Transient behavior of the thermal ion emission from KCl(0 0 1) upon polarity reversal of the electric extraction field

The technique of polarity reversal of the external electric extraction field (strength: 102 V/cm) was applied to study the relaxation of the thermal ion emission from the KCl(0 0 1) single crystal surface. Transient currents of the K+ and K2Cl+ ions upon switching from the emission suppression to the ion extraction mode were recorded as a function of the evaporation time, the temperature, and the time of field reversal. The temperature dependence of the time constants of the K+ ions obtained from the exponential decreases of the emission currents to their steady-state emission resulted as logτh(s)=−(13.39±0.56)+(12.42±0.49)103/T in a high temperature interval of 826–930 K after a prolonged heating period and as logτl(s)=−(20.65±1.04)+(16.77±0.81)103/T in a low temperature interval of 750–801 K at the initial stage of evaporation, with corresponding activation energies of Eh(K+)=2.47±0.14 eV and El(K+)=3.32±0.16 eV, respectively. The transient currents can be interpreted by a partial adsorption of the suppressed ion currents at the kinks of the surface steps. The differences in the high- and low-temperature runs may be attributed to a strong coarsening of the surface at higher temperatures, which occurs as a bunching of monosteps to macrosteps and/or to an enrichment and segregation of divalent impurities at the surface. The transient behavior of the molecular K2Cl+ ions seems to be strongly correlated with that of the K+ ions. This correlation is possibly caused by changes of the strength or the sign of the local electrical field connected with the excess charge at the kinks.