INFLUENCE OF SURFACE-CHARGE ON THERMAL POSITIVE-ION EMISSION FROM POTASSIUM-BROMIDE

The thermionic current (I+) of K+ emitted from thick potassium bromide layers on a metal substrate was measured as a function of the sample temperature (T similar or equal to 700-1000 K) or the extracting voltage (U similar or equal to 0-500 V) in ultra-high vacuum (similar to 3 x 10(-9) Torr). Each plot of In I+ versus 1/T achieved at U = 500 V revealed the existence of a break point at which the slightly curved line was divided into the two almost straight portions. The activation energies (E(des)(+)) for the desorption of K+ were derived to be 2.9-3.1 and 2.1-2.5 eV from the respective slopes of the portions below and above the break-point temperature (T-b). The origin of T-b was elucidated on the basis of defect theory. The larger values of E(des)(+) below T-b are due to the negative surface potential characteristic for the extrinsic temperature range of defect formation, whereas the smaller values of E(des)(+) above T-b are due to the positive surface potential inherent in the intrinsic temperature range of alkali-halide crystals. In the case of a solid solution of KBr-CaBr2 (1000 ppm), such T-b was not observed, yielding E(des)(+) similar or equal to 2.4 eV with a poorer reproducibility.