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

Abstract

When the surface temperature ( T) of catalytic metal (Re, W, Mo, Ta or Nb) was gradually increased in a high vacuum (∼10 −5–10 −3 Pa), the positive ionization efficiency ( β +) of diatomic halide molecule YX (e.g., LiI, KF or TlCl) impinging with a constant flux (∼10 12–10 14 molecules cm −2 s −1) upon the surface steeply increased up to unity at the first boundary temperature ( T 1). Above T 1, β +=1 continued until the second boundary temperature ( T 2), above which β + decreased as T increased. Theoretical analysis of β + around the optimum temperature range ( T 1− T 2) yields the empirical formulae of T 1=( D 1+ I 1− φ 1 +)/ R 1 k and T 2=( φ 2 +− I 2)/ R 2 k. Here, D, I, and φ + 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 T 1 or T 2 in K,and k is Boltzmann's constant. The empirical constants ( R 1=28.4±1.3 and R 2=5.47±0.30) thus determined always hold for any system, irrespective of the difference in species of both sample and catalyst.