Electron Diffraction

Text extracted from opening pages of book: ELECTRON DIFFRACTION by R. BEECHING, A. R. C. S., B. Sc. DEMONSTRATOR IN PHYSICS AT THE IMPERIAL COLLEGE OF SCIENCE AND TEOHNOLOOY with a Preface by G. P. THOMSON, F. R. S. PROFESSOR OF PHYSICS AT THE IMPERIAL COLLEGE OF SCIENCE AND TECHNOLOGY WITH A HALF-TONE FRONTISPIECE AND 39 DIAGRAMS IN THE TEXT METHUEN & CO. LTD. 36 ESSEX STREET, W. C. London First published in 1936 PRINTRD IN GREAT BRITAIN PREFACE ALTHOUGH electron diffraction is a comparatively recent branch of physics, it is easy to justify its inclusion iff this series of monographs. Its interest is twofold. On the one hand, it supplies by far the most direct evidence for the principles of wave - mechanics on which the whole of atomic physics is now based, on the other, its applications provide one of the most promising - lines of approach to the study of surfaces, of molecular structure and of crystal growth. Electron diffraction can supply the same information about the arrangement of atoms in a surface that the diffraction of X-rays gives about their arrangement in bulk. Problems of surface and molecular structure, which now form part of the 110-m ail's - land between physics and chemistry, seem likely to be studied increasingly during the next few years, and it is probable that their solution will provide the most important contributions of physical science to industry of the immediate future. It is therefore most desirable that the possibilities of the method should be presented clearly to as wide a circle of scientific workers as possible, and for this purpose the present book is admirably suited. It is greatly to be hoped that the clear account here given of some of the resultsalready obtained may induce many workers to try their luck with this fas cinating new tool. If they do so they will find the account which Mr. Beeching has given of the technical vi ELECTRON DIFFRACTION methods most valuable. He has used many of them himself, and has had opportunities of seeing most of the others in use and discussing their merits with those who are developing them. The technique of electron diffraction is often supposed to be a difficult one. As regards the examination of surfaces by fast electrons this belief is mistaken, except in so far as any method used for the study of surfaces involves the difficulty of preparing surfaces under conditions which shall be truly reproducible and standard. The difficulties inherent in the vacuum technique have been much reduced by modern pumps and the use of ground and greased metal to metal joints, and are no longer really serious. The book is well adapted for those also, no doubt the majority, who do not intend to use these methods themselves, but who are interested in the far-reaching theoretical implications of the wave character of electrons, or who wish to be able to appreciate the conclusions reached by means of this new technique. G. P. THOMSON August. 1935 CONTENTS CHAPTER PAGK I. THE WAVE PROPERTIES OF THE ELECTRON, AND THE CRYSTAL AS A GRATING .... 1 De Broglie's Theory. Group Velocity. A Crystal as a Grating. Laue's Conditions. Bragg's Law. Structure Factor. II. THE EARLY EXPERIMENTAL WORK . . . .12 Davisson and Germer's Experiments. G. P. Thom son's Work. Kikuchi's Mica Patterns. Origin of Kikuchi Linos. Further Verification of do Broglie's Law. III. DIFFRACTION BY TRANSMISSION THROUGH THIN FILMS 28 Single CrystalFilms. A Small Crystal Block con sidered as a Cross Grating. Effect of Crystal Size. Orientation. Atomic Scattering Factor. Deter mination of Atomic Scattering Factor. Scattering in Monatomic Gases. Scattering by Free Molecules. Mark and Wiorl's Method. IV. DIFFRACTION BY REFLECTION ..... 49 Single Crystal Cleavage Face. Kirchner's Method of considering the Conditions for Diffraction. Inner Potential. Variation of the Apparent Inner Poten tial with Angle of Incidence. Diffraction of Slow Electrons. Dispersion Theory of Electron Diffrac tion. Kikuchi Lines and the Rela