A general picture of threshold effects in LEED

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Abstract
The authors reinvestigate the interpretation of fine structures associated with beam emergence threshold in LEED. This work is based upon a new expansion of the total amplitude matrix of the crystal in a power series of the off-diagonal part of the bulk amplitude matrix. They clearly show that the pure resonance picture is incorrect, even in the favourable case where the fine structures fall inside a forbidden gap. Their alternative picture is that the dominant mechanism is an interference between the waves directly reflected at the barrier or at the bulk, and the waves associated with processes including on metal-metal reflection at the barrier. This interpretation, which proved to be adequate in their previous study of Al(001) and Ni(001), holds equally well for the fine structure measured by Adnot and Carette (1977) on W(001) in the presence of a band-gap. As a consequence the authors discuss why McRae's method (1967) to extract structural information from a 2D surface band structure cannot be used.