Cage formation of N-2 under H2O overlayer on Ru(001)

Water molecules were found to interact strongly with N-2 when coadsorbed on Ru(001). This system was studied by TPD and work function change measurements. It was found that water molecules repel preadsorbed nitrogen at low water coverages (H2O/Ru < 0.3). This is indicated by a shift of the thermal desorption peak to lower temperatures and by displacement of 20% of the saturation nitrogen coverage. At higher water coverages, a unique new N-2 desorption peak gradually emerges and shifts from 105 to 167 K as water coverage increases to 9 bilayers. The extremely narrow molecular nitrogen TPD peak is indicative of the formation of a cage of hydrogen bonded water on top of trapped N-2 molecules. Work function change measurements taken during adsorption and subsequently during sample heating in a Delta phi-TPD mode suggest that the caged nitrogen molecules remain practically unperturbed inside the cage. Site-exchange model between nitrogen molecules escaping from the cage and water molecules which takes place upon sample heating, qualitatively explains the Delta phi-TPD spectra. Deviations of the calculated curves based on this model from the observed data suggest structural rearrangements of the cage during the heating process.