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 (absolut e coverage theta(a) approximate to 2/3) was investigated by thermal de sorption, LEED, and workfunction measurements. In addition to the alre ady known isotope effect in the desorption behaviour [Schmitz et al., Surf. Sci. 186 (1987) 219] which we essentially reproduce, we have als o established isotope effects in adsorption and reaction. We have foun d different dissociation probabilities and different types of long ran ge 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 su rface, the H2O layer is an ordered domain structure consisting of p(ro ot 3 x root 3) stripes approximately 6.5 lattice constants wide. Annea ling the sample to 174 K essentially desorbs the D2O bilayer, while H2 O forms a second domain structure with stripes of similar width and lo cal structure, but now separated by areas of uncovered Ru surface. Str ucture models based on the available experimental data are given for b oth domain structures. The most likely explanation for the structural isotope effect at 150 K is based on the positive Ubbelohde effect whic h 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 i sotopes is reinterpreted.