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We have utilized the dehydrogenation and hydrogenation of cyclohexene as probe reactions to compare the chemical reactivity of Ni overlayers that are grown epitaxially on a Pt(111) surface. The reaction pathways of cyclohexene were investigated using temperature-programmed desorption, high-resolution electron energy loss (HREELS), and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Our results provide conclusive spectroscopic evidence that the adsorption and subsequent reactions of cyclohexene are unique on the monolayer Ni surface as compared to those on the clean Pt(111) surface or the thick Ni(111) film. HREELS and NEXAFS studies show that cyclohexene is weakly pi-bonded on monolayer Ni/Pt(111) but di-sigma-bonded to Pt(111) and Ni(111). In addition, a new hydrogenation pathway is detected on the monolayer Ni surface at temperatures as low as 245 K. By exposing the monolayer Ni/Pt(111) surface to D2 prior to the adsorption of cyclohexene, the total yield of the normal and deuterated cyclohexanes increases by approximately 5-fold. Furthermore, the reaction pathway for the complete decomposition of cyclohexene to atomic carbon and hydrogen, which has a selectivity of 69% on the thick Ni(111) film, is nearly negligible (<2%) on the monolayer Ni surface. Overall, the unique chemistry of the monolayer Ni/Pt(111) surface can be explained by the weaker interaction between adsorbates and the monolayer Ni film. These results also point out the possibility of manipulating the chemical properties of metals by controlling the overlayer thickness.

Articles you may be interested inAnalysis of electronic structure of amorphous InGaZnO/SiO2 interface by angle-resolved X-ray photoelectron spectroscopy Angle-resolved x-ray photoelectron spectroscopy ͑AR-XPS͒ is utilized in this work to accurately and nondestructively determine the nitrogen concentration and profile in ultrathin SiO x N y films. With furnace growth at 800-850°C using nitric oxide ͑NO͒ and oxygen, 10 13 -10 15 cm Ϫ2 of nitrogen is incorporated in the ultrathin (р4 nm͒ oxide films. Additional nitrogen can be incorporated by low energy ion ( 15 N 2 ) implantation. The nitrogen profile and nitrogen chemical bonding states are analyzed as a function of the depth to understand the distribution of nitrogen incorporation during the SiO x N y thermal growth process. AR-XPS is shown to yield accurate nitrogen profiles that agree well with both medium energy ion scattering and secondary ion mass spectrometry analysis. Preferential nitrogen accumulation near the SiO x N y /Si interface is observed with a NO annealing, and nitrogen is shown to bond to both silicon and oxygen in multiple distinct chemical states, whose thermal stability bears implications on the reliability of nitrogen containing SiO 2 .

The adsorption and reactions of vinyl bromide and vinyl iodide on a Cu(100) surface have been studied by temperature-programmed desorption in conjunction with near-edge X-ray absorption fine structure (NEXAFS) and work function change measurements. Vinyl bromide adsorbs molecularly on the surface at 100 K. The polarization dependence of the π*CC resonance indicates that the molecules lie with their π bond within 28 ± 5° of parallel to the surface. Upon heating, both vinyl bromide and vinyl iodide decompose to generate surface vinyl groups, which adopt a tilted orientation on the surface. Both the molecular halides and the surface vinyl groups show a splitting of the π*CC NEXAFS resonance due to the inequivalence of the carbon atoms in these species. The position of the σ*CC shape resonances for these species indicates little change (<0.05 Å) in CC bond length due to adsorption and dissociation to form vinyl groups. Chemical displacement studies show that the CBr bond cleavage in vinyl bromide occurs at 160 K. This dissociation temperature is confirmed by complementary NEXAFS and work function change measurement results. At 250 K, vinyl groups couple to yield 1,3-butadiene with 100% selectivity.

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N  1s core-level shifts from x-ray photoelectron spectra (XPS) are reported for the adsorption products of nitromethane (CH3NO2) on Si(100). Three spectral peaks are identified and these are associated with specific bonding environments for nitrogen by comparison to predicted core-level shifts from density functional calculations on a range of energetically feasible chemical structures. These species can be classified according to the number of N–O bonds (zero, one, or two) that they contain and, in this sense, they are comparable to the species believed to exist in oxynitride films on Si. Since the energetically feasible products of room-temperature CH3NO2 adsorption can be identified with more confidence than those resulting from ion bombardment and high-temperature processing in oxynitride films, nitromethane provides a model system that can aid in correlating spectral features with specific atomic-scale structures. This work supports an earlier proposal that the XPS peak of weakest binding energy is due to species with a dangling bond on nitrogen, while the most intense peak is due to the energetically preferred NSi3 species.

Motivated by a controversy about the proper interpretation of x-ray photoelectron spectra of Si/SiO2 interfaces derived from the adsorption of H8Si8O12 spherosiloxane clusters on Si(100) surfaces, we have studied the adsorption geometry of the H8Si8O12 clusters on deuterium-passivated and clean Si(100) surfaces by using external reflection infrared spectroscopy. Access to frequencies below 1450 cm−1 was made possible through the use of specially prepared Si(100) samples which have a buried metallic CoSi2 layer that acts as an internal mirror. A comparison of the infrared spectrum of the clusters on a deuterium-passivated Si(100) surface at 130 K with an infrared spectrum of the clusters in a carbon tetrachloride solution reveals that the clusters are only weakly physisorbed on the D/Si(100) surface and also provides evidence for the purity of the cluster source. We also present infrared spectra of clusters directly chemisorbed on a clean Si(100) surface and show evidence that the clusters are adsorbed on the Si(100) via attachment by one vertex. A complete assignment of the observed vibrational features, for both physisorbed and chemisorbed clusters, has been made based upon comparisons with the results obtained in ab initio calculations using gradient-corrected density functional methods.