Thermal positive-ionic and electronic emissions from iridium heated in vacua

To clarify the thermionic property of a polycrystalline iridium filament surface heated in vacua (10^-5-10^-3 Pa), the emission current (I⁺) of positive ion (M⁺) produced from alkali halide molecule (MX) impinging upon the filament was measured as a function of surface temperature (T), incident sample beam flux (N) or residual gas pressure (P_r). The current of thermal electron (e^-) was also measured under the same conditions. Theoretical analysis of the experimental data thus obtained yields the following conclusions: (1) in a low-temperature range (T₁⩽1200 K), the ionization efficiency (e^-) of MX decreases steeply with a decrease in T because the work function (φ⁺) effective for the ionization is decreased by adsorption of MX; (2) in a middle temperature range (T₁-T₂≈1200-1300 K), β⁺=1 is attained, thereby yielding I⁺≈10^-5 A/cm² when N is 10¹⁴ molecules/cm² s; (3) in a high temperature range (T₃⩾1500 K), the surface is kept virtually clean, and φ⁺ is constant at 5.73±0.03 eV while the work function (φ^e) effective for emitting e remains at 5.15±0.03 eV; (4) as T decreases from T₃ to T₂, both φ⁺ and φ^e are increased by up to ~0.5 eV; (5) as P_r increases, T₂ and T₃ increase while T₁ decreases, indicating that φ⁺ is increased by adsorption of residual gases (especially of oxygen); (6) the thermionic contrast (φ⁺ -φ^e) is kept constant at 0.57±0.03 eV without depending upon T, N and P_r; and (7) Ir is useful for effectively ionizing those elements whose ionization energy is less than ~6 eV