X-ray photoemissionelectron microscopy (XPEEM)

SPELEEM applications , LEEM , energy filtered XPEEM , XAS-PEEM , microprobe-ARPES

Energy-filtered PEEM XPEEM exploits the characteristics of synchrotron radiation to implement a laterally resolved version of x-ray photoelectron spectroscopy (XPS). This adds lateral sensitivity to the ESCA analysis and ultra-violet photoelectron spectroscopy (UPS). Therefore, synchrotron based PEEM  microscope can probe both the local chemical state and electronic structure of spatially inhomogeneous specimens. Operating principle Energy-filtered imaging. The electron optical conguration is the same as in LEEM, but the energy slit is inserted in the dispersive plane of the analyzer to band pass filter the photoelectron beam. Photons are used to probe the surface. The PEEM detects electrons emitted from atomic core levels with kinetic energy Ekin = hν - Ebin -φ, where Ebin is the core level binding energy, hν the photon energy and φ the work function. Typically hν is kept fixed, in the range provided by the beamline (50-1000 eV). The energy filter is used to select the kinetic energy Ekin of photoelectrons, which allows measuring the binding energies of emitting atoms or accessing the surface electronic structure, including surface states and resonances. The intensity of the photoemission signal is proportional to the number of emitters in the topmost layers within their energy-dependent escape depth, and thus provides straightforward and quantitative information about the surface chemical composition. Typical kinetic energies of the photoelectrons are at the minimum of the inelastic mean free path of the electrons in the matter, ensuring good surface sensitivity through the very reduced mean free path of electrons in the matter. Kinetic energies are in the range 50 to 150 eV, well beyond the broad peak of secondary emission. The PEEM can be operated either in imaging mode, that is when a magnified image of the specimen is projected on the detector, or in diffraction mode, that is when the backfocal plane of the objective lens is imaged. Microprobe XPS. The specimen is probed using soft x-ray or UV radiation produced by the synchrotron for area restricted XPS (X-ray Photoelectron Spectroscopy) measurements. The probed area can be restricted to 2µm by inserting Field Limiting Apertures (FLA) in the image plane in the middle of the beam separator. The last two projectors of the microscope are set to image the dispersive plane of the electron energy filter. The dispersive plane appears as a line, and its intensity profile represents the photoemission spectrum. This operation mode allows fast acquisition times while reaching the best energy resolution of the SPELEEM.

Reviews Recent advances in chemical and magnetic imaging of surfaces and interfaces by XPEEM; A. Locatelli and E. Bauer; J. Phys.: Condens. Matter 20, 093002 (2008). doi: 10.1088/0953-8984/20/9/093002 SPELEEM: combining LEEM and spectroscopic imaging; Th. Schmidt, S. Heun, J. Slezak, J. Diaz, K. C. Prince, G. Lilienkamp, and E. Bauer; Surf. Rev. Lett. 5, 1287-1296 (1998). doi: 10.1142/S0218625X98001626 Elettra SPELEEM (performance) Photoemission electron microscopy with chemical sensitivity: SPELEEM methods and applications; A. Locatelli, L. Aballe, T.O. Menteş, M. Kiskinova, E. Bauer; Surf. Interface Anal. 38, 1554-1557 (2006). doi: 10.1002/sia.2424