Experimental study of low-energy electron-diffraction rotation diagrams from the (001) surface of Al reveals the appearance of systematic structure in the specularly reflected beam. This structure does not seem to be associated either with the secondary Bragg features or with the threshold effect. It has been identified as being due to the coupling between an incident beam and a localized surface state. A simple calculation of the $\varphi$ dependence of the resonance minima based on the Kronig-Penney model confirms this conclusion. The resonance intensity profile is generally in agreement with McRae's prediction.

• Received 21 June 1971

DOI:https://doi.org/10.1103/PhysRevLett.27.735