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, 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 notable results and conclusions. 1) In a low temperature range (T₁ ⩽ 1200 K), the ionization efficiency (β⁺) 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/cm ² 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, clearly 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. 7) Ir is useful for effectively ionizing those elements whose ionization energy is less than ~6 eV