Structure Determination with X- and Cathode-Rays

It is usual, except in special cases, to work with x-rays of 0.5 to about 3.0 A wave lengths or with cathode rays of 10 to 10 volts. The latter coincide, according to the de Broglie relation, with a wave length range of about 0.4 to 0.04 A. The molecules under investigation may be in the free state (gas or solution) or in the form of a homogenous condensed phase (liquid, crystal, mixed crystal). 

The historical development of the experimental procedures began with the examination of molecules in the crystalline state, principally with the aid of x-rays. Here the early work following Max von Laue s fundamental discovery should be cited, particularly that of W. H. and W. L. Bragg and of P. Debye and P. Scherrer, while in subsequent years and up to the present day a great number of eminent research workers have contributed to this field. After the proof by Davisson and Germer and by G. P. Thomson of the wave nature of electrons in rapid motion, this technique was also applied to the study of molecular structure in the crystalline state.  

In contrast, investigation of the gaseous state had been undertaken about ten years previously by Debye and his co-workers with the aid of x-rays, and by Mark and Wierl with electrons. Later, the method was used by other workers and it furnished very valuable experimental data.  

It will be useful to review very briefly the most important features of the different investigation methods employing x- and electron-rays. 

10 w Friedrich, P. Knipping and M. v. Laue, Siiz. ber. bayer. Akad. Wiee., 6, 303 (1912) and for historical development also Naturwiss., 20, 527 (1932) and Current Science Special Number on Laue Diagrams January 1937. 

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