Experimental Attempt to Observe Helium Atom Diffraction from (001) Silver

Abstract

To date, all experiments concerned with light gas scattering from metal single crystals have failed to show identifiable coherent scattering effects, although strong specular peaks are observed. In the present experiment helium diffraction from the (001) plane of lithium fluoride was examined and then a silver crystal was grown expitaxially in situ upon the lithium fluoride substrate and its (001) plane studied. Within the limits of resolution of the experiment, all diffracted rays were extinguished after deposition of the silver, whereas the specular beam intensity increased by nearly an order of magnitude over that for the lithium fluoride. Since the lattice spacing of the (001) plane of silver is very nearly equal to the like ion spacing of lithium fluoride, with which it has been shown to grow in registry, diffraction from the silver might be expected a priori. That diffraction peaks are not observed may be due to a drastic reduction in intensity, or they may be absent altogether. These results are discussed in terms of the periodic electronic and lattice properties of the lithium fluoride surface which may account for the intense diffraction effects which are observed. All of these surface properties are shown to be less localized for metals, thereby obscuring the discrete sites necessary for coherent reflection.