Ultrafast dynamics of o-fluorophenol studied with femtosecond time-resolved photoelectron and photoion spectroscopy

The ultrafast dynamics of o-fluorophenol via the excited states has been studied by femtosecond time-resolved photoelectron imaging. The photoion and photoelectron spectra taken with a time delay between 267 nm pump laser and 800 nm probe laser provide a longer-lived S1 electronic state of about ns timescale. In comparison,the spectra obtained by exciting the S2 state with femtosecond laser pulses at 400 nm and ionizing with pulses at 800 nm suggest that the S2 state has an ultrashort lifetime about 102 fs and reflects the internal conversion dynamics of the S2 state to the S1 state.     

Bonds and Fermi surfaces studied by photoelectron spectroscopy

Photoelectron spectroscopy with synchrotron radiation employing high energy and angular resolutions is a very efficient tool for experimental investigations of the electronic structure of solids and their surfaces. In addition to standard band-mapping applications, photoemission intensity and line-shape analyses provide valuable information about wave functions, bonds and interactions of a many-electron system. In this report we choose covalent semiconductor surfaces as well as metallic clean and nanostructured surfaces of layered materials to serve as model systems for assessing the spatial origin of photoelectrons and the three-dimensional shape of Fermi surfaces. Received: 11 July 2001 / Accepted: 23 July 2001 / Published online: 3 April 2002     

Surface chemistry studied by in situ X-ray photoelectron spectroscopy

Time-dependent X-ray photoelectron spectroscopy is used to study the kinetics and dynamics of simple surface reactions. Combining high-resolution core level spectroscopy with a supersonic molecular beam in one experimental setup, processes such as the dissociative adsorption of methane on both Pt(111) and Ni(111), the coadsorption of water and CO on Pt(111), and the oxidation of CO on Pt(111) have been studied. In the case of methane, the observed vibrational fine structure in C 1s spectra is used to identify the adsorbed species (CH₃) and further thermal dehydrogenation steps. While simple dehydrogenation via CH is observed on Pt(111), a C–C coupling reaction to acetylene is found on Ni(111). In the coadsorbate phase, CO is found to be able to replace predosed water from the bilayer into multilayers. Water, in turn, leads to a site change of the CO molecules, which are preferably adsorbed at bridge sites in the presence of water, as opposed to on-top adsorption on clean Pt(111). For the truly bimolecular surface reaction, the CO oxidation on Pt(111), the ability of the molecular beam to create a relatively high CO pressure was found essential to study the kinetics of the basic step (CO+OCO₂) without influence of adsorption or diffusion rate. An activation energy of 0.53 eV and a preexponential factor of 5×10⁶ s^-1 are found. PACS 68.43.Mn; 79.60.Dp; 82.20.Pm     

Oxidation of cerium and titanium studied by photoelectron spectroscopy

Cerium and titanium are examples of reactive metals that form protective oxide films on their surfaces upon exposure to oxygen. In order to study the building up of these oxide films we have undertaken photoemission measurements with X-ray and ultraviolet radiation (ESCA and UPS). Cerium and titanium levels were studied using ESCA on freshly in situ evaporated metal films. These levels, as well as the O 1s level, were studied after different oxygen exposures. In order to vary the probing depth, spectra have been recorded at two different electron emission angles. All measurements were performed at room temperature. Effects of oxygen exposures upon core levels were quite different in the two metals. In the Ce 3d spectrum a strong peak related to the oxide was observed at an exposure of only 2 L. Such a strong peak was not observed in the Ti 2p spectrum even after an exposure of 3000 L. The valence band spectrum of Ti as observed both by UPS and ESCA measurements, however, changed significantly at much smaller exposures.     

Surface defect centres in irradiated AgCl studied by X-ray photoelectron spectroscopy

Irradiation of silver chloride with both UV and soft X-rays during X-ray photoelectron spectroscopic measurements at ca. 300K produces a new thermally-stable chlorine centre; however, at temperatures below ca. 180K the rate of production of this species is immeasurably slow. Cadmium doping experiments indicate that the centre is closely associated with cation vacancies.     

Pure and Sb-doped SnO 2 nanoparticles studied by photoelectron spectroscopy

We describe photoemission results from pure and Sb-doped SnO₂ nanoparticles deposited on gold substrates. Photoelectron spectra with synchrotron radiation were recorded for Sn 3d, Sb 3d and O 1s core levels and valence bands in the 500-1200 eV energy range. For pure SnO₂ nanoparticles the surface is terminated by an oxygen rich layer with no obvious surface environment for Sn. When doped n-type with 9.1% or 16.7% Sb, dopant atoms are concentrated near the surface of the nanoparticles. The valence state of the dopant atoms is predominantly Sb^V. Plasmon satellite features are also observed in core level photoemission spectra and their intensity relative to the main peak increases with increasing photon energy. Received 30 November 2000     

