HELPFUL EFFECT OF RESIDUAL GASES UPON THE THERMAL POSITIVE-ION EMISSION FROM METAL-SURFACES HEATED IN HIGH VACUA

In readily attainable high vacua (P approximate to 10(-5)-10(-4) Pa), the efficiency (beta(+)) for producing positive alkali ions (M(+)) fro m alkali halide molecules (MX) impinging with a constant beam flux (N) upon a surface (Re, W, Mo, or Ta) was measured as a function of surfa ce temperature (T approximate to 800-2300 K), thus yielding that (1) b eta(+) = 1 is attained only in an intermediate temperature range (T-1- T-2), (2) beta(+) sharply decreases as T decreases from T-1 or as T in creases from T-2 up to another boundary temperature (T-3), (3) above T -3 beta(+) has little dependence upon T, (4) both T-2 and T-3 are near ly independent of N but become higher as P increases, and (5) T-1 beco mes higher as N increases. Theoretical analysis of these data reveals that the residual gases (RG) act decisively as a secret promoter of th e ionization. Specifically, the increase in beta(+) up to unity with a temperature decrease from T-3 to T-2 is caused by a gradual increase in the effective work function for the ionization owing to heavier ads orption of neither MX nor X, but of RG.