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Beamline Description

Overview

The Materials Science Beamline was designed as a multipurpose beamline for surface science using photoemission spectroscopies, with a photon energy ranging from vacuum ultraviolet to soft X-ray radiation (22 - 1000 eV). It was based on a bending magnet radiation source for fast photon tuning in a wide energy range with a single grating monochromator similar in design to an SX-700. The spot size on the sample has a diameter of ~100 μm. Photoelectrons from the sample are analyzed by hemispherical electron energy analyzer.

Photon Source

The Materials Science Beamline uses radiation from bending magnet 6.1 at the Elettra storage ring. It provides to the beamline with polychromatic light in a wide energy range from visible to hard X-ray. Hard X-ray radiation is absorbed at the first mirror and then a monochromator selects light with a desired photon energy and bandwidth. The bending magnet radiation is mostly linearly polarized in the horizontal direction, with only a minor part of the radiation above and below the orbit having circular polarization. The Elettra storage ring parameters can be found here.

Beamline Layout

The beamline is based on grazing incidence reflective optics. All optical surfaces are gold coated. The first optical element is a toroidal prefocusing mirror that focuses the divergent photon beam from the bending magnet sagittally onto the entrance slit and tangentially onto the exit slit. It also absorbs all hard X-ray radiation.  The energy of photons is selected by a plane grating monochromator (PGM). Photon energy resolution is adjusted by the entrance and exit slit widths. From the exit slit the beam is focused again by the refocusing mirror onto the sample. Technical details of the optical concept of the MSB are described in [1].

MSB_scheme.gif


Monochromator

The beamline monochromator uses the well proven SX-700 Plane Grating Monochormator (PGM) design concept with a single grating of groove density 1200 l/mm. The monochromator consists of a rotating plane mirror, rotating plane grating and fixed spherical focusing mirror with an entrance arm 1.4 m and exit arm 7.6 m long. This solutions allows a large energy range (22 - 1000 eV) to be covered with fine, fast and continous tuning. The resolving power is naturally better at lower energies, allowing the photon flux to be further increased by opening both the entrance and exit slit wider than is the typical values (100 μm for entrance and 200 μm for exit slit). In this case the resolving power is still higher than 1000 and photon energy resolution below 100 meV. MSB_Monochromator.jpg

MSB_resol_RP.gif

 


Beamline Flux

The experimental flux curve for a storage ring energy of 2.0 GeV normalized to 300 mA ring current (typical top-up conditions) is shown below. The data were collected from a photodiode inserted at the end of the beamline with the entrance slit open to 100 μm and exit slit to 250 μm (typical setting). Below 50 eV the flux can be increased by factor of 10 by opening the slits without significant impact to energy resolution.

MSB_flux.gif



End Station

The top view of the end station illustrates a configuration with the main chamber in the centre (with the Phoibos analyzer in bottom left corner) and the preparation chamber on the right side (load lock with transfer at right edge of the image). There are gate valves between the main chamber and the beamline, between the main chamber and the preparation chamber and between the preparation chamber and the load lock.

The Main Chamber

The fundamental instrument in the experimental chamber is the hemispherical energy analyzer (SPECS Phoibos 150 [2]) inclined 60° from the beamline axis. Samples aer mounted in a holder on a manipulator with X, Y and Z linear axis and azimutal and polar rotations. Furthermore, the manipulator also allows heating and cooling of the sample together with Ar+ sputtering, gas lines with precision leak valves and set of evaporators this allows the preparation of well-defined surfaces of various materials in-situ. The chamber is pumped by a combination of a turbomolecular pump and a Ti sublimation pump. Residual atmosphere as well as gasses desorbed from sample can be measured by a quadrupole mass spectrometer. The base pressure of the main chamber is in low 10-10 mbar range.

MSB_End_Station.jpg

The Preparation Chamber

A small preparation chamber is available for sample treatment with a sputter gun available. The sample is hosted on a transfer rod. Several CF 40 and CF 63 flanges are available for users' equipment. Sample heating is not available in the preparation chamber. The sample preparation chamber is equipped with a load-lock that allows the transfer of samples in 30 minutes from air to a vacuum of better than 10-8 mbar. The preparation chamber is pumped by a turbomolecular pump and a Ti sublimation pump. Base pressure is in 10-9 mbar range.

References

  1. Mechanical design aspects of a soft x-ray plane grating monochromator
    Vasina R, Kolarik V, Dolezel P, Vondracek M, Mynar M, Slezak J, Prince KC, Chab V, Comicioli C
    NUCL INSTRUM METH A, Vol. 467, pp. 561-4 (2001)
    doi: 10.1016/S0168-9002(01)00412-0
  2. SPECS Phoibos 100/150
    official website
     http://www.specs.de/cms/front_content.php?idart=122
Last Updated on Wednesday, 21 December 2011 18:02