M. Neumann

Adsorption and reaction of CO2 and CO2/O CO-adsorption on Ni(110): Angle resolved photoemission (ARUPS) and electron energy loss (HREELS) studies

Abstract Molecular adsorption is observed on a Ni(110) surface at 80 K. The relative binding energies of the valence ion states as determined by ARUPS are consistent with those in the gas as well as in the condensed phase, and indicate that the electronic structure of the adsorbed molecule is only slightly distorted upon adsorption at this temperature. The adsorbate spectra show E versus k ∥ dispersions indicating some long-range order in the adsorbate. The variations in relative ionisation probabilities of the ion states as a function of electron emission angle suggest that the molecular axis is oriented parallel to the surface within ± 20°. Upon heating the adsorbate to above 100 K. (i.e. 140 K) the spectrum changes. A new species causing an increase in work function by 1 eV can be identified. Comparison with calculations suggests that it is an anionic bent CO; molecule. Electron energy loss studies on this intermediate species support the proposed bent CO 2 geometry and favour a coordination site with C 2v symmetry. The bent CO 2 moiety is stable up to 230 K. Further heating to room temperature leads to dissociation of the bent CO 2 molecule into adsorbed CO and O. The CO molecule is oriented with its axis perpendicular to the surface. The bent CO 2 species appears to be a precursor to dissociation. Results on CO 2 adsorption on an oxygen precovered surface show that CO 2 interacts with oxygen at 85 K. Upon heating the co-adsorbate to near room temperature a reaction product is formed the nature of which cannot yet be clearly identified.

CO2 adsorption and reaction on Fe(111): An angle resolved photoemission (ARUPS) study

Abstract Molecular CO 2 adsorption is observed on an Fe(111) surface at 85 K. For the main fraction of molecules the relative binding energies of the valence ion states as determined by ARUPS are consistent with those in the gas as well as in the condensed phase, and indicate that the electronic structure of that fraction of adsorbed molecules is only slightly distorted upon adsorption. There is a fraction of adsorbed molecules at 85 K that can be identified as bent, anionic CO 2 − species. While the weakly adsorbed, linear CO 2 molecules desorb at low temperature, the CO 2 − species is stable up to 160–180 K. The latter is proposed to be a precursor to dissociation. Above this temperature adsorbed carbon monoxide and oxygen are observed on the surface, and at room temperature the CO 2 − signals have disappeared. Heating above room temperature dissociates the CO molecules into carbon and oxygen.

Electronic and geometric structure of CO on Ni(110): Experiment and theory

Abstract Chemisorption of CO on Ni(110) results at saturation in a well ordered (2 × 1) p2mg structure observed in LEED. The strong lateral interaction between the CO molecules leads to the formation of a two-dimensional band structure, which has been investigated in great detail by angle resolved photoelectron spectroscopy using monochromatized light from the storage ring BESSY at Berlin. Due to the low symmetry and the existence of two CO molecules in the unit cell eight valence bands have been observed, namely the 4 σ + , 4 σ − , 5 σ + , 5 σ − , 1 π x + , 1 π x − , 1 π y + and 1 π y − bands. The experimentally determined band structure can be well reproduced by tight binding calculations of an unsupported CO overlayer of p2mg symmetry, if the interaction with the substrate is included so as to account for 4 σ -5 σ mixing upon chemisorption as well as for the 5 σ -1 π hybridization. Certain deviations between experiment and theory allow us to deduce information about the possible influence of indirect intermolecular interactions. The dispersion of the 1π x bands turned out to be very sensitive to the tilt angle of the CO molecules. An inclination of 17 ± 2° from the normal along the [001] direction alternating to both sides has been evaluated, consistent with a structure of p2mg symmetry rather than p1g1 symmetry.

Photoemission of Molecular Adsorbates

In photoemission the observed changes in band intensities as a function of light polarization, electron emission direction, and photon energy can be used to deduce information, for example, about geometry, two-dimensional band structures, intermolecular interactions and chemical reactivity of molecular adsorbates. To characterize the ion states of molecular adsorbates in the so-called inner valence electron region we discuss results from autoionization spectroscopy as a method that yields complementary information to photoemission in this spectral region. We use results on CO, N2, and CO2 adsorbates to discuss the various aspects of photoemission of molecular adsorbates.

Influence of alkali co-adsorption on the adsorption and reaction of CO2 on Pd(111)

The Pd(111) surface has been extensively used as a model substrate to study CO oxidation. The reverse reaction, CO2 dissociation, however, has not been investigated to a comparable extend on this surface. We report angle-resolved photoemission studies on the system CO2 (+Na)/Pd(111). On a clean Pd(111) surface at 85 K no CO2 adsorption takes place. Upon small precoverages of Na, adsorption of CO2 occurs at T = 85 K. Via an analysis of the angular dependence of the photoelectron spectra we obtain information about the orientation of the adsorbed molecules. Upon annealing the surface to 125 K, CO2 reacts to form co-adsorbed CO and O. For a thick Na film on Pd(111) we also find CO2 adsorption but the reaction of the adsorbed species occurs along different reaction channels, ending at adsorbed carbonate species.

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.

The adsorption of N2 on Fe(111): Angle resolved photoemission and theoretical model studies

Angle resolved photoelectron spectra excited by synchrontron radiation from the BESSY storage ring have been obtained for adsorbed N 2 on Fe(111) as a function of temperature. The two previously identified phases of molecular N 2 , i.e. the γ-and α-phase, can be clearly differentiated by their photoemission characteristics. From the angular profiles the γ-phase can be shown to consist of N 2 molecules oriented perpendicular to the surface plane. The α-phase, on the other hand, shows spectra that are only consistent with N 2 whose axis is strongly inclined from the surface normal. These findings corroborate the conclusions drawn from HREELS and XPS that the N 2 in the α-phase is “π-bonded” to the surface. We also present the results of ab initio generalized valence bond calculations for the A 3 u + excited, state of the N 2 molecule and suggest that this state of N 2 may be stabilized by the Fe(111) surface resulting in the species responsible for the α-phase. A microscopic model for the π-bonding” state is proposed.

Photoemission of benzene on Os (0001 ): A synchrotron radiation study

Abstract The molecular structure and orientation of benzene on Os (0001 ) has been investigated by angle-resolved UV photoemission spectroscopy (ARUPS) using synchrotron radiation. At temperatures below 285 K benzene adsorbs in a molecular state and ARUPS indicates C 3v symmetry. This implies that the adsorbed benzene molecules are oriented parallel to the metal surface with a trigonal distortion of their aromatic rings. Bonding to the surface occurs through the benzene π system.

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.

Bremsstrahlung isochromat spectroscopic study of the system CO/Ni(110): Strong intermolecular interaction

Adsorption of CO on Ni(ll0) at monolayer coverage results in a (2X 1) p2mg overlayer structure with the CO molecular axis tilted by 150-20° with respect to the surface normal. The low symmetry of the overlayer leads to splittings of all CO-derived levels at the center of the Brillouin zone. As a result of the high coverage and the concomitant strong lateral interaction between the CO molecules the level splittings are as large as 2 eV for both occupied and empty CO-derived levels. The same order of magnitude has been estimated from tight binding calculations for a free unsupported CO layer in p2mg structure.