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 (T700–1000 K) or the extracting voltage (U0–500 V) in ultra-high vacuum (∼ 3 × 10−9 Torr). Each plot of ln 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 (Tb). The origin of Tb was elucidated on the basis of defect theory. The larger values of E+des below Tb are due to the negative surface potential characteristic for the extrinsic temperature range of defect formation, whereas the smaller values of E+des above Tb 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 Tb was not observed, yielding E+des2.4 eV with a poorer reproducibility.