Mass-spectrometric study of the temperature variation in the dimer-to-monomer ratio in the free-surface vaporization fluxes from alkali halide single crystals

A mass-spectrometric method is used to investigate the kinetics of the vaporization of LiF (970-1070 K), NaCl (722-889 K), KCl (780-900 K), KBr (724-918 K), and Csl (656-838 K) single crystals. A dimer-to-monomer ratio, J(D)/J(M), in the fluxes vaporizing from free surfaces of these crystals are calculated, using the currents of M+, MX+, and M2X+ ions formed via electron impact ionization of the molecules MX and M2X2 (M is the alkali atom; X is the halogen). It is found that the dimer-to-monomer ratio increases with temperature at a continually increasing rate in the LiF and NaCl cases. In the KCl, KBr, and Csl cases, this rate passes through a maximum. Such a specific temperature variation in J(D)/J(M) is discussed through the analysis of proposed mechanisms of dimer formation in light of the terrace-ledge-kink and surface charge models.