Critical analysis of the excitation map concept for studying threshold effects in VLEED

The excitation map concept introduced by Solbrig to discuss threshold effects in very low energy electron diffraction (VLEED) is reinvestigated. In detail the relevance of such excitation maps (|det (1 - M-+R+-)| where M-+ is the reflection matrix of the atomic planes and R+- is the internal reflection matrix of the surface barrier) for characterizing surface resonances in VLEED is examined. It is shown that, in most cases, the use of this concept leads to an overestimate of the resonance contribution in VLEED spectra. The excitation map concept is replaced by a new relevant one. This result is exemplified by reinvestigating the region of Solbrig's excitation map on Ni(001) corresponding to the normal incidence, just below the emergence of the first four diffracted beams (10), (01), (1-bar0), and (01-bar). Even in this case, in favour of the resonance interpretation according to Solbrig's analysis, it is shown that the picture of the waveguide located at the surface, channelling the pre-emergent waves, falls down. This conclusion is strengthened by an analysis of the reflection amplitude matrix of the crystal based upon a second-order expansion in which the actual role played by resonances can be easily examined. In the case of Ni(001), threshold effects are only due to a simple interference process. (author)