ANISOTROPIC SECONDARY EMISSION AND OTHER DYNAMICAL EFFECTS IN LOW ENERGY ELECTRON DIFFRACTION.
POLYTECHNIC INST OF BROOKLYN N Y DEPT OF PHYSICS
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It is observed that the total secondary electron emission from single crystals depends on the incident direction of the primary electron beam. A dynamical diffraction model is proposed to explain these effects and is applied to other dynamical effects observed in LEED. By means of the reciprocity theorem and Brillouin zone diagrams maxima in the secondary emission are predicted when the incident beam is in a low index direction the angular width of these maxima are also predicted. Furthermore the theory predicts conditions under which the peaks may vanish, and the existence of maxima which do not correspond to low index directions. It is concluded that the maxima in emission are artifacts resulting from the negative contribution to the secondary emission due to forward diffraction. A formulation in terms of two beam dynamical theory is proposed which qualitatively describes the shift in the position of the dips from the Brillouin zone boundary, and the temperature dependence of the maxima and the minima. From computer calculations based on this model and the best fit with the data, it is concluded that the incident electrons penetrate to the depth of the order of the escape depth for secondary electrons, which is of the order of 20-200 angstroms. The observation of multiple scattering minima in elastic Bragg intensities, different from resonance effects previously described in the literature are described in terms of the excitation of low index reflections. Author
- Physical Chemistry