The trapping of K and Na atoms by a clean W(110) surface trajectory calculations

The fraction of K and Na atoms initially trapped by the W(110) surface has been measured as a function of the incident energy (0.5–15 eV) and as a function of the incident angle. The trapping probability equals one at low incident energies (E_i ? 0.5 eV) and decreases with increasing energy. The measurements show an increase of trapping with increasing angle of incidence θ_i (measured from the surface normal). Simultaneously the desorption energies Q_i were determined from the temperature dependence of the measured mean residence time on the W(110) surface. We obtained for K: Q_i = 2.05 ± 0.02 eV, and for Na: Q_i = 2.60 ± 0.04 eV.The trapping phenomenon at a solid surface was approximated in a theoretical way by calculating the in-plane trajectory of a projectile scattered from a diatomic surface-molecule. The important feature which showed up was the conversion of tangential to normal momentum of the projectile, and thus the inapplicability of cube models. As a function of the angle of incidence two regimes can be distinguished: at the smaller angles the scattering is governed by simultaneous interaction of the projectile with two neighbouring surface atoms, and at the higher angles of incidence the single particle interaction contributes most to the momentum transfer.