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Manuscript Title: Calculation of the impact scattering contribution to electron energy loss spectra.
Authors: G.C. Aers, J.B. Pendry
Program title: EELSOV
Catalogue identifier: AARU_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 25(1982)389
Programming language: Fortran.
Computer: IBM 370/165.
Operating system: OS, COS.
RAM: 300K words
Word size: 32
Keywords: Electron energy Loss spectroscopy, Impact scattering, Solid state physics, Crystal, Overlayer, Surface, Surface vibrations, Surface phonons, Multiple scattering.
Classification: 7.2.

Nature of problem:
To calculate the impact scattering contribution to the electron energy loss current, from one or more adsorbate layers on a single crystal surface, as a function of vibrational frequency and polarisation, incident electron energy and direction and scattered electron direction.

Solution method:
Assuming the initial and final state energies to be the same, the energy loss matrix element is calculated to first order in the absorbate displacement, and in the rigid muffin-tin approximation, for a simple harmonic surface vibration of given amplitude and polarisation. The initial and final state wavefunctions are calculated with full multiple scattering for one or more adsorbate layers, on a crystal described by sufficient identical layers to obtain the bulk reflectivity.

Restrictions:
1) The program assumes only one atom per unit cell in any given layer and that the bulk crystal is composed of identical layers.
2) Each layer including the adsorbate must be a sublattice of the layer below. This is of course satisfied by the bulk crystal layers.
3) The surface barrier is assumed to refract but not to reflect electrons. This assumption becomes poorer as the incident electron energy is lowered.
4) The dipole interaction is neglected. This program will only give a small part of the total current near the specular direction for modes of vibration perpendicular to the surface.
5) The incident and scattered electrons are assumed to have the same energy. This is reasonable for many systems where the phonon frequency is typically ~~100 meV compared to incident electron energies of a few eV.
6) Degenerate modes parallel to the surface must be treated with care since the program assumes a non-degenerate vibration with a well defined polarisation vector as the source of a particular loss peak. For vibrating atoms in two-fold sites e.g. W(100)p(1*1)H, where degenerate vibrations occur on seperate sites, the total result for a parallel mode can be obtained for all angles by adding the intensities obtained from two possible sites. For atoms in four-fold sites e.g. Ni(100)p(1X1)CO, where degenerate vibrations occur on the same site, only scattering angles in the plane of incidence can be considered and for the scattering plane lying parallel to one direction of vibration. In this case the contribution to the current from vibrations perpendicular to the plane of incidence is zero.