Dislocations as sources of ions from NaCl and KCl (1 0 0) surfaces

The thermal emission of atomic M+ and molecular M2Cl+ ions (M: alkali) from (1 0 0) surfaces of NaCl and KCl crystals under the exposure to alkali chloride molecular beams is studied. Transient emission currents after release and interruption of the impinging fluxes are recorded for different crystal temperatures and molecular beam fluxes. The fact that the host ion currents can be effectively suppressed even by relatively low impinging guest fluxes leads us to the conclusion that not the terrace, ledge or kink sites but dislocations ending at the evaporating surfaces represent the sources and sinks for the ions. Similarities between the ion emission and diffusion at dislocations and grain boundaries as well as the mathematical analysis of the courses of the transients support this assumption. Different models for the formation of the molecular ions on the terraces are discussed.