Free evaporation of alkali halide crystals

Abstract

A study has been made of the evaporation of alkali halide crystals directly into vacuum to determine (1) whether or not the velocity distribution of the evaporating molecules is Maxwellian, (2) the chemical composition of the vapor, and (3) the evaporation coefficients and heats of evaporation of the various molecular species. Single crystals of CsBr, CsI, LiF, and NaCl were used. The velocities of the evaporating molecules were analyzed by means of the rotating cylindrical velocity selector used by Miller and Kusch. It was found that within relatively narrow limits of error the velocity distributions were Maxwellian. The vapor of CsI was found to contain only monomers; the others contained polymers in abundances different from those found in the saturated vapors. The heats of evaporation of NaCl monomers and dimers are, respectively, 61.7±4.6 and 67.2±4.6 kcal/mole, which yields heats of activation for evaporation of about 9 kcal/mole. Heats of activation also exist for LiF, but could not be determined from the data. The evaporation coefficients of the CsI and CsBr monomers are about 0.3; the evaporation coefficients of the polymers of NaCl and LiF are temperature dependent and are in the range 0.2 to 1.