Multiple diffraction origin of low energy electron diffraction intensities. II

Abstract

Strong minima in the rotation diagram about the 550 Bragg reflection from the (110) surface of tungsten are interpreted as being due to the excitation of forward diffracted Laue reflections having low index and large wave amplitude. The orientations of these minima correspond to the geometry for the excitation of the three beam case.In a recent article (Gervais, Stern & Menes, 1968) the structure in the rotation diagrams from a tungsten (110) surface was interpreted in terms of multiple diffraction. At that time we were mostly concerned about what appeared to be strong, sharp maxima, attributing their origin to the presence of simultaneous reflections. A detailed systematic investigation of the nature and origin of the structure observed in rotation diagrams indicates that this interpretation was misleading; the multiple diffraction causes sharp minima in the intensity. This can be seen from the following qualitative argument. If a Bragg maximum is excited in the specularly reflected beam, then that reflection remains excited during a rotation about the surface normal. If no other reflections are considered to be excited, then during the rotation the incident beam remains totally reflected. When dynamical interactions are included the excitation of other reflections will cause a decrease in the specularly reflected intensity on the basis of conversation of current.It is found that the origin of the minima in the rotation diagrams is the excitation of strong forward scattered Laue reflections, having for the most part an index H(hkl) such that hE+k2+12<_12. This conclusion is justified on the basis of the diffraction geometry determined by the incident beam orientation, and an ana]ysis of the measured total secondary emission current from the crystal during both rotation diagrams and rocking curves. * Supported by USAFOSR Grant 1263-67.In each case, the excitation of a low-index Laue reflection is accompanied by a strong reduction in the amplitude of the secondary emission and in the specularly reflected Bragg intensity. The mechanism for the reduction in the secondary emission is the reduction in the net absorption of the wave field propagating in the crystal during the excitation of the Laue reflection; the forward scattered beam carrying the field deep into the crystal. Secondary electrons produced far from the surface have a reduced escape probability and hence do not contribute to the secondary current (Stern & Taub, 1968).That the Bragg intensity is reduced because of the large amplitude of the Laue wave field can be shown quantitatively by solving the secular equation in the three beam approximation of Bethe (1928); in particular the ratio of the plane wave amplitudes can be determined for a given geometry. It has been shown that for certain diffraction geometry the wave field amplitude associated with a sheet of the dispersion surface can become zero (Niehrs, 1954). Furthermore, Stern, Perry & Boudreaux (1969) have shown in the three-beam approximation for the mixed Bragg-Laue case, that there exist diff...