Oxidation of nickel studied by UV and X-ray photoelectron spectroscopy

The nature of argon-ion bombarded nickel surfaces (polycrystalline, and (111), (110) and (100) single crystals) and their subsequent interaction with oxygen at ordinary temperatures have been studied using X-ray and UV photoelectron spectroscopy, including angular variation measurements and the determination of work function changes, in combination in the same apparatus. Variations between the HeI spectra of the four clean substrates were taken to confirm the presence of substantial order within the depth sampled by UPS. The four surfaces exhibited similar but not identical behaviour during oxidation, resembling that reported by other workers from studies of both annealed single crystals and evaporated polycrystalline films. The initial process was deduced to be essentially dissociative chemisorption: no evidence supporting a previous suggestion of associative adsorption at low coverages was found. Oxygen commenced to penetrate below the surface of all samples before oxygen equivalent to a monolayer had been taken up (~10 L exposure) and further substantial uptake followed resulting in the formation of a stable film (~18 Å) of nickel oxide by ~100 L exposure. This oxide layer was not stoichiometric nickel(II) oxide: it was characterized by the presence of two distinct O 1s signals, the relative intensities of which depended on the crystallographic nature of the surface. It is tentatively suggested that the oxygen signal with the higher BE be associated with Ni^III. Comparison of the X-ray and UV spectra suggests that the oxide film is very non-uniform in thickness, some Ni metal remaining very close to the surface.     

Electronic structure of black phosphorus studied by X-ray photoelectron spectroscopy

The X-ray photoelectron spectrum of black phosphorus has been measured for the first time. The features in the valence band spectrum are found to be in good agreement with those of the valence state density recently calculated on the basis of a pseudopotential method. The 2s and 2p core spectrum of black phosphorus is also discussed.     

Electronic structure of carbyne studied by X-ray photoelectron spectroscopy and X-ray emission spectroscopy

The electronic structure of amorphous carbyne has been investigated by X-ray photoelectron spectroscopy and X-ray emission spectroscopy. Carbyne band structure has been calculated semiempirically and the experimental data have been interpreted on the basis of the calculation results. The valence band width was found to be about 20 eV which is the same as that in all other condensed carbon structures. The fine satellite structure near the 1s line of carbon has been studied. It is shown that the energy bands in carbyne are arranged in a mirror-like way relative to the Fermi level. The real carbyne structure is susceptible to conformations which affect primarily the π-subband structure.     

An equivalent cores calculation of solid-state and adsorbate shifts in X-ray photoelectron spectroscopy

An equivalent cores model is used to calculate the observed binding energy shifts in the X-ray photoelectron spectra of atoms when bonding to form a solid phase. Good agreement is found with experimental data both for the formation of elemental metals and for the adsorption of gases on metal surfaces.     

Valence bands of aluminium noble-metal alloys studied by X-ray photoelectron spectroscopy

The valence bands of aluminium noble-metal alloys are studied by XPS using Al Kα-radiation. The alloys were prepared by evaporation followed by interdiffusion of the components under ultra-high vacuum within the analyzer chamber of the spectrometer. In the absence of surface contamination the alloys exhibit more intense spectra, with sharper features, than those observed for previously studied bulk samples.     

Initial oxidation of Mn surface studied by ultraviolet photoelectron spectroscopy and metastable induced electron spectroscopy

Metastable induced electron spectroscopy (MIES) was combined to ultraviolet photoelectron spectroscopy (UPS) to study the initial steps of manganese oxidation. Oxygen exposure directly led to the formation of MnO with no intermediate states. The MnO feature saturation observed by MIES and UPS techniques showed noticeable differences and proved the formation of several oxide layers. The oxidation kinetics was studied by measuring MnO features by UPS, which depend on the surface coverage by oxygen. We observe a decrease of oxygen adsorption probability with oxygen exposure. Oxidation proceeds by oxygen dissolution into the first layers to form a three-dimension MnO. This hypothesis was confirmed by our work function measurements.     

Electronic properties of free and supported metal clusters studied by photoelectron spectroscopy

The electronic properties of free and supported metal clusters are studied by photoelectron spectroscopy. Experimental as well as theoretical results clearly demonstrate a dramatic dependence of the level structure on the cluster size. By this an interesting way might be opened to modify the electronic, optical and chemical properties of surfaces.     

Electronic band structure of graphite studied by highly angle-resolved ultraviolet photoelectron spectroscopy

The electronic valence band structure of graphite has been experimentally determined with considerable accuracy for the first time by the highly angle-resolved ultraviolet photoelectron spectroscopy. The photoemission measurements were performed on a synthesized single-crystalline graphite (kish graphite) with the He II resonance line as an exciting source. Some high symmetry points in the Brillouin zone have been successfully identified in the experimentally determined band structure. The present experimental result has been compared with early photoemission experiments and some band calculations presented so far.