Department of Chemistry,
University College London,
T: +44 (0)20 7679 1003
Surface X-ray Diffraction (SXRD)
Surface structure is well known to influence, and in some cases dominate, phenomena such as expitaxial growth, catalytic action, semiconductor device performance etc. Surface X-ray Diffraction (SXRD) uses X-ray scattering from surfaces and interfaces for structure determination. The weak interaction of x-rays with matter gives SXRD distinct advantages over more common electron based techniques, allowing operation in more realistic environments where surfaces are buried (interfaces) or under high ambient gas pressure. Intense sources are required, however, since the critical surface layer diffracts only ~10-7 of the signal arising from bulk scattering. For this reason synchrotron radiation has to be employed.
Low Energy Electron Diffraction (LEED)
The fundamentals for Low Energy Electron Diffraction (LEED) were shown by Davisson and Germer in the 1920s ; when it was demonstrated that the elastically scattered electrons from a monochromatic beam of electrons incident on a surface have a spatial distribution that reflects the underlying symmetry of the surface. During a LEED experiment, the scattered electrons are usually analysed in the range of 20-200 eV. This corresponds to an inelastic mean free path of around 10Å, and hence the electrons can only travel a few atomic layers into the surface. This energy range corresponds to de Broglie wavelengths between 2.74 and 0.87Å, which are of the same order of magnitude as the interatomic spacings of atoms at a surface. As a result, a periodic surface structure can lead to a diffraction pattern of the scattered electrons. A schematic of the experimental setup is shown below.A beam of monochromatic electrons is produced by an electron gun whose energy can be varied between 0-1000eV, this beam is then incident upon a sample which is earthed to prevent charging problems. The diffracted electrons then travel through 4 grids toward a phosphor screen. The inner and outer grids are both earthed to create a ‘field free region’ for the scattered electrons to travel through. The inner pair of grids form a filter by being held at a negative potential V = -Ep + ΔV, where Ep is the energy of the incident beam, and ΔV is in the range 0-10V. This filter ensures that only the elastically scattered electrons make it through to the screen, where they cause bright spots on a dark background.The intensities in the diffraction pattern produced on the screen can be analysed quantitatively in so-called IV-LEED, but this requires complex modelling in most cases, and as such LEED is most often used in a qualitative manner.
Photoelectron Diffraction (PED)
Photoelectron diffraction is a method used to determine the geometry of molecules or atoms on single crystal surfaces. The knowledge of structure parameters like the adsorption site, bond lengths and bond angles enables a detailed insight into basic physical or chemical processes on surfaces such as catalytic reactions.The principle of photoelectron diffraction is comparable with the principle of optical holography: a "reference" electron wave and different "object" electron waves (which emerge from the reference wave being scattered at atoms in the substrate or the adsorbate) interfere in a suitable detector (analyzer). The electron source is an atom in the adsorbate or on in the surface. The electron wave is created by exciting a core hole electron with synchrotron radiation or x-rays. Part of it will reach the detector directly without interacting with any other atoms. This is the "reference" wave. Other parts will get there after being scattered off other atoms. These are the "object" waves. In the analyzer these different parts of the same electron wave interfere with each other. The structural parameters can be extracted from the interference pattern by comparing the experimental pattern with patterns simulated using theoretical methods and computer models of the surface. We can perform the photoelectron diffraction experiment in two possible modes.
- (a) Scanned energy scan mode. Here we measure the interference pattern (or better interference spectra) as a function of the kinetic energy of the emitted electrons in fixed emission directions. The kinetic energy of the electrons can only be varied by changing the photon energy. This requires a synchrotron light source.
- (b) Scanned angle mode. An alternative method is to measure the interference pattern as a function of the emission angle at a fixed kinetic energies. This is easy to do with a relatively cheap lab based X-ray source. In our case we use an High Power (800W) VG X-ray source with a Magnesium anode, giving us a photoenergy of 1253.6 eV.