The Study of Mean Residence Time of Alkali Metal on Tungsten Surface by Means of Molecular Beam-An Attempt of Analysis of Experimental Values

Abstract

Mean residence time of potassium atoms on polycrystalline tungsten filament surface has been studied. Beam pulse shorter than those of the measurements reported previously were used to improve time resolution. In order to measure the delay time of desorbed ions with an acceptable signal-to-noise ratio, sampling method was employed. From the dynamics of desorption of potassium atoms and ions, taking into consideration the shape of the gate function which has seldom been considered, the following formulas are obtained : G (p) /F (p) = (K+1/K) τ+p+ (1/β'+1) =θp+1/β : G (p) =α∫ ∞ 0 e-ptg (t) dt, F (p) =∫ ∞ 0 e-pt f (t) dt, where p is a parameter of Laplace transformation. G (t) and f (t) are gate function and intensity function of desorbed ions, respectively. The quantity a can be determined from the experiment. K is an equilibrium constant between the transient adatoms and adions : M (s) _??_ M+ (s) + e-β' is the degree of ionization and is considered to be constant under the same surface condition and temperature. A relation τ=βθ is introduced to make a comparison with the values in published papers. Scheer et al. estimated β by Saha-Langmuir equation using value of the work function of metals (Φ+) and the ionization potential (I) of alkali atoms, whereas β can be determined directly from experimental data. The temperature dependence of residence time of adions will be expressed by Frenkel equation of the formτ = τ0exp (l/kT).From this equation desorption energy (l) and the corresponding pre-exponential factor (τ0) are found to be 1.85 eV and 2.2×10-12 sec, respectively. These values are in good agreement with those of potassium ions on contaminated tungsten surface published by other authors.