Diffraction of Low-Speed Electrons by Single Crystals of Copper and Silver

Diffraction beams for normal incidence on the (100) faces of copper and silver crystals.—Previous results for a copper crystal are checked and extended, by using another crystal of exceptionally pure copper. All of the expected diffraction beams which are x-ray analogues in the two principal azimuths and in the range below 325 volts are found. These beams require values of refractive index greater than unity, but the associated values of inner potential are not constant. Another class of weak beams is found characteristic of the copper lattice, but which require a refractive index of approximately unity for the first order beams of the two principal azimuths. These weak beams accompany the main beams on the high voltage side as satellites. Conditions for orders higher than the 1st are more complicated and indicate that the above method of classification is not sufficient. For a silver crystal the number of experimental maxima is considerably greater than the number of theoretical beams in the low-voltage range. These maxima cannot be classified as main beams requiring a refractive index greater than unity, and weak satellites requiring unit refractive index as in case of a copper crystal, but whenever two or more experimental maxima are to be associated with a given theoretical beam, they are, in general, grouped as components of fine structure of a single diffraction beam.

Intensity measurements on diffraction beams as a function of angle of incidence.—For copper, as the angle of incidence is changed by only a few degrees from normal, large intensity changes of the various diffraction beams are observed. Some beams increase while others decrease in intensity for a given change in angle of incidence. Primary voltage and collector angle are adjusted for each observation. For silver, as the angle of incidence is changed from normal the relative intensities of the components associated with a particular beam change rapidly. Some components may disappear and others appear at different voltages, as the incident angle is changed a few degrees. The plane grating formula is approximately satisfied over considerable ranges in the angle of incidence, but there are other ranges for which it is not satisfied. The above variations for various beams do not correspond; neither do those for the beams associated with different orders of reflection from the same set of Bragg planes. Intensity measurements are given for most of the beams in the two principal azimuths below 350 volts for ranges in the angle of incidence, including normal incidence, through which the beams are observable.

Regular reflection of electrons from the (100) set of planes.—For copper, with angle of incidence equal to angle of reflection, curves obtained by measuring collector current as a function of primary voltage show many irregularities. For silver, the curves contain many irregularities which do not correspond to those for a copper crystal under similar conditions. For the same angle of incidence and the same set of reflecting planes, the results are not the same for the plane of reflection in the (100) and the (111) azimuths, respectively.

Diffraction beams due to a surface gas lattice on the (100) faces of copper and silver crystals.—For copper, "additional" beams which were formerly reported have since been found to decrease in intensity after prolonged heat treatment of the copper crystal at temperatures near its melting point. These beams indicate a simple-cubic, single-spaced lattice with the same constant as that for the copper lattice. They require values of refractive index greater than unity. When the surface gas lattice becomes thin it changes in structure from a simple-cubic, single-spaced lattice to a face-centered, double-spaced lattice. The beams characteristic of the latter structure require unit refractive index. Hydrogen also forms in either of the above two lattices on a copper crystal, depending on the pressure of the hydrogen while the lattice is formed. The lattice which is characteristic of the thicker layer is very unstable and soon changes to that of the thinner layer after the pressure is removed. For silver, only a few very weak beams due to a surface gas lattice are found below 50 volts after heating the crystal a short time at red heat. They indicate a double-spaced, face-centered structure.

Inner potential. The measured value of the inner potential is neither constant nor a continuous function of the voltage. It is found to change discontinuously, by several volts in some cases, as the angle of incidence is varied. Results for the crystals of copper and silver which have the same lattice structure are found to differ widely. Hence they appear to be a function of the type of atom.

Surface action. Experimental methods of distinguishing between effects due to surface action and those due to a space lattice are discussed.