Optimum temperature range for positive ion production from metal halide molecules incident upon heated metal catalysts

When the surface temperature T. of catalytic metal Re, W, Mo, Ta or Nb. was gradually increased in a high vacuum ;10y5–10y3 Pa., the positive ionization efficiency bq.of diatomic halide molecule YX e.g., LiI, KF or TlCl. impinging with a constant flux ;1012–1014 molecules cmy2 sy1. upon the surface steeply increased up to unity at the first boundary temperature T1.. Above T1, bqs1 continued until the second boundary temperature T2., above which bq decreased as T increased. Theoretical analysis of bq around the optimum temperature range T1yT2.yields the empirical formulae of T1s D1qI1yfq1.rR1k and T2s fq2yI2.rR2k. Here, D, I, and fq are the dissociation enthalpy of YX, the ionization enthalpy of Y and the effective work function for the ionization on each metal surface employed, respectively, in eV at T1or T2in K,and k is Boltzmann’s constant. The empirical constants R1s28.4"1.3 and R2s5.47"0.30.thus determined always hold for any system, irrespective of the difference in species of both sample and catalyst. q1999 Elsevier Science B.V. All rights reserved