition software

The experimental apparatus is controlled via a LabVIEWTM based software, developed by the beamline staff. The program fully controls the instrumentation in the experimental chamber: besides  photoemission spectra acquired by the electron energy analyzer, for each data set the software registers also the sample temperature, the pressure in the chamber and a reference current for flux calibration. In case of photoelectron diffraction measurements, the program guarantees

the synchronization between sample rotations and spectra acquisitions.
Moreover the program  interacts also with the Elettra Beamline Control System (BCS) for setting undulator gap and changing the photon energy, allowing in this way the maximum flexibility for different kinds of experiments.

 


Branch-line layout


When the manipulator is retracted in the preparation chamber, the photon beam can pass through the SuperESCA end-station and reach the new SuperESCA branch line.

In the branch line a toroidal mirror re-focuses the beam at the center of a sencond experimental station.
The spot size in the second experimental chamber is the same as in the main end-station, i.e. 100 x (exit slit dimension) µm2.

In this way the light delivered by the SuperESCA beamline can be used in an experimental system equipped with instrumentation complementary to that availble in the main end-station.


 


CoSMoS end-station

The Combined Spectroscopy and Microscopy on Surfaces (CoSMoS) facility, belongs to the National Institute of Materials Physics (NIMP) in Magurele, Romania. CoSMoS is installed on the branch-line of the SuperESCA beamline, within a cooperation agreement between Elettra and NIMP. It is a single set-up from SPECS including:
(i) a photoemission chamber equipped with a 150 mm Phoibos electron energy analyzer with spin detection system and standard preparation tools and lab sources (sputter, flood gun, monochromatized Al Kα/Ag Lα (about 2.98 keV), high power UV-lamp, etc.). The sample is supported by a five-axis manipulator (x, y, z, θ, φ) with cooling capabilities down to 80 K and heating up to about 1300 K;
(ii) a variable temperature (80-600 K) SPECS Aarhus STM with sample cartridge with heating stage;
(iii) a MBE system equipped with the same manipulator of the photoemission chamber, LEED, RHEED, thickness monitor, RGA, 6 evaporation cells, and a RF discharge plasma source.

Different transfer arms allow moving samples between the three chambers and a fast-entry-lock is used for inserting samples in UHV from air in a few minutes. About 10-12 samples can be accommodated on different storage facilities of the whole setup, which means that all these samples might be prepared and characterized well in advance and ready for the synchrotron radiation measurements. Once fully operating the CoSMoS facility will be made available also to external users. It will feature the rare requisite to combine synchrotron-based photoemission techniques with STM and an MBE deposition system, all the preparation and measurement operations being performed in situ.

Last Updated on Friday, 31 March 2017 13:54