A quantitative study of the reflexion of X-rays by sylvine

Abstract

In recent papers on the intensity of reflexion of X-rays from rocksalt crystals it has been shown that, from the temperature of liquid air up to about 500° abs., the dependence of the intensity of reflexion upon temperature is in accord quantitatively with a formula of the type originally deduced by Debye, if the modification suggested later by Waller is introduced, although the decrease of intensity for higher temperatures is much greater than that indicated by the law. In dealing with quantitative results of experiments on reflexion from crystals, it is convenient to consider the quantity usually denoted by F, which is a measure of the scattering power, in a given direction, of an atom for X-rays. In the course of experiments with rocksalt it has been possible to determine F both for Na and Cl, and the values so obtained, when corrected for temperature, agree very closely with the F factors calculated from Hartree’s Schrodinger density-distribution for the ions Cl - and Na + . The calculation is based upon the theoretical result, due to Wentzel and Waller that, to obtain the coherent scattering from an electron in an atom, the electron must be represented by its corresponding Schrodinger charge-density, each element of which must be supposed to scatter classically. In order to get agreement between the calculated and observed F curves, it is necessary to assume the existence of zero-point energy of amount half a quantum per degree of freedom, which is of course required by the new quantum mechanics. The agreement between theory and experiment in the case of rocksalt is extremely interesting, but, in order to place the quantitative treatment of X-ray reflexion on an entirely satisfactory basis, it appears to be of some impor­tance to see whether a similar agreement can be obtained with other crystals. This is not quite so easy as might be supposed.