R.M. Jaeger

Formation of a well-ordered aluminium oxide overlayer by oxidation of NiAl(110)

We have investigated the electronic and geometric structure of a thin oxide film grown by oxidation on NiAl(110) using electron spectroscopic techniques, i.e., LEED, EELS, XPS and ARUPS. This film is inert to adsorption of, respectively reaction with many molecules up to temperatures of about 800 K. It is well ordered as deduced from the LEED pattern and covers the whole surface. We find that the oxide film is about 5 A thick, consisting of aluminium oxide as shown by EELS, XPS and ARUPS. It is most likely formed of two aluminium layers and two quasihexagonal oxygen layers with oxygen surface termination. Since the oxide film is rather thin it only shows a two-dimensional band structure which has been investigated using ARUPS. For the electronic levels of the oxide strong periodic dispersions are observed with bandwidths compatible to dispersion bandwidths calculated for the ΓX direction of α-Al2O3.

Adsorption and reaction of molecules on surfaces of metal—metal oxide systems

Abstract Molecular adsorption on oxide surfaces is gaining increasing interest both experimentally and theoretically. Adsorption studies on model systems, where well ordered thin oxide films grown on a metal substrate to avoid sample charging in connection with electron spectroscopic measurements, were used, are reported. Two oxide systems are compared: (i) a reactive transition metal oxide surface of Cr2O3(111) where it is shown that the surface contains Cr2+ ions which trigger its reactivity; (ii) a non-reactive simple metal oxide surface of γ-Al2O3(111) which is used as a support model surface. The adsorption of various molecules on both surfaces has been examined, and how the properties of the surface are modified when metals are deposited on the oxide surface have been studied. The results of alkali metal deposits on Cr2O3(111) and Pt deposits on γ-Al2O3(111) are presented. The applied methods include LEED, STM, TPD, ARUPS, ELS, XPS, HREELS and ISS.

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.

Vibrational structure of excited states of molecules on oxide surfaces

Abstract A more detailed knowledge of excited states of molecules adsorbed on solid surfaces is interesting from various points of view. To understand, for example, photochemistry at surfaces, the lifetimes of excited states are important parameters. We report ELS spectra showing vibrational structure for CO on a thin (thickness: 5 A) γ-Al2O3(111) film grown on a NiAl(110) surface. The molecules desorb from the surface in the range 35 K

Lifetimes of electronically excited states of molecules on oxide versus metal surfaces

Abstract The optically forbidden 1 Σ + → 3 Π transition of CO adsorbed on an epitaxially grown Al 2 O 3 (111) surface has been studied by means of electron energy loss spectroscopy. On this surface several CO adsorption states exist with adsorption enthalpies ranging from 88 (multilayer) to 170 meV. For the first time even in the (sub) monolayer range the vibrational fine structure of the energy loss peaks due to the 1 Π + →a 3 Π electronic transition of adsorbed CO could be clearly resolved. The half widths of the loss peaks have been used to estimate the lifetimes of the excited states.

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.

Angular dependence of loss intensities in inelastic electron scattering for physisorbed CO on Ag(110)

Abstract The a 3 Π and A 1 Π excitations of a physisorbed disordered bilayer of CO on Ag(110) have been investigated by means of electron energy loss spectroscopy (EELS) as a function of electron incidence angle and scattering angle and primary electron energy. Whereas in the (sub)monolayer regime these electronic excitations are quenched because of adsorbate-substrate interactions, strong losses with a clearly resolved vibrational splitting are observed at coverages above a monolayer. The intensities of these excitations behave differently as a function of the experimental geometry, which can be explained by the different characters of the excitations, i.e. one is optically allowed whereas the other is optically forbidden. The role the electron trajectories play for the ratio of the cross-sections of these two excitations is discussed, including electron-image charge interaction and diffraction effects.

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.