Influence of the flux of H2 and D2 molecules on kinetics of low-temperature (down to 5 K) adsorption on the W(1 1 0) surface

The molecular beam method is applied to study the kinetics of H2 and D2 adsorption on the W(1 1 0) surface with the substrate temperature TS∼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 H2 and D2 adsorption. H2 case, the increase of F from 5×1012 to 2×1014 molecules/cm2 s modifies the coverage dependence of the sticking probability S(θ) both qualitatively and quantitatively, i.e., (i) the initial sticking probability S0 increases more than by a factor of two; (ii) the saturation coverage θS 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. case of D2 adsorption, the changes of F in approximately the same range produce relatively weak effect on the adsorption kinetics. In particular, both S0 and θS are changed insignificantly. crease of S0 with rising F is assumed to be caused by the interaction of H2 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 S0. The growth of θS with increasing F is associated with the instability, for TS∼5 K, of the physisorbed H2 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 H2 molecules and the surface under the formation of a weakly bound molecular adlayer. vealed distinctions in the character of the molecular flux influence on the kinetics of H2 and D2 adsorption for TS∼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 D2 molecule.