G. Illing

The influence of defects on the Ni 2p and O 1s XPS of NiO

The Ni 2p and O 1s XPS of NiO single crystals were measured using monochromatic Al K alpha radiation. Different treatments of the crystals allow the authors to give a description of the influence of defects on these spectra. The Ni 2p3/2 spectrum of in-situ-cleaved NiO exhibits clearly visible features at 854.1 ev 855.6 eV and 861 eV. The intensity ratio changes after ion bombardment and an additional peak at 852.2 eV appears. Apart from a slight increase in linewidth the O 1s spectrum remains unchanged. The O 1s spectrum from an in-situ-cleaved NiO single crystal exhibits only a single peak at 529.4 eV which can be fitted by a Gaussian line profile. Further measurements allow the authors to attribute the well known O 1s satellite at 531.2 eV to emission from oxygen-containing species adsorbed at defects.

Adsorption and reaction of CO2 on Ni{110}: X-ray photoemission, near-edge X-ray absorption fine-structure and diffuse leed studies

Using three different techniques X-ray photoemission (XPS), near-edge X-ray absorption spectroscopy (NEXAFS) and diffuse LEED we have studied the adsorption and reaction of CO, on Ni(ll0). In agreement with previous angle-resolved photoemission (ARUPS) and vibrational electron energy loss (EELS) data both a linear, physisorbed molecule and a bent, chemisorbed species CO,Sare found. An evaluation of the XPS line intensities shows that the stoichiometry of the chemisorbed species is 1 to 2 in carbon and oxygen. The polarisation dependence of the NEXAFS indicates that the physisorbed molecule lies with its axis parallel to the surface and that the molecular plane of the CO;species is perpendicular to the surface. There is no clear preferential azimuthal orientation. This is in agreement with a diffuse LEED analysis where equal numbers of bent molecules adsorbed on atop sites and oriented along the (100) and (110) directions gave the best fit to the data.

CO2 activation and reaction with hydrogen on Ni(110): formate formation

Abstract Upon adsorption of CO2 at T ≈ 90 K on Ni (110), the molecule weakly chemisorbs and undergoes a geometrical distortion, i.e. it bends into a nonlinear geometry. This geometrical distortion is triggered by electron transfer from the substrate to the molecule, accompanied by formation of an anionic COδ−2 molecule. It is shown via HREELS spectra that COδ−2 reacts with adsorbed H atoms in low concentration (⩽0.1 l ) via a Langumuir—Hinshelwood mechanism to form the formic acid anion, i.e. formate, which remains adsorbed on the surface. Hydrogen coverages, which allow the surface ot reconstruct, inhibit CO2 chemisorption.

Adsorption of NO on an oxygen precovered Ni(100) surface

Abstract We have employed high resolution electron energy loss spectroscopy (HREELS), angle resolved photoelectron spectroscopy (ARUPS), and near edge X-ray absorption fine structure (NEXAFS) measurements to study the adsorption of nitric oxide (NO) on a clean and on oxygen precovered Ni(100) surfaces at T = 95 K. The adsorption behaviour on both the clean and the oxygen precovered surfaces is very complex. On the clean surface adsorption at low coverage starts in hollow sites with the NO axis oriented perpendicular to the surface. Consecutively, bridge sites are populated with both perpendicular and bent NO molecules. Finally, terminally bound, linear NO adsorbs on the surface. On the oxygen precovered surfaces we find the same adsorbate sites. Depending on whether we have chosen p(2 × 2) or c(2 × 2) oxygen precoverage we find a smaller percentage of molecules adsorbed in hollow sites, because oxygen occupies these sites in both layers but twice as many hollow sites in a c(2 × 2) layer. A particularly interesting observation concerns an oxygen influenced site in which the NO molecules are adsorbed with a bent orientation. This is corroborated via NEXAFS measurements. This NO species is more strongly bound to the surface than on the clean Ni surface. A section is included in the paper where we discuss some general aspects of NO bonding towards metal atoms in a linear versus bent orientation and the influence of coadsorbed species on the orientation of the molecular axis.

Thermal evolution of benzene adsorbate phases on a Os(0001) surface

The molecular structure and orientation of adsorbed benzene (C6H6) on Os(0001) has been investigated by angle resolved UV photoemission spectroscopy (ARUPS), low energy electron diffraction (LEED) and thermal desorption spectroscopy (TDS) in the temperature range 200 290 K at saturation coverages. Dehydrogenation occurs via C6H5 and C6H4 species which are expected to have an intact carbon ring system up to a temperature of T ≈ 500 K. Cracking of the carbon ring occurs at T > 500 K where C-H fragments are formed at the substrate surface. Increasing the temperature leads to a successive loss of H atoms. For T > 830 K H2 desorption ends and an ordered (9 × 9) graphitic carbon structure was detected. The characterisation of the different adsorption phases was performed by calculating ARUPS spectra. A reaction and decomposition path for the system C6H6 + Os(0001) is proposed.

