Effect of constituent halogens upon the thermal alkali ion production from alkali halides on tungsten

To clarify the effect of halogen (X) upon the production of ion (M‡) from alkali halide (MX) on a tungsten surface heated in vacuum, the ionization ef®ciency (b‡) of MX was measured as a function of surface temperature (T), and the effective work function (f‡) of the surface was determined, thus yielding the conclusions: (1) above 1950 K, the surface was kept virtually clean at f‡ ˆ 5:2 eV; (2) as T decreased from 1950 to 1400 K, b‡ and f‡ were increased to unity and above 6 eV, respectively, mainly by adsorption of neither MX nor X but of residual gases (RG) and (3) below 1300 K, each of b‡ and f‡ showed different patterns among MXʹs because of coadsorption of MX and RG. The work function (fe) for thermal electron emission exhibited the same temperature dependence as that of f‡ and f‡ ÿ fe was constant at 0.7 eV above 1600 K, supporting the above conclusions. # 2001 Elsevier Science B.V. All rights reserved.