Reactivity and structure of CF3I on Ru(001)
A variety of surface-sensitive techniques are used to elucidate the reaction pathways, as well as adsorbate structures, associated with thermal activation of CF{sub 3}J following adsorption on Ru(001) at 100 K. XPS shows that the C-I bond of CF{sub 3}I dissociates below 200 K to form CF{sub 3}(ad) and I(ad); the subsequent reactions of CF{sub 2} are best viewed as being regulated by the availability of surface sites. CF{sub 3}(ad) dissociates to CF{sub 2}(ad) below 200 K. Further CF{sub 3} dissociation, some of which is activated by H(ad), occurs between 200 and 400 K until all available sites are filled. Desorption of the remaining CF{sub 3}, peaking at 705 K, once again opens surface sites for decomposition. This is followed by recombination of the products to form CF{sub 3}(g). No evidence for CF(ad) is ever observed. Hydrogen coadsorption studies explain interesting features associated with fluorine evolution. HREELS and ESDIAD results indicate that CF{sub 3} adopts a tilted configuration on Ru(001). 53 refs., 11 figs., 3 tabs.
Desorption induced by electronic transitions: Some recent progress
Abstract In analogy to electron impact or photon-induced dissociation of molecules, electronic excitation of adsorbate complexes leads to desorption of ions and neutrals. Both processes require a primary excitation to a repulsive potential curve, by which conversion of electronic excitation to nuclear motion can occur. However, in the surface case strong competition with the desorptive process occurs by transfer of the excitation away from its primary location, because of strong coupling to the many modes of surface layer and substrate; this leads to very strong dependence of cross sections on bonding mode, excitation energy, isotope mass, and coverage. Recently, considerable progress has been made in the understanding of primary, secondary, and tertiary processes by detailed investigations of energy and polarization dependencies of cross sections, and of angle and energy distributions of products, for primary valence as well as core excitations, and for neutral and ionic products. The importance of localization of the excitation and its dependence on many-body interactions becomes obvious. These aspects are reviewed, and some applications sketched.
The structure of CO on Ni(111)
Electron stimulated desorption ion angular distributions (ESDIAD), low energy electron diffraction(LEED), and temperature programmed thermal desorption(TPD) have been used to study the adsorption of CO on Ni(111) in the temperature range 80−300 K. For low coverages, the CO layer is disordered; a c(4×2) pattern appears at coverages ϑ∼0.5, the maximum coverage at 300 K. At temperatures 220–240 K, a well‐ordered (√7/2×√7/2)R 19° LEED pattern forms at saturation (ϑ∼0.57). At 80 K, the CO saturation layer is characterized by a ’’complex’’ LEED pattern. Only one binding state is seen in TPD for ϑ≲0.40 (peak temperatures 450–430 K); species having lower desorption temperatures populate at higher coverage. At 300 K adsorption the only ESD ion observed is O+, with desorption centered about the direction perpendicular to the surface. The O+ ion yield shows a maximum at intermediate coverages. CO+ ions are also observed at adsorption temperature < 300 K at higher coverages. The ESDIAD patterns for saturation coverage in the range 80–260 K indicate off‐normal ion emission in addition to the normal component. The data suggest that for ϑ≲ 0.5, CO is adsorbed in multiply coordinated sites with the molecular axis perpendicular to the surface. At temperatures < 300 K, a fraction of CO can adsorb in singly coordinated sites as well. Upon heating the CO layer formed at 80 K (ϑ?0.57) the CO+ ion yield maximizes at ∼240 K when the (√7/2×√7/2)R 19° LEED pattern is well defined. The CO+ yield therefore reflects the ordering behavior in the adsorbate layer at temperatures below the onset of desorption and indicates a shift of a fraction of CO molecules from multiply coordinated to singly coordinated sites. Finally, a model is proposed involving resonance charge exchange between adsorbed CO molecules to account for the observed variations of the O+ yield with surface coverage.