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/sup +/) of positive ion (M/sup +/) 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/sub r/). The current of thermal electron (e/sup -/) 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 (T/sub 1/ /spl les/ 1200 K), the ionization efficiency (/spl beta//sup +/) of MX decreases steeply with a decrease in T because the work function (/spl phi//sup +/) effective for the ionization is decreased by adsorption of MX. 2) In a middle temperature range (T/sub 1/-T/sub 2/ 1200-1300 K), /spl beta//sup +/=1 is attained, thereby yielding I/sup +/ /spl ap/ 10/sup -5/ A/cm/sup 2/ when N is 10/sup 14/ molecules/cm/sup 2/ s. 3) In a high temperature range (T/sub 3/ > 1500 K), the surface is kept virtually clean, and /spl phi//sup +/ is constant at 5.73/spl plusmn/0.03 eV while the work function (/spl phi//sup e/) effective for emitting e/sup -/ remains at 5.15/spl plusmn/0.03 eV. 4) As T decreases from T/sub 3/ to T/sub 2/, both /spl phi//sup +/ and /spl phi//sup e/ are increased by up to /spl sim/0.5 eV. 5) As P/sub r/ increases, T/sub 2/ and T/sub 3/ increase while T/sub 1/ decreases, clearly indicating that /spl phi//sup +/ is increased by adsorption of residual gases (especially of oxygen). 6) The thermionic contrast (/spl phi//sup +/-/spl phi//sup e/) is kept constant at 0.57 /spl plusmn/ 0.03 eV without depending upon T, N and P/sub r/. 7) Ir is useful for effectively ionizing those elements whose ionization energy is less than /spl sim/6 eV.