Trapping probabilities and desorption kinetics of alkali atoms on a clean and an oxygen covered tungsten (110) surface

Abstract

Abstract Alkali atoms were scattered with hyperthermal energies from a clean and an oxygen covered (θ ≈ 0.5 ML) W(110) surface. The trapping probability of K and Na atoms on oxygen covered W(110) has been measured as a function of incoming energy (0–30 eV) and incident angle. A considerable enhancement of trapping on the oxygen covered surface compared to a clean surface was observed. At energies above 25 eV there are still K and Na atoms being trapped by the oxygen covered surface. From the temperature dependence of the mean residence time τ of the initially trapped atoms the pre-exponential factor τ0 and the desorption energy Q were derived using the relation: τ = τ 0 exp ( Q kT s ) . On clean W(110) we obtained for Li: τ 0 = (8 ± 8 4 ) × 10 −14 sec , Q = (2.78 ± 0.09) eV; for Na: τ0 = (9 ± 3) × 10−14 sec, Q = (2.55 ± 0.04) eV; and for K: τ0 = (4 ± 1) × 10−13 sec, Q = (2.05 ± 0.02) eV. Oxygen covered W(110) gave for Na: τ0 = (7 ±3) × 10−15 sec, Q = (2.88 ± 0.05) eV; and for K: τ 0 = (1.3 ± 0.9 0.6 ) × 10 −14 sec , Q = (2.48 ±0.05) eV. The adsorption on clean W(110) has the features of a supermobile two-dimentional gas; on the oxygen covered W(110) adsorbed atoms have the partition function of a one-dimen-sional gas. The binding of the adatoms to the surface has a highly ionic character in the systems of the present experiment. An estimate is given for the screening length of the non-perfect conductor W(110):ks−1≈ 0.5 A.