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

• Published in: Applied surface science 1999-04, Vol.144, p.404-408
• Main Authors: Kawano, Hiroyuki, Ohgami, Katsushi, Matsui, Seiji, Zhu, Yongfa
• Format: Article
• Language: English
• Subjects: Catalysis; Catalysts: preparations and properties; Chemistry; Diatomic metal halide beams; Exact sciences and technology; General and physical chemistry; Optimum temperature range; Refractory metals; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; Thermal positive ionization efficiency; Work function change
• ISSN: 0169-4332; 1873-5584
• DOI: 10.1016/S0169-4332(98)00829-0

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.