An electron diffraction study of alkali chloride vapors

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Abstract
Monomers and dimers of the four alkali chlorides NaCl, KCl, RbCl, and CsCl have been studied in the vapor phase using the counting method of high energy electron diffraction. Nozzle temperatures from 840–960 K were required to achieve the necessary vapor pressures of approximately 0.01 Torr. Large mean amplitudes of vibration (as big as 0.26 Å) and the similarity of the monomer and dimer bonded distances complicate the analysis, although the refined structures yield good fits to the data, especially for the stronger scatterers RbCl and CsCl. Using harmonic calculations for the monomer and dimer l values, a consistent set of structures for all four molecules is obtained. The corrected monomer re distances reproduce the microwave values very well. The planar rhombic structure of the dimers shows a uniform decrease of the Cl–M–Cl angle as the alkali M increases in size. For the better data sets some of the l values were refined to gain perspective on the validity of the calculated ones. The experiment also yields information on the amount of dimer present in the vapor, and these results are compared with thermodynamic values.