Chemisorbed ordered oxygen overlayer on AL(111): An angular resolved photoemission study

Chemisorbed oxygen atoms on aluminum induce a strong O 2p derived surface resonance. On the (111) crystal face the oxygen atoms form an ordered overlayer of (1x1) symmetry. The O 2p resonance in this system has been studied using angle resolved photoemission. The results indicate one of the threefold centered hollow sites as the probable site of chemisorption. At normal electron emission, the O 2p resonance is sharp and symmetric. Strong dispersion effects are seen at non-normal emission indicating appreciable oxygen-oxygen interaction in the chemisorbed layer.     

Angle resolved photoemission study of the Ce/Pd(1 1 1) interface

X-ray and UV excitation angle resolved photoemission spectroscopy of ultra-thin films of cerium deposited on a Pd(1 1 1) single-crystal surface has been carried out. Both core level and valence band spectra show a formation of a surface alloy exhibiting d- and f-electron orbital hybridization. An azimuth and polar angle mapping of X-ray excited photoemission intensities results in a surface-geometrical structure information. Mapping of ultra-violet photoelectron intensities as a function of emission angles is used for a band mapping and an electronic structure determination. The d-, f-hybridization occurs even for 0.7 ML of Ce deposited at room temperature. The surface annealing at 260 °C enhances this behavior. A shift of Pd 4d-derived states to higher binding energy in the Ce-Pd systems is observed.     

Angle resolved photoemission study and calculation of the electronic structure of the Pt(111) surface

Angle resolved photoemission spectra of Pt(111) were measured along the direction of the surface Brillouin zone (SBZ). The electronic structure of a semi-infinite Pt(111) crystal was calculated applying the LMTO-TB approximation to aid interpretation of the spectra. The experimental spectra are well described by the calculated bulk band structure. Both the experiment and calculation reveal a surface state near the Fermi level in the neighborhood of the point of the SBZ.     

Selenium and sulfur bonding on Ni(111); angular resolved photoemission

The bonding mechanism of selenium and sulfur on Ni(111) has been investigated by observing the variation of the normal component of the photocurrent as the angle of incidence of the light is varied. The application of simple arguments leads to the assignment of symmetries to the orbitals observed in the photoemission spectra. An interpretation in terms of a Ni₃ cluster model is made and consequences of this model are discussed.     

Surface energy bands observed in photoemission from a (1 × 1)-N structure on Cr (100)

The surface structure obtained on Cr (100) by diffusion of nitrogen from the bulk is studied by angle resolved photoemission. It is found that nitrogen is bonded in an ordered (1 × 1) configuration with a crystallographic point group C_4v. The data display a strongly dispersive two dimensional Bloch character for the N2p derived resonances. For these bands the symmetry at the $\text{Γ}$ point and the dispersion along the $\text{Δ}$ and $\text{Σ}$ lines of the surface Brillouin zone are determined. The results indicate a strong interaction of the nitrogen atoms with the substrate and favor the intersticial fourfold hollow position.     

Experimental band structure of GaSe obtained by angular resolved photoemission

A very detailed experimental band structure of the layered compound GaSe has been obtained in all the symmetry directions of the Brillouin zone for photon energy varying between 21 and 45 eV. It is shown that in this energy range the dispersion curves E (K//) are independent of the final state and that it is also possible, assuming a free electron like conduction band, to have information along the normal to the layers.     

Atomic geometry and electronic properties of the Ge(1 1 1)2 × 1-Sb surface studied by scanning tunneling microscopy/spectroscopy and core-level photoemission

By scanning tunneling microscopy and spectroscopy (STM/S) and high-resolution core-level photoemission using synchrotron radiation, we have investigated the atomic structure and electronic properties of Sb-induced 2 × 1 reconstruction on Ge(1 1 1). Our results support well the zigzag-chain model proposed for this phase in the literature; in particular, the STM images visualize the Sb zigzag (Seiwatz) chain in a real space, and the STS I-V spectrum suggests this surface to be semiconducting, in good agreement with the atomic configuration proposed. However, a closer inspection of the STM results does not support the buckling of Sb chains reported in earlier studies. Moreover, the analysis of the Sb 4d core-level line shape of the (2 × 1) reconstruction shows that the bonding state of the Sb atoms is very similar, suggesting an unbuckled Seiwatz chain. In addition, the Ge 3d core-level emission reveals only one component, giving evidence for the ideal bulk-terminated structure of the Ge substrate.     

