A Novel Form for Adsorbed Water: Changing the Overlayer Structure by Preadsorbing Different Quantities of Oxygen on Rh(111)


    Water adsorbed on clean Rh(111) forms an ordered (31/2 x 31/2)R30° bilayer (BL) structure. The preadsorption of small quantites of disordered oxygen improves the long-range ordering of the water overlayer. When a well-ordered half-monolayer of oxygen is grown on the Rh(111) prior to water exposure, there is no evidence of any long-range ordering of the water. However, when water is adsorbed on a (1x1)-O/Rh(111) surface, where there is a well-ordered monolayer (ML) of adsorbed oxygen, the adsorbed water forms an entirely new structure exhibiting a (1x1) diffraction pattern.


Figure 1 shows surface diffraction spectra along two principle symmetry directions of the Rh(111), taken with elastic He scattering. When the Rh(111) is predosed with only ~0.02 ML of randomly adsorbed oxygen, the spectra are what is expected for a (31/2 x 31/2)R30° structure for the adsorbed water. When an ordered 1 ML of oxygen is grown before exposure to water, the diffraction pattern is (1x1), with some small extra superlattice features.
Figure 2 shows some temperature-programmed thermal desorption (TPD) spectra, where the signal of desorbing water is monitored as a function of surface temperature, which was ramped at 200 K/sec. Figure 2a shows spectra for two amounts of adsorbed water on Rh(111). The small high temperature peak is due to ~0.02 ML of preadsorbed oxygen. Figure 2b shows the TPD spectrum when an ordered (1x1)-O overlayer is adsorbed before water exposure. 
Proposed p(2x1)-H2O/(1x1)-O/Rh(111) structure.  The (2x1) unit cell and underlying crystal directions are indicated. Water is shown in ball and stick form to indicate bond directions.



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