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 (beta(+)) of TICl on W and its effective work function (ph i(+)) during the ionization, the current of Tl+ was measured as a func tion of either surface temperature (T), incident sample beam flux, or residual gas pressure. Theoretical analysis of the data thus obtained indicates that (1) beta(+) is less than 0.1 both above similar to 1500 K and below similar to 1000 K, (2) as T increases from similar to 135 0 K, beta(+) decreases from similar to 1 to typically similar to 0.01 at similar to 1600 K because phi(+) is decreased from similar to 7 to 6 eV mainly by oxygen desorption, (3) the patterns of T versus beta(+) and versus phi(+) are essentially the same as those found previously with alkali halide/W systems, and (4) the vapor pressures measured wit h TlCl at several Knudsen cell temperatures (similar to 550-600 K) acc ording to beta(+) = 1 at T approximate to 1300 K well agree with liter ature values determined by quite different methods, thus confirming th e validity of our experimental method and theoretical model of the ion ization.