ISOTOPE EFFECTS IN STRUCTURE AND KINETICS OF WATER ADSORBATES ON RU(001)

The adsorption of H2O and D2O on Ru(001) in the bilayer range (absolute coverage theta(a) approximate to 2/3) was investigated by thermal desorption, LEED, and workfunction measurements. In addition to the already known isotope effect in the desorption behaviour [Schmitz et al., Surf. Sci. 186 (1987) 219] which we essentially reproduce, we have also established isotope effects in adsorption and reaction. We have found different dissociation probabilities and different types of long range order for D2O and H2O in LEED for the saturated bilayer produced at 150 K. While D2O forms an extended p(root 3 x root 3) layer on the surface, the H2O layer is an ordered domain structure consisting of p(root 3 x root 3) stripes approximately 6.5 lattice constants wide. Annealing the sample to 174 K essentially desorbs the D2O bilayer, while H2O forms a second domain structure with stripes of similar width and local structure, but now separated by areas of uncovered Ru surface. Structure models based on the available experimental data are given for both domain structures. The most likely explanation for the structural isotope effect at 150 K is based on the positive Ubbelohde effect which consists in an elongation of hydrogen bonds upon deuteration: while bulk ice does not show any Ubbelohde effect, it is induced here by the compression of the bilayer under the action of the adsorptive forces. On the basis of these findings, the desorption behaviour of the two isotopes is reinterpreted.