Influence of surface charge and surface structure on the molecular and ionic sublimation of KCl single crystal

Abstract

A mass spectrometric method was used to study a kinetics of potassium chloride single crystal sublimation in the form of neutral molecules and positive and negative ions. In electron-impact-ionization mass spectrum K +, KCl + and K 2Cl + ions originated from KCl and K 2Cl 2 molecular precursors were detected in the temperature range 780–900 K. The curvature exposed by the temperature dependencies of ion currents, ln I i T−1/ T, was interpreted as a result of surface charge influence on the rate of sublimation. Theoretical and experimental retardation factors were analyzed for free sublimation of KCl and NaCl crystals [J.E. Lester, G.A. Somorjai, J. Chem. Phys. 49 (1968) 2940]. In thermal ionization mass spectrum K +, K 2 +, K 2Cl +, K 3Cl 2 + positive ions and Cl −, Cl 2 −, and KCl 2 − negative ions were identified in the ranges 710–1020 K and 950–1020 K, respectively; the negative ion currents were much weaker than positive ones. During an induction period, a gradual decrease of isothermal ion currents was observed. The temperature dependence, ln I(K +)−1/ T, measured at successive stages of vaporization revealed (i) the existence of knee temperature, above and below which the ion emission is characterized by different activation energies for ion desorption, E +, (ii) the values of E + were found to be dependent on the time elapsed from the vaporization start, i.e., on the developing surface morphology. The comparison of mass spectra of both types obtained in this work with those measured under equilibrium conditions showed the similarity of the electron-impact-ionization mass spectra and the significant difference between the thermal ionization ones. All the results are discussed in light of terrace-ledge-kink model of vaporization and surface charge concept that are a corollary of defect theory.