J. Onsgaard

Adsorption and reactivity of CO2 on the K/Cu(110) interface and the effect of photon irradiation

Abstract Adsorption and reactions of CO 2 on Cu(110) with preadsorbed potassium were studied with synchrotron radiation spectroscopy (SRS), X-ray photoelectron spectroscopy (XPS), thermally programmed desorption spectroscopy (TPD) and workfunction measurements. The amount of K was varied between 0.1 ML and several monolayers, and the CO 2 exposures were in the 0.1–10 L range. Although CO 2 does not react with a clean Cu(110) surface, several reaction channels open in the presence of the alkali metal depending on the coverage. Different CO 2 states and dissociated states can be assigned on the basis of energy positions and peak intensities of C(1s) and O(1s) core lines. Both physisorbed CO 2 and a chemisorbed anion, CO − 2 , were observed at 110 K. The effects of heating were followed with SRS. The chemisorbed CO − 2 anions react according to 2 CO − 2 → CO 2− 3 + CO a with CO desorbing at 200 K. Carbonate is formed at all K coverages. The presence of CO 2 on the surface stabilizes the alkali metal as demonstrated by the use of selected state thermal desorption. Decomposition of the carbonate occurs in the temperature range 450–550 K due to the reaction CO 3 → CO 2 + O. A comparison is made with published data on CO 2 adsorption on alkali-modified transition metal surfaces with the conclusion that the adsorption characteristics and reactions are similar. It is demonstrated that a photon-induced reaction takes place, involving conversion of a physisorbed CO 2 state to a negatively charged, chemisorbed state. The cross section was determined to be 10 7 barn.

Coadsorption of K and CO on Cu(110)

Abstract Adsorption of K and CO on Cu(110), and coadsorption of K and CO on Cu(110), have been studied by the methods of thermally stimulated desorption (TSD), low-energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). The evolution of the TSD-spectra of K as a function of coverage is characterized by four K-states that reflect the change in binding from ionic to metallic. The heat of adsorption for the CO/Cu(110) interface, has been studied by a complete analysis of the TSD-spectra. The temperature corresponding to the maximum desorption rate shows a decrease with increasing CO coverage. In agreement with this finding, a marked decrease in activation energy with increasing coverage is observed. Coadsorption results in increased stability of CO on the surface with new coupled TSD-states at different temperatures, dependent on the alkali precoverage. The O(1s) photoemission peak shape is strongly influenced by the presence of K and a −2.0 eV binding energy shift, relative to the position of CO on clean Cu, is measured. A survey of the data for the CO/alkali-metal/Cu-substrate systems, Cu(100) and Cu(110), shows strong analogies.

Adsorption and compound formation of Yb on Ni(100)

Abstract The adsorption and compound formation of Yb on Ni(100) is studied by AES, ISS, LEED and XPS. The deposition of Yb is followed at two substrate temperatures, room temperatures and 670 K. Yb deposition at 670 K results in the formation of ordered structures. An incommensurate structure is attributed to adsorbed Yb, a c(10 × 2) and a slightly distorted hexagonal structure result from the formation of ordered Yb Ni compound layers. Intermixing between Yb and Ni occurs only above a certain Yb threshold coverage. Different valence states of the Yb atoms are revealed by XPS. Divalent Yb is found at the surface for all Yb depositions. Trivalent Yb is located in the bulk of the compound and, in certain coverage regions, also at the surface. Room temperature deposition of Yb does not lead to any ordered structures. At low coverages the room temperature structures can in many respects be considered as disordered versions of the 670 K structures. At higher Yb deposition no strong intermixing with the Ni occurs. Instead an Yb film starts growing.

Spectroscopic and structural investigations of the YbAl(110), YbNi(110) and YbSi(111) interfaces as a function of temperature

Abstract The interfaces were established by evaporation of very thin films of Yb on single crystals of Al, Ni and Si. The development of the interface by heat treatments was followed by the surface sensitive techniques Synchrotron radiation induced Photoemission Spectroscopy (SPS), Auger Electron Spectroscopy (AES), Ion Scattering Spectroscopy (ISS) and Low Energy Electron Diffraction (LEED). Similarities and differences between the three systems are discussed. The sharpness of the interface is found to decrease in the order Ybue5f8Ni, Ybue5f8Si and Ybue5f8Al. Yb exists in the Ybue5f8Ni and the Ybue5f8Si cases as divalent surface layers which are shown to form ordered structures by heat treatments to temperatures where the bulk Yb are diluted to infinity. At the same temperature segregation of Yb is shown to take place.

The interaction of CO2 with potassium-promoted Cu(100): adsorption, reactions and radiation induced dissociation of CO2

Abstract The coadsorption of CO 2 and K on Cu(110) has been studied by the methods of photoelectron spectroscopy using synchrotron radiation, high resolution electron energy loss spectroscopy (HREELS) and work function measurements (ΔΦ). Both weakly adsorbed molecular carbon dioxide, chemisorbed carbonate and carbon monoxide are found at 107 K. It was found that the presence of free copper sites is necessary for the adsorption of CO and the large amount of physisorbed CO 2 in comparison with the copper surface completely covered with potassium. A coherent picture of the adsorption and reactions of CO 2 with a potassium modified Cu(110) surface is obtained by combination of the above mentioned spectroscopies with earlier thermally programmed desorption measurements and binding energy determinations of C 1s and O 1s. Irradiation with 125 eV photons causes a transformation of physisorbed CO 2 to chemisorbed CO and O. The kinetics of this photoinduced process was followed via the intensity variations of the O 1s characteristic of physisorbed and reacted CO 2 . The cross-section for the reaction was determined to 3 × 10 7 barn at a photon energy of 125 eV. Both the large cross-section and the photon energy dependence indicate that photon generated secondary electrons play a dominant role in the transformation process.

