The conventional electron energy in the low-energy electron ionization source is 7UeV, and the electrons below this energy are called low-energy electrons.High energy reactions that do not occur under low-energy electron conditions reduce the average of molecular ionsinternal energy, easy to obtain high abundanceMolecular ion peak。[1]
Low-energy electron diffractionLEED is to incident monochromatic electrons with energy ranging from 5 to 500 eV on the sample surface. Through the interaction between electrons and crystals, some electrons are reflected into the vacuum in the form of coherent scattering. The formed diffracted beam enters a movable receiver for intensity measurement, or is accelerated to the fluorescent screen to give an observable diffracted image.In the diffraction image, the first grid is grounded, so that the diffracted electrons fly freely through the space between the sample and the grid;Add tens of volts of negative voltage to the second gate, which can be filteredInelastic scatteringElectronics.The high voltage of kilovolt is applied to the fluorescent screen to enable electrons to have enough energy to excite fluorescent substances.Because the scattering of electrons by matter is much stronger than that of X-ray, low-energy electrons have high surface sensitivity.
Give the crystal mesh image of the reciprocal space on the crystal surface, or directly give the crystalReciprocal latticeIt can be explained by the kinematic theory of diffraction on the basis of a two-dimensional model (seeTheory of diffraction dynamics）。The reciprocal lattice of an infinite two-dimensional crystal is an array of reciprocal bars perpendicular to the two-dimensional crystal plane.Incident wave vector parallel to this crystal planek"And scattering wave vector（ki) The difference "is equal to the two-dimensional reciprocal lattice vector of this crystal planeGi，Immediate（ki)〃－k〃=GI,
Meet the diffraction enhancement conditions.As incident wave vectorkMake a ball for the radius (called the Urval ball), and the intersection point of the ball and the reciprocal bar will give the wave vector of the diffracted beamki。In the corresponding positive space, the diffraction enhancement condition is the Bragg formulaasinφ=hλ，bsinφ┡=kλ，
Wherea、bIs the length of the 2D translation vector.The surface translation vector can be determined from the diffraction patternAs、Bs,Various types of surface ordered structures are studied, and the corresponding space groups are given[2]。
Analysis is the core problem of determining the atomic position in the surface cell using LEED.Since the kinetic energy of slow electrons is close to the scattering potential in crystals, the usual kinematic theory or modified kinematic theory dealing with high-energy electron diffraction cannot be used for low-energy electron diffraction.The comparison between theoretical calculation and experimental data shows that the analysis of the behavior of low-energy electrons in crystals must consider the atoms, electrons andphononThe interaction with it and the multiple scattering of low-energy electrons in the crystal.All these interactions are expressed as an effective potentialV(r）The Hamiltonian of low-energy electrons is written asH=Ho+V(r)，
HO is the kinetic energy of the incident electron.This reduces the multi-body problem to solving a single electronSchrodinger equation：Hψ=Eψ。
The diffraction intensity to be solved is equal to the second power of the mode of the eigen wave function|ψ|。Many modern low-energy electron diffraction theories are analyzed from multiple scatteringGreen's functionStarting from the method, various model assumptions are made for the specific scattering process, and several effective methods are developed, such as KKR method, Becky T-matrix method, renormalized forward scattering method, double-layer method, chain method and other perturbation methods.Low energy diffraction technology has been extended to study surface defects and two-dimensional phase transition, and its theoretical analysis method is also otherSurface analysis technologyLessons learned.
Low energy electron diffractometerIt is often used in conjunction with a variety of surface analyzers to comprehensively analyze the element composition and surface atomic structure of clean and adsorbed surfaces of various metals and semiconductors.
