Photo-Ionization in Liquids and Crystals and the Dependence of the Frequency of X-Ray Absorption Edges on Chemical Constitution

Abstract

The mechanism giving rise to the electron affinity E of a liquid or a crystal is discussed, and it is shown that the relation E≅αW0 should hold, in which α is a numerical factor lying between ½ and 1, and W0, the average potential energy of a free electron in the liquid or crystal (with the sign changed), is related to the specific diamagnetic susceptibility of the liquid or crystal by the equation W0=-6.43×10^6κ (in volt-electrons). Franck and Scheibe's interpretation of the ultra-violet absorption of aqueous solutions of halides as showing photo-ionization of halide ions is used in deducing a value of 3.81 v.e. for the electron affinity of water, corresponding to α=0.82. Observed values of long wave-length limits for ultra-violet absorption leading to photo-ionization in crystals of five alkali halides lead to electron affinities for the crystals corresponding to values of α of about 0.80. Assuming the relation E=0.80 W0 to hold in general it is found that the diamagnetic susceptibilities of the silver halides are of the correct order of magnitude to give electron affinities of the crystals sufficient to allow photo-ionization of the halide ions by visible light. This is presumably the primary process in photosensitization of the photographic plate. The equation hνabs=hν0+eΦ(1)-Ecrystal, in which νabs is the x-ray absorption edge of an ion in a crystal ν0 that for a free ion, Φ(1) the electrostatic potential at the center of the ion due to surrounding ions, and Ecrystal is given by E≅0.80 W0, is used to predict shifts in the K edge of chlorine in various crystals and of potassium in its halides. The predicted shifts agree well with those observed for the series LiCl, NaCl, KCl, RbCl and KF, KCl, KBr, KI, supporting the contention that the factors of importance in producing shifts of the edges in these cases are the electrostatic potential and the electron affinity of the crystals.

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