Influence of the flux of H 2 and D 2 molecules on kinetics of low-temperature (down to 5 K) adsorption on the W(1 1 0) surface

Abstract

The molecular beam method is applied to study the kinetics of H 2 and D 2 adsorption on the W(1 1 0) surface with the substrate temperature T S ∼5 K as a function of the molecular flux F. The character of the effect of flux changes on the adsorption kinetics is essentially different in the cases of H 2 and D 2 adsorption. In the H 2 case, the increase of F from 5×10 12 to 2×10 14 molecules/cm 2 s modifies the coverage dependence of the sticking probability S( θ) both qualitatively and quantitatively, i.e., (i) the initial sticking probability S 0 increases more than by a factor of two; (ii) the saturation coverage θ S considerably increases; (iii) the monotonic (for the lowest F) dependence S( θ) transforms into a dependence with a maximum whose height grows as F increases. In the case of D 2 adsorption, the changes of F in approximately the same range produce relatively weak effect on the adsorption kinetics. In particular, both S 0 and θ S are changed insignificantly. The increase of S 0 with rising F is assumed to be caused by the interaction of H 2 molecules in the intrinsic precursor state which leads to the nucleation of the 2D condensed phase. The greater F, the higher the nucleation probability. The 2D condensation suppresses the thermodesorption from the precursor state and thus leads to the increase of S 0. The growth of θ S with increasing F is associated with the instability, for T S ∼5 K , of the physisorbed H 2 layer whose dynamically equilibrium coverage increases as F rises. The appearance of a maximum in the S( θ) dependence can be explained by greater efficiency of the energy exchange between the incident H 2 molecules and the surface under the formation of a weakly bound molecular adlayer. The revealed distinctions in the character of the molecular flux influence on the kinetics of H 2 and D 2 adsorption for T S ∼5 K are associated with differing quantum properties of these molecules, namely, by the deeper position of the zero-point vibrational level for the heavier D 2 molecule.