Technical Description

 Source:   

The source for the Circe beamline is a Pure Permanent Magnet APPLE II helical undulator, inserted in the center of the 9th medium straight section of the ALBA storage ring. This device is capable of delivering linearly polarized light in any direction, as well as circularly polarized light (left- or right-handed). Optics: The Circe beamline has a variable angle plane grating monochromat or. A plane mirror and three diffraction gratings are used to cover the full energy range of 100 to 2000 eV. After the grating chamber, there are two toroidal mirrors which focus the beam on the exit slit of the PEEM or NAPP branch. The energy resolution of the monochromator is around 8000 at an exit slit of 20 um and Cff=2.25.

Apple II undulator

 CIRCE deflecting mirror (left) and monochromator (right).            Two branches after deflecting mirror.    

PEEM Endstation:

The PEEM endstation consists of an Elmitec LEEM/PEEM III with a Kirkpatrick-Baez refocusing mirror pair focusing the beam down to 12x36 um² (footprint on the sample). The microscope can be operated as LEEM, LEED, PEEM (X or UV) with a spatial resolution of 10 nm in LEEM and UV-PEEM modes and down to 20 nm in XPEEM . In X-PEEM mode, XMCD and XMLD effects can be used as mechanisms to reveal ferromagnetic or antiferromagnetic contrast, while scanning the incoming photon energy or the electron analyzer energy, local XAS and XPS spectra with submicron spatial resolution and electron energy resolution ~ 0.2 eV can be obtained. Rotation of the manipulator around the surface normal permits any relative orientation of the polarization and the sample.

The measurement temperature ranges from 100K up to about 1500K, while for preparation short time flashes of substrates up to 2000K are possible. The sample holders are equipped with a filament for electron bombardment, a W:Re thermocouple, and two extra contacts. Two ports in the PEEM chamber allow installation of custom equipment like e-beam evaporators for in situ layer growth with real time imaging or gas crackers for reactivity studies. Additional preparation options exist in the preparation chamber (ion sputtering gun, another port e.g. for organic materials). Precision leak valves for gas dosing and residual gas analyzers are available on both chambers.

The KB refocusing mirror (left) and the Elmitec LEEM/PEEM at Circe.

NAPP Endstation:

The Near-Ambient Pressure Photoemission (NAPP) endstation is an Ultra High Vacuum (UHV) setup equipped with a hemispherical electron energy analyzer for X-ray Photoelectron Spectroscopy (Phoibos NAP150 from SPECS) that can operate at a sample pressure range from UHV up to 25 mbar thanks to a differential pumping system which ensures a pressure difference of 10⁹ between detector and sample [http://rsi.aip.org/resource/1/rsinak/v73/i11/p3872_s1]. Therefore the usual characterization capabilities of XPS are extended to the study of gas-solid and gas-liquid interphases, with applications to in-situ characterization of heterogeneous catalysts, corrosion processes, wetting, fuel cells, photovoltaics, etc.

Near Edge X-ray absorption (NEXAFS or XAS) will also be possible either in Auger mode using the electron energy analyzer, or in total electron yield mode, measuring sample current. A UHV gas inlet system (Puregas from SPECS) will allow dosing gases and mixes with specified purity level better than ppb in the analysis chamber, with automatic flow and pressure control. Additionally, for finer dosing, two leak valve lines are also available in the analysis chamber.

A mass spectrometer is installed in the differential pumping system of the analyzer for simultaneous analysis of the reaction gases during the experiments, to confirm gas composition and investigate reaction products. A motorized 5-axes manipulator allows to vary sample polar and azimuth angles in the analysis chamber. The incident photon beam is deflected 3o downwards, making it possible to measure with the sample in horizontal position, which is of interest in the case of powder samples. It is also possible to transfer samples in horizontal position all the way to the measuring position.

The NAPP endstation at Circe.

Regarding electron emission geometry, the analyzer axes is at 54.7^o with respect to the incident beam (Magic Angle for both linear vertical and horizontal light polarizations). During the data acquisition, the samples can be heated up to aprox. 1000 K either by means of a infrared laser or using a sample holder with an encapsulated filament. For experiments requiring cooling, there will be an alternative manipulator that will allow to cool the sample down to 223 K (-50 C) using a Peltier element.

The NAPP setup includes also a Surface Science preparation chamber currently equipped with an ion sputtering gun and ports for installation of custom equipment (evaporators, sample cleavers, etc) and UHV sample heating up to aprox. 1500 K.