Empirical formulae of the threshold temperature range for dissociative positive ionization of alkali halides on heated metal surfaces

The ionization efficiency (β+) of alkali halide molecules (MX) incident upon W or Nb heated in a high vacuum (∼0.2 μTorr) always attained an appreciable value (∼0.01) at the appearance temperature (T0) and steeply increased up to unity at the saturation temperature (T1). To find the main factors governing the threshold temperature range (T0–T1), the experimental data onβ+were analyzed by our theory on dissociative positive surface ionization (DPSI), yielding withMX/Wsystems that T0 =(D0 + I0 − Φ0+)/(41.5 ± 2.0)kandT1 =(D1 + I1 − Φ1+)/(28.7 ± 1.1)k. Here, D, I, andΦ+are the dissociation energy of MX, the ionization energy of M and the effective work function for DPSI, respectively, in eV at T0 or T1in K, and k is Boltzmannʹs constant. The empirical constants are essentially equal to the values (41.1 ± 1.0 and 27.6 ± 0.9) obtained with MX/Nb systems and also to those (41.7 ± 2.2 and 29.1 ± 1.4) found previously with MX/Re systems at ∼0.2-2.0 μTorr, indicating that our empirical formulae expressing the range generally hold with any MX/metal system.