Main factors decisive of the threshold temperature range causing a sharp change in thermal positive ionization efficiency of alkali halide molecules incident upon a rhenium surface

Abstract

The thermal positive ionization efficiency (β+) of sodium or lithium halide molecules (MX) incident upon a polycrystalline rhenium surface attained an appreciable value (ca. 0.01) at the appearance temperature (T0) and reached essentially unity at the saturation temperature (T1). The threshold temperature range (T0−T1) which caused a sharp change in β+ depended on the species of MX, the incident sample beam flux (N) and the residual gas pressure (Pr) around the surface. Analysis of experimental data leads to the conclusion that (1) the main factors governing the range are the dissociation energy (D) of MX, the ionization energy (I) of M and the effective work function (φ+) of the surface, (2) φ+ depends upon N and Pr and hence upon T, and (3) T0 and T1 are given by the following empirical formulae (in degrees Kelvin and electron-volts), which hold for the ranges N ≈ 1012–1014 molecules cm−2 s−1 and Pr ≈ 0.02–2 mPa studied in this work: T0 = (D0 + I0 - φ+0)/(41.7 ± 2.2) k T1 = (D1 + I1 - φ+1)/(29.1 ± 1.4) k where D0 and D1, for example, represent the values of D at T0 and T1, respectively, and k is Boltzmann's constant. The formulae can be used to calculate φ+0 and φ+1 from experimental data on T0 and T1, respectively, for a given condition, as D and I at the relevant temperatures are available from thermochemical tables.