THE YB/AL(110) INTERFACE STUDIED BY ELECTRON-SPECTROSCOPY

Abstract Thin films of Yb overlayers on an Al(110) surface have been studied with different methods of electron spectroscopy. The applicability of using Auger electron spectroscopy (AES), reflection electron energy-loss spectroscopy (ELS) and X-ray photoelectron spectroscopy (XPS) for recognizing mixed-valent Yb in the Yb/Al interface is discussed. Comparison between Yb + O 2 and Yb/Al shows that core hole interactions play a strong role in the ionization process which involves an ionization of the 4d shell in Yb. Diffusion of Yb into Al(110) can be described as a two step process.

Formation of the PtSi(111) interface

Abstract We have studied the atomic structure and some of the electronic properties of the Ptue5f8Si(111) interface as it forms under evaporation of Pt onto a clean silicon surface near room temperature, using Auger electron spectroscopy (AES), photoemission with synchrotron radiation, low energy electron diffraction (LEED), and Auger/sputter profiling. Growth of a single homogeneous Pt monolayer is observed at sufficiently low rates of deposition of Pt. For Pt coverages above one monolayer a reaction occurs between Pt and Si resulting in silicide-like features in the Si(L 23 VV) Auger spectrum and in valence band and core level photoemission excited with synchrotron radiation. An inward diffusion of Pt is observed in cases of elevated substrate temperatures and during ion bombardment. For slow deposition of Pt on a 7 × 7 surface, a change of the 7 × 7 pattern into a weak √3 × √3 pattern is observed. Further experiments were conducted to investigate the annealing behaviour of monolayer deposits with LEED and AES. With annealing to ∼ 800°C sharp √3 × √3 patterns are found. At intermediate annealing temperatures (320°C). a √7 × √7 pattern is sometimes observed.

The YbNi interface studied with photoemission spectroscopy

Abstract The development of the ytterbium valence band region was followed with Synchrotron radiation induced Photoemission Spectroscopy (SPS) by interdiffusion of Yb into a Ni (110) single crystal in order to identify the valence conditions of Yb in the bulk and on the surface. During this process, also the width of the Ni L 3 VV Auger transition was investigated with X-ray induced Photoemission Spectroscopy (XPS), reflecting the electron donation of Yb to the Ni valence band. By comparison between theory and experiment, strong multiplet splittings were found to take place in the 4d and 5p core level spectra of Yb due to the promotion of one 4f electron to the valence band by reaction with Ni. The 5p level is demonstrated to resonate strongly at hν =181 as a consequence of the 4d–4f giant resonance.

Structural investigations of the Yb Si(111) - 2x1, 5x1 and 3x1 overlayers

Abstract The Ybue5f8Si interface has been investigated in the sub-monolayer regime employing Ion Scattering Spectroscopy (ISS), Auger Electron Spectroscopy (AES) and Low Energy Electron Diffraction (LEED). Three different structures, Ybue5f8Si(111) 2x1, Ybue5f8Si(111) 5x1, and Ybue5f8Si(111) 3x1, have been established by heat treatments of the interfaces. The structures consist of a stable overlayer of Yb atoms on the Si(111) surfaces. The distance of the Yb atoms to the uppermost layer of Si atoms has been estimated by comparing the Ybue5f8Si ISS intensity ratio with the predictions of a model based on classical scattering theory and a Thomas-Fermi-Moliere potential. The height of the Yb atoms relative to the substrate toplayer was found to be 1.9 ± 0.3→.

Mechanisms for oxygen adsorption on the Si(110) surface studied by Auger electron spectroscopy

Abstract Oxygen adsorption on the Si(110) surface has been studied by Auger electron spectroscopy. For a clean annealed surface chemisorption occurs, with an initial sticking probability of ∼6 × 10−3. In this case the oxygen okll signal saturates and no formation of SiO2 can be detected from an analysis of the Si L2,3VV lineshape. With electron impact on the surface during oxygen exposure much larger quantities are adsorbed with the formation of an SiO2 surface layer. This increased reactivity towards oxygen is due to either a direct effect of the electron beam or to a combined action of the beam with residual CO during oxygen inlet, which creates reactive carbon centers on the surface. Thus in the presence of an electron beam on the surface separate exosures to CO showed adsorption of C and O. For this surface subsequent exposure in the absence of the electron beam resulted in additional oxygen adsorption and formation of SiO2. No adsorption of CO could be detected without electron impact. The changes in surface chemistry with adsorption are detectable from the Si L2,3VV Auger spectrum. Assignments can be made of two main features in the spectra, relating to surface and bulk contributions to the density of states in the valence band.