Influence of the defects of a thin NiO(100) film on the adsorption of NO

Defects often play an important role for the adsorption and reaction behavior of a surface. This, however, is not the case with the adsorption of NO on NiO(100), as our experiments with a thin NiO(100) film grown on a Ni(100) substrate demonstrate. This film possesses a three‐dimensional band structure, which is comparable to that of a NiO(100) single crystal. A spot profile analysis low‐energy electron diffraction investigation shows that the film consists of crystallites, which are tilted with respect to the Ni substrate. The film must exhibit a high defect density where the crystallites border on each other. Moreover, x‐ray photoelectron spectroscopic (XPS) measurements indicate the presence of O− or OH species on the surface. We have studied the adsorption of NO on a NiO(100) film via high‐resolution electron energy‐loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS), and XPS. With HREELS and TDS we could only detect one kind of NO species on the surface. Comparing the TD and XP spectra ...

NO2 adsorption on Ni(100): A comparison of NO2 with CO2 adsorption

NO2 adsorption has been studied on Ni(100) at temperatures between 90 and 400 K via HREELS, ARUPS, XPS and NEXAFS. It is shown that NO2 dissociates at low temperatures and small exposures forming atomic oxygen and molecularly adsorbed NO. HREELS data of NONi(100) in comparison with those of NO + ONi(100) indicate that the molecular axis of NO in the coadsorbed layer is tilted away from the surface normal. After saturation of the dissociative adsorption NO2 will chemisorb on the surface. This has been followed by HREELS and XPS. NEXAFS data indicate that the chemisorbed NO2 moiety is adsorbed with the molecular plane perpendicular to the surface plane and the nitrogen end down. At high NO2 exposures and at low temperatures physisorbed N2O4 is formed on top of this relatively complex chemisorbed layer. It is likely that the molecular plane of N2O4 is oriented parallel to the metal surface. The adsorption of NO2 on Ni(100) is compared with other NO2 adsorption systems, and it is shown in comparison with the HREELS data of another triatomic, i.e. CO2, that the vibrational spectra represent finger prints of the adsorption geometry of these triatomic molecules.

Adsorption, thermal and photochemical reactions of NO on clean and oxygen precovered Ni(100) surfaces

We have studied the adsorption, thermal and photochemical reactions of NO adsorbed on clean Ni(100), epitaxially grown NiO, and Ni(100) precovered with chemisorbed oxygen. The electronic and geometric structure of the substrate surfaces and the adsorbed NO molecules were investigated by electron spectroscopic techniques, i.e. HREELS, NEXAFS and LEED, whereas the thermal and photochemical properties of the adsorbate layer were probed using TPD and laser induced desorption, respectively.

Electron spectroscopy of adsorbates via autoionization of core-tobound excited states: experiment and theory

Abstract We use autoionization Spectroscopy after core-to-bound excitation of adsorbed molecules to get information about the spectral distribution of valence electrons in adsorbates and about the nature of shake up satellite states. Examples are presented for a physisorbed system (CO 2 /Ni(110)), a weakly chemisorbed system (N 2 /Ni(110)) and a strongly chemisorbed system (CO/Ni(110)). It is shown that in the first case detailed calculations of the autolonization rates of the isolated molecule can basically explain the experimental findings. For N2/Ni(110) autoionization spectroscopy is shown to verify an assignment put forward for the photoemission spectrum, i.e. that satellites contribute heavily to the valence ionization of many systems. In the third example, the angular dependences of the autoionization lines are used to deduce Information on the symmetries of ion states.

Adsorption and UV-Laser Desorption of NO/O/Ni(100)

We have studied adsorption as well as thermal and UV-laser (193 nm) desorption of NO adsorbed on Ni(100), on O(c2×2)/Ni(100) and on epitaxially grown oxides NiO(111) and NiO(100) using HREELS, XPS, NEXAFS, LEED and TPD. We find NO on NiO to be weakly chemisorbed. Laser desorption is only induced from the oxidic surfaces. Laser desorption cross sections are two orders of magnitude higher than gas phase photoabsorption cross sections for NO. NO2/NiO(100) has been studied via XPS, LEED and TPD. NO is probably the predominant desorption product under UV laser impact /1/. Reaction also takes place under XPS influence and under impact of secondary electrons from the X-ray gun. Possible desorption processes are discussed.