General applicability of our empirical formulae expressing the threshold temperature range for dissociative positive ionization of halide molecules on heated metal surfaces

The positive ionization efficiency of alkali halide (MX) incident upon Mo or Ta heated in a high vacuum (∼ 0.2 μTorr) always attained an appreciable value (∼ 0.01) at the appearance temperature (T0) and then steeply increased up to unity at the saturation temperature (T1). The data on the threshold temperature range (T0–T1) were analyzed to find the quantitative relation between each boundary temperature and the main factors governing it, thereby yielding the empirical formulae of T0 = (D0 + I0 − φ0+)R0k and T1 = (D1 + I1 − φ1+)R1k. Here, D, I and φ+ are the dissociation energy of MX, the ionization energy of M and the effective work function for the ionization on each surface, respectively, in eV at T0 or T1 in K, and k is Boltzmannʹs constant. The empirical constants (R0 = 42.1 ± 1.0 and R1 = 28.2 ± 1.1) thus determined are essentially equal to our previous values (41.7 ± 2.2 and 29.1 ± 1.4 for MX/Re and 41.5 ± 2.0 and 28.7 ± 1.1 for MX/W) and also present ones (39.5 ± 2.9 and 27.8 ± 1.1 for TlX/Re, InI/W, etc.) determined in high vacua (∼ 0.2–20 μTorr). These results indicate that our empirical formulae expressing the range are generally applicable to any diatomic halide molecule/metal surface system and also that the most probable values of R0 and R1 are 41.3 ± 2.2 and 28.4 ± 1.3, respectively.