Beamline description

In electron spectroscopy for chemical analysis (ESCA) microscopy, the X-ray emitted photo electrons are detected and analyzed in terms of their kinetic energy. Small shifts in element specific core or valence energies signalize changes and can give detailed information about bond bindings. ESCA is a method sensitive to the chemical information of few monolayers and offers therefore a special instrument for surface sensitive chemical analysis of heterogeneous materials.

The ESCA microscopy beamline at Elettra houses a worldwide unique instrument allowing combining chemically surface sensitive measurements with high spatial resolution. (see Scanning photoelectron microscope (SPEM) )

The beamline is a branch line sharing an undulator photon source with another beamline (SuperESCA). The recently (December 2010)  installed 98 periods undulator consists of two equal modules which produce horizontally polarised light. By setting the gap value from a minimum of 13.5 mm up to 40 mm the photon energy can be varied in the range 90÷1500 or 130÷1800 eV when the Elettra storage ring works at 2.0 or 2.4 GeV, respectively.

The beamline was installed and commissioned in 1994.

Scanning photoelectron microscope (SPEM)

With respect to the other photoelectron microscopy techniques a Scanning photoelectron microscope (SPEM) uses the most direct approach to photoelectron spectromicroscopy which is the use of a small focused photon probe to illuminate the surface (see Zone plate focusing optics).

The SPEM at the Elettra synchrotron light source can operate in two modes: imaging and spectroscopy. In the first mode the sample surface is mapped by synchronized-scanning the sample with respect to the focused photon beam and collecting photoelectrons with a selected kinetic energy. The second mode is photoelectron spectroscopy from a microspot.

Experimental & Preparation Chambers

The measurement station consists of three UHV chambers: the experiment chamber, hosting a Scanning photoelectron microscope (SPEM), a fast entry lock connected to the small preparation chamber, where 5 sample holders can be kept in vacuum contemporarily and simple preparation procedures can be performed and a main preparation chamber with facilities for specimen preparation / characterization. The specimen mounted on a sample holder is transported using magnetic arms and wobble sticks. the preparation chamber includes LEED, AES, PEEM, a sputter gun, plasma source, a gas inlet system, an evaporation source, a mass spectrometer, an arm for sample transport, and facilities for indirect and direct sample heating. Additionally, a high pressure chamber can be attached to the cross of 3 above mentioned chambers.

Beamline

Zone plate focusing optics utilizes only coherent flux of the beam determined by the central source brightness. Taking into account relatively low photoemission cross sections SPEM has become possible thanks to appearance of high brightness 3d generation synchrotron radiation sources such as ELETTRA. Accordingly, prefocusing and monochromator optics were designed with the scope to convey the beam to the exit slit of the monochromator and not to destroy its spatial coherence.