Interatomic Distances of the Alkali Halide Molecules by Electron Diffraction

Abstract

A determination of the interatomic distances of the alkali halide gas molecules has been made by the electron diffraction method. Heating the salts to a temperature of approximately 1200\ifmmode^\circ\else\textdegree\fi{}C provided a molecular beam of sufficient density to give electron diffraction photographs. The nuclear distances were obtained from visual measurements of the interference maxima. Data from ${\mathrm{I}}_{2}$ and ${\mathrm{Cl}}_{2}$ provided a calibration of the visual measurements since the internuclear distances for these molecules are well known from band spectra. The interatomic distances for the sodium, potassium and rubidium halides were found to be about 10 percent less than the corresponding known distances in the crystal lattice. For the case of the caesium halides the distances obtained were approximately 14 percent smaller than the separations found in the crystals. Tables showing complete data on twelve molecules are given.