Influence of Electronic and Geometric Structure on Desorption Kinetics of Isoelectronic Polar Molecules: NH3 and H2O

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Consideration of the thermal desorption kinetics for isoelectronic NH3 and H2O from certain metal surfaces reveals an interesting contrast, which relates to geometrical and electronic structural effects in the adsorbed layers. Both NH3 and H2O are polar molecules bonded to the surface via lone pair orbitals on N and O, respectively. Hydrogen - bonding attractive interactions between neighboring H2O molecules lead to formation of 2-d and 3-d clusters; thermal desorption kinetics of H2O are characterized by sharp desorption peaks over narrow temperature ranges [the full width at half maximum (fwhm) is ΔT<10K in some cases]. In distinction, lateral interactions between neighboring NH3 molecules are largely repulsive (dipole-dipole interactions) and the thermal desorption spectra are considerably broader in temperature than for H20 (fwhm ΔT~ 7 OK to 15OK, depending on substrate). In the following paragraphs, we summarize results obtained in our laboratory during the last few years which illustrate these points for desorption of NH3 and H2O from metal surfaces, including Ni(111), Ru(0001) and Ag(110). In each of these cases, the adsorption of NH3 and H2O is molecular, and desorption proceeds without dissociation.