In the following, a brief description of the main SPELEEM imaging modes is provided.
X-ray Photoemission Electron Microscopy (XPEEM)– energy-filtered imaging
The technique can be used for slow secondaries electrons (utilizing a work function contrast) as well as for electrons whose energy is characteristic for the material studied. When primary electrons from the atomic core level are excited by the X-rays at a fixed energy, hν, they escape from the sample. These photoelectrons with a certain kinetic energy, Ekin= hν − Ebin – φ, are selected by the hemispherical energy analyzer to form an XPEEM image, where Ebin is a core level binding energy and φ the work function. By varying the Ekin, one can probe the chemical state of the emitting atoms for a specific element allowing performing elemental/chemical mapping.
Micro-X-ray Photoemission Spectroscopy (µ-XPS)
Photoelectron spectroscopy from extremely small areas down to a fraction of a micron can be performed. The high flux on the samples allows both high spatial and high energy resolution.
Micro-X-ray Absorption Spectroscopy (µ-XAS)
The microscope images the secondary electron emission at fixed kinetic energy as a function of the photon energy hv. In combination with linear and circular chroism, XPEEM has become the main tool for imaging the magnetic state of surfaces, thin films, and buried interfaces.
X-ray Magnetic Linear Dichroism (XLCD) and X-ray Magnetic Circular Dichroism (XMCD)
Taking advantage of elliptically polarized undulator, ferromagnetic and antiferromagnetic domains in magnetic materials can be studied with spatial resolution down to a couple of nanometers. It is important to note that, due to the experimental configuration with normally incident X-ray beam, the beamline provides high sensitivity to the out-of-plane component of the magnetic moment.
The intensity of a core level line as a function of energy and emission angle is measured. The technique can provide spatially resolved information on the surface crystallographic structure and it is therefore complementary to LEED and STM. If the valence band electrons form the diffraction pattern, the band and Fermi surface mapping in the full cone become possible (µ-ARPES technique).
Low Energy Electron Microscopy (LEEM)
This is the most powerful technique for the study of the morphology of crystalline surfaces. Several contrast mechanisms (including Dark Field Imaging) allow the determination of the lateral dimensions of regions with a given crystal structure, the thickness distribution of thin overlayers with monolayer resolution, the imaging of monoatomic surface steps and other morphological features.
Micro-Low Energy Electron Diffraction (µ-LEED)
By simply switching one lens and removing the contrast aperture the LEED pattern of the imaged area can be obtained. The imaged area can be as small as 250 nm, so the diffraction pattern from such a small area can be obtained.
Mirror Electron Microscopy (MEM) mode
In this mode the specimen is more negative than the electron source so that the electrons are reflected in front of the surface. Contrast is determined by field distribution above the surface which depends upon the surface topography and the charge distribution on the surface. In the case of magnetic specimens also the magnetic field distribution above the surface can be imaged by proper illumination and imaging conditions.