Electron Diffraction and the Imperfection of Crystal Surfaces

Abstract

Bragg reflections are obtained by scattering fast electrons (0.05A) from the etched surfaces of metallic single crystals. The surfaces studied are a (100) face of an iron crystal, (111) face of a nickel crystal and (110) face of a tungsten crystal. In each case the reflections occur accurately at the calculated Bragg positions with no displacement due to refraction. A given reflection is found, however, even when the glancing angle of the primary beam differs considerably from the calculated Bragg value—by over 1° in some cases—so that several Bragg orders occur simultaneously. The accuracy with which this glancing angle must be adjusted is a measure of the degree of imperfection of the crystal. From the electron experiments, estimates are made of the widths at half maximum of electron rocking curves. These widths are 0.8° for the iron crystal, 1.5° for the nickel crystal and somewhat over 1° for the tungsten crystal. X-ray rocking curves for these same crystals are much narrower, although the observed widths vary considerably with the treatment of the surfaces. It is concluded that the values obtained from the electron measurements apply to projecting surface metal only, and that the degree of misalignment is much greater at the surface than deep down within the crystal. Furthermore, even the x-rays [Mo $K\alpha$ radiation-0.71A] are not sufficiently penetrating to yield values certainly characteristic of these metal crystals.