Thermal positive ion production from thallium chloride molecules impinging upon a tungsten surface heated in high vacua

To study the effect of various conditions upon the positive ionization efficiency (β+) of TlCl on W and its effective work function (Φ+) during the ionization, the current of Tl+ was measured as a function of either surface temperature (T), incident sample beam flux, or residual gas pressure. Theoretical analysis of the data thus obtained indicates that (1)β+ is less than 0.1 both above ∼ 1500 K and below ∼ 1000 K, (2) asT increases from ∼ 1350 K, β+ decreases from ∼ 1 to typically ∼ 0.01 at ∼ 1600 K becauseΦ+ is decreased from ∼ 7 to 6 eV mainly by oxygen desorption, (3) the patterns ofT versusβ+ and versusΦ+ are essentially the same as those found previously with alkali halide/W systems, and (4) the vapor pressures measured with TlCl at several Knudsen cell temperatures (∼ 550–600 K) according toβ+ = 1 atT ≈ 1300K well agree with literature values determined by quite different methods, thus confirming the validity of our experimental method and theoretical model of the ionization.