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Manuscript Title: Symmetrization and calculation of LEED intensity patterns.
Authors: J. Rundgren, A. Salwen
Program title: LEED BEAM-SYMMETRIZED
Catalogue identifier: ACKG_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 9(1975)312
Programming language: Fortran.
Computer: IBM 360/75.
Operating system: OS MVT.
RAM: 194K words
Word size: 32
Keywords: Solid state physics, Surface structure, Adsorbed layers, Crystal symmetry, Two dimensional Space groups, Low energy electron Diffraction, Leed, Layer-by-layer method.
Classification: 7.2.

Other versions:
Cat Id Title Reference
ACKE_v1_0 LEED BY LAYERS AND PERTURBATION CPC 7(1974)369

Revision history:
Type Tit le Reference
correction 000ACORRECTION 12/09/75 See below

Nature of problem:
Experiments with low energy electron diffraction(LEED)are used for finding the composition of the unit mesh of crystal-overlayer (crystal plus overlayer) structures. For open structures having a large unit mesh the number of spots of the diffraction pattern rises rapidly with increasing energy. By performing the diffraction experiment at normal incidence the symmetry in the spot pattern can be utilized to reduce the number of beams to be calculated.

Solution method:
It is supposed that the crystal-overlayer structure can be sliced into well separated layers. The interaction of the beams of electrons with a particular layer is governed by von Laue's diffraction conditions characteristic of that layer. The behaviour of von Laue's diffraction conditions characteristic of that layer. The behaviour of von Laue's diffraction conditions under the operations of the two-dimensional space group of the crystal overlayer structure is established by a group- theoretical method. The method applies to dense or open crystal-over- layer structures having any of the 17 two-dimensional space group symmetries. The LEED program is extended to use the least possible number of beams for the calculation of symmetric intensity patterms. The program can be run both in a symmetrized and in an unsymmetrized mode.

Restrictions:
One atom per unit mesh of the individual layers; the unit mesh of the crystal-overlayer structure equal to that of the overlayer; 36 beams; 8 phase shifts.

Running time:
Eg. for a primitive 2 X 2 structure on the (100) surface of a fcc crystal symmetrization will reduce 69 spots to 13 spots whose intensities are calculated in roughly 10 seconds on an IBM 360/75 computer, if 6 phase shifts are used. Parts of the program take times scaling as (l+1)**4 p and (l+1)**2 n**2 p, where l is the number of phase shifts minus unity, n the number of beams or symmetrized combinations of beams and p the crystal period normal to the surface.

CORRECTION SUMMARY
Manuscript Title: Unpublished correction to symmetrization and calculation of LEED intensity patterns.
Authors: J. Rundgren, A. Salwen
Program title: 000ACORRECTION 12/09/75
Catalogue identifier: ACKG_v1_0
Distribution format: gz
Classification: 7.2.