Influence of surface charge on thermal positive ion emission from potassium bromide

Abstract

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 ( 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-CaBr 2 (1000 ppm), such T b was not observed, yielding E + des2.4 eV with a poorer reproducibility.