THERMAL ION EMISSION FROM POTASSIUM FLUORIDE DEPOSITED ON A SUBSTRATE-METAL SURFACE

Thermal emission rates of K+ and F- ions from thick potassium fluoride layers on a metal substrate were measured alternately as a function of the sample temperature (670-990 K) or the extracting voltage (0-500 V) in ultra-high vacuum (approximately 3 x 10(-9) Torr). In the course of an induction period, a gradual decrease of isothermal ion currents with time and ion accumulation at the sample surface during application of the retarding electrical field against either ions were observed prior to attaining a steady-state ion emission. The results thus obtained were discussed in light of (1) the terrace-ledge-kink model of vaporization, (2) the morphological development on alkali halide surface and (3) the concept of surface charge in ionic crystals. The activation energies for the desorption of K+ and F- ions, changing from about 2.6 to 2.2 eV and from 2.2 to 2.8 eV, respectively, with increasing temperature, were derived from the respective slopes of Arrhenius plots. The ion emission rates were analyzed on the basis of a theoretically deduced equation of ion emission, thereby elucidating the influence of the surface electrical potential and the relative importance of surface kinks to the ion emission. The non-stoichiometry in kink-site occupancy by cations and anions, favoring the emission of K+, was demonstrated to be of defect-related origin.