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Research

Research
 

The Bending magnet for Emission, Absorption and Reflectivity beamline (BEAR) is operational since the beginning of 2003 at the Elettra Synchrotron Light Laboratory. The beamline aims to provide a number of spectroscopic tools to investigate the basic properties of the electronic states in solids or molecules through the measurement of the optical absorption, the reflectivity and the photoemission yield in a UHV environment. The apparatus is based on a bending magnet as a source, a beamline optics delivering photons from near visible up to about 1600 eV with selectable degree of ellipticity and an end station featuring, in an UHV environment, a large variety of scattering geometries. The UHV end station has a movable hemispherical electron analyzer and a set of photodiodes to collect angle resolved photoemission spectra, optical reflectivity and fluorescence yield, respectively. The photoemission/scattering geometries can be chosen with a wide flexibility thanks to the movability of the detectors within the UHV chamber.

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Optical elements

The use of suitable sequences of stratified media represents the currently adopted technological solution in the construction of optical devices acting as band-pass mirrors or filters. The advancement of experimental capabilities in a variety of areas including opto-electronics, transport and handling of lasers, synchrotron and FEL beams, instrumentation for aero-space and astronomical observation often stems from performance and reliability of these kinds of systems. The control of the sharpness and the stability of capping layers and buried interfaces is one of the key aspects in this field. Synchrotron radiation characterization based on spectroscopy can be applied in the  optimization and monitoring of these kind of systems. Typical applications are specular reflectivity of EUV Multilayer mirrors and thin film mirrors, transmittance of UV pass band filters, calibration of spectrometers and CCD detectors, calibration of Streak cameras. All these require great accuracy.  For these applications, the beamline is equipped to have higher order  purity or using filters (SiO2, LiF, In, Sn, Al, Si, Ag) or choosing an accurate monochromator deviation angle, the control of the light polarization (Linear, Right circular and Left circular) by direct measurement of the Stokes parameters. Typical S/N ratio in the Mo-Si range is 1%.
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Blind tests: specular reflectivities of EUV multilayer mirrors



 


Organic ultra thin films

 

Collaborations with the Dipartimento di Ingegneria dei Materiali e dell'Ambiente e Dipartimento di Chimica Università di Modena e Reggio Emilia (L. Pasquali et al.), and the Saha Institute of Nuclear Physics Kolkata (A. Datta and M. Mukherjee)


X-ray reflectivity (XRR) offers a powerful tool to study a wide class of surfaces and buried interfaces with molecular scale resolution. Resonant soft x-ray reflectivity exploits the characteristic absorption spectra of atoms and molecular states to access element- and bond-specific information beyond the pure electron density distributions. In contrast to established absorption spectroscopy techniques which are based on electron/photon yield, resonant reflectivity is quantitative in nature and the data carry full depth-dependent information with nanometer resolution through structures up to several hundred nm thickness ranging from buried interfaces to the free surface. Reflectivity data recorded in s and p incidence across carbon K-edge absorption resonances allow to study oriented functional groups in an ultrathin self-assembled monolayers. The dielectric tensor and the molecular orientation can be obtained from polarization-dependent resonant XRR experiments using an indigenously developed simulation code OPAL.
This technique open the possibility to study single molecules electronic devices, that has attracted much attention in recent decades. It is known that the detailed geometrical configuration of metal−molecule−metal junctions plays a key role in the charge transport properties. For instance, self-assembled monolayers of dithiols are promised to have different applications and have been extensively studied. However, the accurate determination of the configuration of dithiols adsorbed on a surface is a long-standing and interesting problem.


 


The 1,4 benzenedimethanethiol (BDMT) molecule


Experimental results compared to calculations


 


Magnetic materials 


 

Collaboration with the Laboratory Oxide-MBE IOM-CNR (B. Davidson, A. Petrov, A. Verna) and the Laboratoire de Chimie Physique Univ. Paris - CNRs (P. Jonnard, J.M. Andrè and K. Le Guen)

Synchrotron  radiation experiments are helpful to investigate  the electronic and magnetic properties of materials. Reflectivity from Circular Polarization light is a technique sensitive to the magnetic state of Multilayer systems and their interfaces. The  interface properties can be probed with atomic cell resolution (interference effects in reflected radiation). This kind of measurements allows to determine the dielectric tensor of each layer, contains all the information on the electric and magnetic properties of the material. The determination of the magnetization profile can be done with a fitting procedure. An important class of materials are manganite based colossal magnetoresistance devices, which require a high control of the growth process, in particular the crystalline structure and stochiometry, high quality heterostructure, metallic characteristics well above Curie Temperature.