Spatially resolved STM light emission spectra of cleaved (1 1 0) AlGaAs/GaAs heterostructures

We have measured the spectra of light emitted from individual single GaAs quantum wells of cleaved (1 1 0) AlGaAs/GaAs heterostructures using the STM (scanning tunneling microscope) tip as a local electron injection source. The cross-sectional STM images of the quantum wells were obtained, and the light emission spectra were measured by locating the STM tip over individual quantum wells of interest. Single emission peaks were observed to shift to the high-energy side with decreasing well width. The peak positions agree with the calculated transition energies for the corresponding well width. The thermalization length of the injected electron was estimated by observing the change in the emission intensity as the tip is moved at different distances from a given well.     

Mapping of conduction bands in GaAs by angle-resolved photoemission

Using angle-resolved photoemission spectra in the photon energy range 12 ? hv ? 20 eV, we have mapped selected conduction bands for the (110) direction in GaAs using a simple direct-transition analysis. At higher energies the spectra suggest a one-dimensional density of states interpretation. The two mechanisms are discussed.     

Surface core-level shifts for chlorine covered GaAs (1 1 0) surfaces

Photoemission measurements show that upon chlorine adsorption the Ga 3d surface core-level remains essentially unchanged whereas the As 3d level is shifted by 820 meV to higher binding energy changing the sign of the shift with respect to the bulk level. Chlorine is thus bound to As in a single chemisorption state. The shift is due to charge transfer. The adsorption does not change the surface relaxation.     

Interpretation of photoemission from Na (1 1 0)

A self-consistent calculation of the electronic structure of the Na (1 1 0) surface, using an embedding method to treat the semi-infinite system, gives a prominent surface resonance peak at 0.75 eV above the Fermi energy. The tail of this resonance extends below E_F at the surface, and it is suggested that surface photoemission from this tail is responsible for the peak in the photocurrent at E_F observed experimentally, A photoemission calculation, with a non-self-consistent surface potential, reproduces the enhancement of the peak as a direct transition moves through it.     

Surface reactions of 2-iodopropane on GaAs(1 0 0)

The surface reactions of 2-iodopropane ((CH₃)₂CHI) on gallium-rich GaAs(1 0 0)-(4 × 1), was studied by temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). CH₃CHICH₃ adsorbs molecularly at 120 K but dissociates below room temperature to form chemisorbed 2-propyl ((CH₃)₂CH) and iodide (I) species. Thermal activation causes desorption of the molecular species at 240 K, and this occurs in competition with the further reactions of the (CH₃)₂CH and I chemisorbed species. Self-coupling of the (CH₃)₂CH results in the formation of 2,3-dimethylbutane ((CH₃)₂CH-CH(CH₃)₂) at 290 K. β-Hydride elimination in (CH₃)₂CH yields gaseous propene (CH₃CHCH₂) at 550 K while reductive elimination reactions of (CH₃)₂CH with surface hydrogen yields propane (CH₃CH₂CH₃) at 560 K. Recombinative desorption of the adsorbed hydrogen as H₂ also occurs at 560 K. We observe that the activation barrier to carbon-carbon bond formation with 2-propyls on GaAs(1 0 0) is much lower than that in our previous investigations involving ethyl and 1,1,1-trifluoroethyl species where the β-elimination process was more facile. The difference in the surface chemistry in the case of 2-propyl species is attributable to its rigid structure resulting from the bonding to the surface via the second carbon atom, which causes the methyl groups to be further away from the surface than in the case of linear ethyl and 1,1,1-trifluoroethyl species. The β-hydride and reductive elimination processes in the adsorbed 2-propyl species thus occurs at higher temperatures, and a consequence of this is that GaI desorption, which is expected to occur in the temperature range 550-560 K becomes suppressed, and the chemisorbed iodine leaves the surface as atomic iodine.     

Surface quantum well state at the striped Cu(1 1 0)(2 × 1)O surface studied by angle resolved photoemission spectroscopy

We have performed an angle resolved photoemission spectroscopy with high energy and high momentum resolutions and have observed the k dependent energy dispersion curves of the striped Cu(1 1 0)(2 × 1)O surface. It is found that the Shockley surface state electron is confined in the clean surface along the perpendicular direction to the stripes and forms a quantum well state (QWS). It has also been clearly observed that an electron of Cu-O antibonding state is confined within the oxygen covered surface.     

High-resolution angle-resolved photoemission study of Ni(1 1 1) surface state

High-resolution angle-resolved photoemission spectroscopy (ARPES) has been conducted to study the Shockley state (SS) in ferromagnetic Ni(1 1 1) located at the point of the surface Brillouin zone. We have determined the Fermi wave vector and Fermi energy of the state with excitation photon energies of hν = 6.9-27.5 eV. On the basis of ARPES spectral shape analyses, we have found significant electron-electron interaction in the SS.     

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

Molecular CO₂ 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 CO₂⁻ species. While the weakly adsorbed, linear CO₂ molecules desorb at low temperature, the CO₂⁻ 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 CO₂⁻ signals have disappeared. Heating above room temperature dissociates the CO molecules into carbon and oxygen.