Mean Adsorption Lifetimes and Activation Energies of Silver and Gold on Clean, Oxygenated, and Carburized Tungsten Surfaces

Abstract

Direct measurements of the mean adsorption lifetimes and the activation energies of silver and gold have been made on a polycrystalline tungsten substrate. The lifetime was determined by the time constant of the exponential increase of the surface concentration when an atomic beam impinged on the substrate. The activation energy was determined by the slope of the logarithm of the lifetime as a function of the inverse substrate temperature. The adsorption lifetime of atoms on a metal surface is τ=(1/ν0) exp (E/kT), where ν0 is the vibrational frequency and E is the activation energy. The experiments were conducted in an ultrahigh-vacuum system free of hydrocarbon contamination. A clean substrate surface was obtained by heating in a partial pressure of oxygen for a long period of time and flashing to 2500°K before each data point was taken. The well-defined surface condition and the precisely known temperature allow reliable interpretation of the experimental results. The experimental results for silver and gold on clean, oxygenated, and carburized tungsten substrates when τ is in seconds and E is in electron volts are the following: Ag on W, τ=(1/3.6×1012) exp (2.9/kT); Ag on O–W, τ=(1/1.43×1012) exp (2.06/kT); Ag on C–W, τ=(1/8.2×1012) exp (3.18/kT); Au on W, τ=(1/8.2×1013) exp (4.54/kT); Au on O–W, τ=(1/4.7×1011) exp (1.99/kT); Au on C–W, τ=(1/1.73×1014) exp (4.725/kT).