Mass-spectrometric study of the kinetics of ionic and molecular sublimation of sodium chloride single crystals

A mass-spectrometric method is used to study the kinetics of ionic and molecular sublimation of sodium chloride single crystals. The positive ions Na+, Na-2(+), and Na2Cl+ were identified in the mass spectrum in the temperature range 820-970 K. Up to 970 K no emission of negative ions was observed. The temperature dependence of thermal ion currents, InIi - (1/T), exhibits a maximum and a minimum successively with increasing temperature. These features are identified with a change in the electrical properties of the surface. It is inferred that in the intrinsic temperature range the sign of the surface charge in NaCl is negative and thus the Gibbs free energy of formation of cation vacancies is greater than that of anion vacancies. In the electron impact ionization mass spectrum of molecular fluxes vaporized from an open crystal surface, Na+, NaCl+, and Na2Cl+ ions originating from NaCl and Na2Cl2 precursors are detected in the temperature range 779-982 K. The dimer-to-monomer ratio is found to be of the same order of magnitude in the cases of equilibrium and free-surface vaporization. However, the dependence on temperature of this ratio is different in these two cases. Mechanisms which account for this observation are discussed in light of the terrace-ledge-kink and surface charge models.