 Dichroic reflectivity at Mn L2,3



 


Radiation Damage 

 

Collaboration with Sincrotrone Trieste (A. Bianco, C. Svetina, N. Mahne)
 

The handling and transport of photon beam at the new free-electron lasers (FELs) light sources present demanding technological requirements mainly because of radiation damage which can be induced in optical coatings. Theoretical and experimental studies are crucial to understand the mechanisms of damage process occurring at optical surfaces irradiated by XUV ultra short pulses of extremely high fluences. Of particular interest is the response to the high intensity XUV FEL pulses of multilayer coated optics, due to their useful and widely exploited properties as high reflectivity mirrors working at near normal incidence. These optics are absolutely promising for FEL experiments, where they must satisfy extremely demanding requirements in space and time handling of the wavefront. Different techniques, as specular and diffuse reflectivity, X-ray Photoemission Spectroscopy (XPS) and Total Electron Yield (TEY) in the EUV and soft X-ray range, Standing wave enhanced XPS can give important indications on the evolution of the first interfaces of irradiated mirrors.

Standing wave enhanced XPS on a radiation damaged multilayer mirror: the analysis of the peak intensity gives quantitative indications of the evolution of the surface and of the buried interfaces 




Buried interfaces

The quantitative characterisation of degree of intermixing at interfaces is crucial to understand the properties of optical mirrors. A study has been done on Ru/Si interface. The experiment consisted in recording photoemission from Ru3d core levels at fixed photon energy while going through the Bragg resonance of multilayer following the evolution of component of Ru in silicide from Ru as metal itself. Results have been compared with a model calculation using the OPAL code giving the distribution of the electric field inside the sample as a function of depth and incidence angle. The evolution of Ru in silicide (red curve) and Ru (metal) (blue curve) was fitted to the experiment deriving the degree of ruthenium-silicon intermixing at the Ru-Si buried interface. It was possible to observe that the minima for ruthenium silicide and ruthenium metal were shifted. By comparing model calculations with the experiment it was possible to conclude that the intermixing profile has a σ of the order of 1nm.


 Experimental setup for buried interface analysis



 
Evolution of the Ru 3d and C 1s XPS peaks in correspondence of the Bragg peak


 


Optical constants


 

Collaboration with Instituto de Fisica Aplicada CSIC Madrid (J.I. Larruquert, J.A. Aznarez), Laboratorio Regionale IFN-CNR Luxor Padova (L. Poletto), Dipartimento di Elettronica Università di Pavia (M. Malvezzi)
 

The availability of optical constants n and k is a crucial aspect in the design of optical devices as interferential multilayer mirrors. Experimental data of the optical constants of most compounds and even several single elements are not available in important parts of the spectrum, such as lanthanides and rare earths in the EUV region. A Spanish-Italian collaborating group at BEAR beamline worked on the experimental determination of the optical constants of materials, mainly lanthanides and rare earths in the UV-Soft X range. The low absorption below O2,3, N4,5, and M4,5 absorption edges of lanthanides and rare earths turns them promising materials for the realization of multilayer mirrors.The experiments consisted mainly on the deposition in UHV of a thin film of the material under study onto a special substrate followed by the transmittance measurement of the film over a broad spectrum in situ in ultra high vacuum; several films of various thicknesses have been deposited. The transmittance as a function of film thickness is fitted for each wavelength and the fitting results in the direct determination of k at the given wave-length. The real part of the refractive index n has been obtained from k data using the Kramers-Kronig dispersion analysis. For this analysis, data on k, necessary in the whole spectral range, was measured from ~3 to 1,600 eV.

Extinction coefficient of promising materials in the EUV 50-105 nm spectral range


Typical setup for absorption measurements

Image of the sample: thin film deposited on a carbon film (10 nm)/ Ni grid (on the left). The glass witness sample allows to determine the film thickness by means of a θ 2θ reflectivity scan. 


 

 

Last Updated on Tuesday, 08 May 2012 15:32