Call for Proposals and User information is published by the ALBA User Office and the ALBA Industrial Office.


DESCRIPTION

CIRCE is a variable polarization soft X-ray beamline dedicated to advanced photoemission experiments. A plane grating monochromator covering the energy range 100 - 2000 eV is shared between two independent branches with dedicated experimental endstations: PEEM (photoemission electron microscopy) and NAPP (near ambient pressure photoemission). A couple of deflecting mirrors directs the beam to one branch or to the other.

Two endstations of CIRCE  Detail of CIRCE


STATUS 

The PEEM endstation has been receiving users since October 2012. Relevant results have been obtained in XPS-PEEM, XMC(L)D-PEEM, IV-LEEM and u-LEED modes, combining state of the art spatial resolution with spectroscopic measurements (spectromicroscopy). Since then, the beamline control system has been continuously updated to improve acquisition speed and user friendliness. Electrical poling experiments with out-of-plane fields and in-plane electrodes have been performed and sample holders with in-situ electromagnets (in-plane, out-of-plane, biaxial in-plane and combined in-plane/out-of-plane) have been added. We continue the beamline and endstation development according to user needs.

The NAPP endstation has been in users operation since Nov 2013. First results have been obtained in catalysis and surface science. Since April 2014, the Peltier cooling system is also operative. A gas line for dosing CO into the analysis chamber will be operative in the second half of 2015.

For more information, please also refer to the detailed description further below on this page, the documents provided here or contact the beamline staff. 

 

TECHNICAL SPECIFICATIONS

Photon energy:

100 - 2000 eV 
Polarization Variable
Energy resolution  ~ 8000
Photon flux       ~ 1013 ph/s 
Beam spot size at PEEM position (VxH) (FWHM): variable, min 3.2 x 36 µm
Beam spot size at NAPP position (VxH) (FWHM):  ~ 20 x 100 µm2

 

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

CIRCE beamline has a variable angle plane grating monochromator. 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 onto 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.

Deflecting mirror and monochromator of CIRCE

PEEM ENDSTATION

The PEEM endstation consists of an Elmitec LEEM/PEEM III with a Kirkpatrick-Baez refocusing mirror pair focusing the beam down to 12 x 36 µm2 (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 a ferromagnetic or anti-ferromagnetic 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 of temperature ranges from 100 K up to about 1500 K, while for preparation of short-time flashes of substrates it is possible for this range to go up to 2000 K. The sample holders are equipped with a filament for electron bombardment, a W:Re thermocouple, and two extra contacts. Special sample holders and HV rack inserts for in-situ application of small (bipolar) magnetic fields and voltages up to 220 V are now available.

Two ports in the PEEM chamber allow installation of custom equipment, e.g. 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, for organic materials for example). Precision leak valves for gas dosing and residual gas analyzers are available on both chambers.

IM-CIRCE_KBMirror

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 (A differentially pumped electrostatic lens system for photoemission studies in the millibar range). Therefore the usual characterization capabilities of XPS are extended to the study of gas-solid and gas-liquid interphases, with applications for "in-situ" characterization of heterogeneous catalysts, corrosion processes, wetting, fuel cells, photovoltaics, etc.

Near Edge X-ray absorption (NEXAFS or XAS) is also possible either in Total Electron Yield mode (measuring sample current) or in Auger mode using the electron energy analyzer. A UHV gas inlet system (Puregas from SPECS) for dosing pure gases or gas mixtures in the analysis chamber, with purity level better than ppb, will be operative in the second half of 2017. This system will allow dosing up to three different gases simultaneously with automatic flow and pressure control. Additionally, for finer dosing, three leak valve lines are also available in the analysis chamber.

A mass spectrometer is installed in the second stage of the differential pumping system of the analyzer, making it possible to simultaneously analyze the reaction gases during the experiments in order to confirm gas composition and investigate reaction products.

A motorized 5-axes manipulator allows the variation of sample polar and azimuth angles in the analysis chamber. The incident photon beam is deflected 3o downwards, making possible to measure with the sample in horizontal position; this 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.

Regarding electron emission geometry, the analyzer axis is at 54.7o with respect to the incident beam (Magic Angle for both linear vertical and horizontal light polarizations). During data acquisition, the samples can be heated up to approx. 1000 K either by means of an infrared laser or by using a sample holder with an encapsulated filament. For experiments requiring cooling, there is an alternative manipulator that allows cooling the sample down to -23 ˚C by using a Peltier element.

The NAPP setup also includes a surface science preparation chamber currently equipped with ion sputtering gun, LEED and a flange for the installation of evaporators. A quartz balance will be available in 2017. Other ports are available for installing additional equipment brought by the users (sample cleaver, etc).

 

 NAPP Endstation at CIRCE beamline

 

LAYOUT

CIRCe layout