Materials Science Beamline at synchrotron ELETTRA, Trieste

M. Vondráček¹, V. Cháb¹, V. Matolín², K. C. Prince³

¹Institute of Physics of the ASCR, v. v. i., Cukrovarnická 10, CZ-162 00 Praha 6, Czech Republic

²Charles University, Department of Surface and Plasma Science, V Holešovičkách 2,

CZ-180 00 Praha 8, Czech Republic

³Sincrotrone Trieste S.C.p.A, Strada Statale 14 - km 163,5 in AREA Science Park,

34012 Basovizza, Trieste, Italy

vondrac@fzu.cz

Keywords: Materials Science Beamline, synchrotron, ELETTRA, photoemission spectroscopy

Introduction

Materials Science Beamline at ELETTRA, Sincrotrone Trieste is probably the most complex device used in synchrotrons, which was proposed, designed and manufactured mainly by Czech companies and institutes and financed from European and Czech funds. In the present time it is operated in close cooperation of three partners - Institute of Physics of the ASCR, Charles University and Sincrotrone Trieste by the international team with Czech participation. This 10 years long experience with construction, operation, maintenance and never-ending improvement of MSB would be useful for currently proposed ambitious project of Central European Synchrotron Laboratory (CESLAB) in Brno.

ELETTRA

Synchrotron ELETTRA was proposed as a soft X-ray complement of ESRF Grenoble in 80's. A decision to build the two 3^rd generation synchrotron facilities was taken in 1986 and construction of ELETTRA began in 1991, reaching commissioning in 1993. In contrast to older facilities, 3^rd generation sources are dedicated for synchrotron radiation not only from bending magnets, but also from many of insertion devices (undulators, wigglers). They have low emittance and then small beam size. ELETTRA storage ring with a circumference 259 m has double bend achromate structure with 24 bending magnets and 11 long straight sections dedicated for insertion devices [1]. Energy of circulating electrons was originally proposed to 1.5 GeV with full energy injection from linac, but early it was increased to 2.0 GeV with injection at 1.0 GeV. Later, the new mode with 2.4 GeV energy was introduced (25% of user time). The latest development represents a new full energy injection device consisting of 100 MeV linac with a 2.4 GeV booster. Abandoned old linac will be rebuilt and become the core of the FERMI@elettra 4^th generation light source (free electron laser).

Materials Science Beamline

History

The idea to build relatively simple and cheap beamline using bending magnet radiation with photon energy in the range 20-1000 eV for the wide range of application in materials science jointed together people from many institutions leaded by Institute of Physics in Prague. Materials Science Beamline was proposed for the first time as Italian-Czech project for financing from Central European Initiative in 1996. The second attempt in 1997 was successful and MSB was supported by EU grant for transfer of high technologies to the east European countries. Main components of the beamline (UHV chambers for all optical elements with fine mechanics) were designed and manufactured by Delong Instruments, Brno in period 1998-99. Final assembly began in 1999 and commissioning followed in 2000. First experimental station attached to the Materials Science Beamline was UHV chamber with Scienta 200 analyzer from Karl Frenzens University, Graz (2000-2002). From 2002 it was replaced by the chamber from ELETTRA fitted with Phoibos 150 electron analyzer and other equipment from Charles University, Prague. This experimental chamber is continuously upgraded and works on MSB up to now. In 2008 a new grating chamber manufactured in Bestec Berlin was installed. It conserves optical concept, but introduces state-of-the-art components of the fine mechanics including actuators and high precision angular encoders.

Beamline description

Materials Science Beamline is attached to the bending magnet exit 6.1 on ELETTRA storage ring. To cover photon energy range 20-1000 eV a grazing angle reflective optics with gold coating in the UHV chambers was used. The first optical element is a toroidal mirror focusing light sagitally onto the entrance slit and tangentially onto the exit slit. Due to the incidence angle 4° from the optical surface, hard X-ray is absorbed and only visible, ultraviolet and soft X-ray light reflects downstream. A plane grating monochromator based on the SX-700 concept has plane mirror determining the angle of incidence on the plane grating, while the spherical mirror focuses the diffracted light onto the exit slit. After exit slit the light beam is refocused to the sample by the toroidal mirror that deflects the beam in vertical plane [2]. Schematic optical layout is on figure 1.

Figure 1. Optical layout of MSB, side view (not in scale).

End station

The key component of the beamline is end station allowing not only experiments with synchrotron light, but also in-situ sample treatment and its characterisation by supplemental methods. In the experimental chamber it is possible to use Ar⁺ sputtering, LEED, HeI lamp for UPS, Mg+Al X-ray tube for XPS, various evaporators and off-course sample heating and cooling during all experiments. High pressure expositions and sample cleaning can be done in neighbouring preparation chamber. Fast entry allows sample entering from ambient with atmospheric pressure to the UHV chambers in 20 minutes.

Synchrotron radiation photoemission spectroscopy

The main experimental method used at MSB is photoemission spectroscopy. In contrast to UPS, when excitation energy is 21.218 eV (HeI lamp) or XPS with excitation energy 1253.6 eV (Mg K-α) and 1486.6 eV (Al K-α), the synchrotron radiation on MSB represents a tuneable light source covering the whole gap between classical UPS and XPS. There are more advantages – small and selectable energy bandwidth (10-500 meV) and focus to the small spot on the sample (100 x 100 µm). The practical result is, that synchrotron radiation in combination with SPECS Phoibos 150 electron energy analyzer can distinguish not only between different elements, but also between different sites of atoms by analyzing of chemical or surface core level shifts only ~0.1 eV large. Tuning of excitation energy changes information depth (due to variation of electron inelastic mean free path) as well as focusing on specific element (due to photoionization cross section dependence). Near Edge X-ray Absorption Fine Structure (NEXAFS) can be measured only with tuneable photon source. Photon flux on the sample as a function of photon energy is displayed on figure 2.

Figure 2. Photon flux as a function of photon energy for resolving power E/dE = 2000.

Scientific output

During its seven years history MSB produced nearly 50 scientific papers and other 10 are submitted or in press. As an example can be mentioned work about valence-charge fluctuations in the Pb/Si(111) system [3] or photoemission study of CO adsorption on ordered Pb/Ni (111) surface phases [4].

References

[1] http://www.elettra.trieste.it/accelerator/parameters.html

[2] R. Vašina, V. Kolařík, P. Doležel, M. Mynář, M. Vondráček, V. Cháb, J. Slezák, C. Comicioli and K.C. Prince, Nuclear Instruments and Methods in Physics Research A, 467–468 (2001) 561–564.

[3] V. Dudr, N. Tsud, S. Fabík, M. Vondráček, V. Matolín, V. Cháb and K.C. Prince, Physical Review B, 70, 155334 (2004).

[4] V. Matolín, I. Matolínová, N. Tsud, S. Fabík, J. Libra, V. Dudr, V. Cháb and K.C. Prince, Physical Review B, 74, 075416 (2006).

Acknowledgements

Materials Science Beamline was supported by 3^rd FP EU (CIPA-CT-0217), by Grant Agency of the Czech Republic (project GV202/98/K002) and by Ministry of Education of the Czech Republic (projects INGO LA 151 and LC06058).