Measurements of Mean Residence Time of Potassium on Clean Tungsten Surfaces by Means of Molecular Beams

Abstract

Mean residence time(τ)of potassium on atomically clean and gas-covered tungsten surfaces has been studied. Beam pulses shorter than those of previously reported measurements were used to improve the time resolution. Taking into consideration the shape of the gate function, which has seldom been considered, the following formulas were obtained: G(p)/F(p)=k+1/Kτ+ p+(1+1/β')=θp+1/β G(p)=a∫∞0ep1g(1)dt. F(p)=∫∞0ep1f(t)dt. where p is a parameter of the Laplace transfbrmation, g(t)and f(t)are the gate function and the intensity function of potassium ions, respectively. The quantity a can be determined from the experiment. K is an equilibrium constantbetween the transient adatoms and adions M(s)↔M+(s)+c. where s represents the surface. The quantity β, the ionization coefficient, is considered to be constant under the same surface condition and temperature. In order to make a comparison with the published values, the relationτ=βθ was introduced. From the Arrhenius type expressions of the from τ=τ0 exp(l/kT)andτ0+ exp(l+/kT), ignoring the temperature change of β, the desorption energies(l+) were found to be 2.51 eV for the clean tungsten surface and 1.63 eV for the strongly gas covered tungsten surface. Considering β, the desorption energies(l)were fbund to be 2.30 eV for the clean tungsten surface and 1.85 eV for the strongly gas-covered tungsten surface. The corresponding pre-exponential factors(τ0)were estimated to be 3.24×10-14 sec and 2.2×10-12 sec, respectively.