M. H. Pirenne, The diffraction of X-rays and electrons by free molecules. The Cambridge series of physical chemistry. Cambridge, University Press, 1946. 154 pp., 82 figures, 6 tables, bibliography of 187 titles.

M. H. Pirenne, Diffraction of X-rays and Electrons by Free Molecules.

This small monograph is the first specialized book devoted to the diffraction of electrons and X-rays in gases. The experimental facts, as well as the physical foundations and results of the theory of electron and X-ray scattering, are set out in the book with exceptional clarity and consistency. Details of the mathematical calculations are absent from the book. The book is intended mainly for the experimental physicist.

In the first chapter the foundations of the classical theory of electron scattering are set forth. An isolated electron and a rigid system of many electrons are considered in sequence; the definition of the atomic factor is introduced, and methods for its exact and approximate calculation are discussed.

In the second chapter, incoherent scattering is considered—first by an electron, then by a hydrogen atom and by other atoms. All the results of the theory are compared with experiment. The formulas needed for such comparison are given; where necessary, numerical tables are provided that make it possible to compute the required quantity quickly (for example, a table of the scattering function according to Heisenberg–Bewilogua).

The third chapter, devoted to characteristic radiation, is hardly necessary. It briefly sets out certain experimental facts and points to the necessity—well known to specialists working in this field—of avoiding the characteristic radiation of the object when studying the scattering of electrons or X-rays by selecting a suitable primary wavelength.

In the following two chapters the scattering of X-rays and slow electrons by polyatomic gases is presented. Here too the investigations of Brett and his school, devoted to determining atomic factors by measuring scattering by a crystal, are discussed. These experiments, unfortunately, were discussed only in general terms: a whole series of interesting experimental works remained outside the author’s attention.

In the subsequent two chapters diffraction in gases and liquids is examined very concisely and, at the same time, with complete clarity. Of particular interest is the account of works devoted to clarifying the influence of the size of the atom (the blurring of the electron cloud) on the diffraction pattern. Separate chapters consider the effect of intermolecular interference (so essential for liquids) and the allowance for thermal vibrations of atoms. The ninth chapter, devoted to the question of the influence of chemical bonding (that is, properly speaking, the deviation of the form of the electron cloud from spherical), is of no special interest; it contains little factual material.

Chapters ten and eleven are devoted to questions of analyzing the structure of the molecule. In these chapters the methods of electron and X-ray diffraction are constantly compared. The author seeks to show that X-ray diffraction by gases is in many respects a more powerful means of investigating the structure of a molecule than electron diffraction. The opposite point of view is considerably more generally accepted. However, the theoretical and experimental facts assembled by the author are highly convincing.

The author considers only the X-ray experimental technique for the study of gases. Here he shows that, with carefully performed experiments, X-ray photographs of gases can be obtained with quite acceptable exposures—of the order of one hour. Until now it has been assumed that it is practically impossible to obtain satisfactory X-ray photographs with exposures of less than many hours.

The book is written in a clear and simple language and is well illustrated. The bibliography is fairly complete, apart from a certain neglect of the works of the American school of Brockway. Between the lines one senses that the measurements and determinations of molecular structure made in these investigations do not seem to the author of the book to deserve full confidence.

The book is supplied with a foreword by N. Debye, warmly recommending this work. It seems to us that Pirenne’s work should be translated into Russian.

A. Kitaigorodskii