Phase diagram of hydrogen adsorbed on Ni(111)

An explanation for the phase diagram of the H/Ni(111) system is proposed, where the existence of the p(2 × 2) phase is assumed in addition to the (2 × 2) phase. Using the position-space renormalization-group theory with prefacing transformation, a lattice gas on a honeycomb lattice, including up to 6th nearest neighbor interactions, is treated. Another explanation previously put forward by Domany et al. is also examined, where the occurrence of the (2 × 2) ordered phase and the gas + p(2 × 2) coexistence phase is assumed. The phase diagrams thus obtained for these two cases are exhibited and discussed.     

Laser induced charge transfer from Ni to CO adsorbed on (111) and (110) surfaces

Ni(110) and (111) surfaces covered with up to one monolayer of CO were irradiated with the light of a dye laser in the photon energy range 2.0 to 3.4 eV. Two-photon photoemission was observed when the laser light was focussed. Upon defocussing a signal was measured which did not depend on the potential of the sample and showed a linear intensity dependence. It is caused by electrons transferred from the Ni substrate into adsorbate states. The signal vanishes for photon energies below 2 eV. This shows that the adsorbate state lies at most 2 eV above the fermi level. The lifetime τ of the electrons in the adsorbate states is estimated to be 10^?10 < τ < 10^?7 s. No fluorescence in the photon energy range above 1 eV could be detected.     

Inverse photoemission of pyridine on silver (111)

Inverse photoemission, together with UPS and EELS, is used to obtain information on the position of the affinity level of pyridine adsorbed on Ag(111). The unresolved a₂ and b₁ affinity levels (AL)(1.20 and 0.62 eV above the vacuum level for free pyridine) are found at E_F+(2.9±0.2) eV (indicating a Coulomb relaxation of about 2.1 eV). The reported phase transition in the first adsorbed monolayer of pyridine had no influence on the position and halfwidth of the AL nor on the electronic excitation intensities. The assignment of UPS structure is discussed. Relaxation is different for unoccupied and occupied states. The second layer “initial state” model of photoionization is not confirmed.     

Giant Raman scattering by pyridine and CN− adsorbed on silver

The giant RS by pyridine and CN^? on Ag is accompanied by a strong RS continuum which is attributed to inelastic light scattering by charge carrier-excitations. The enhanced RS by the adsorbed molecules and by the charge carrier-excitations are attributed to surface roughness enhanced EM fields at the metal surface resulting from the excitation of transverse collective electron-excitations and surface-EM modes, and to surface roughness-induced radiative-excitation and radiative-recombination of particle-hole pairs.     

Infrared spectra of CO adsorbed at low temperatures on Ni

At low temperatures (1.5K–40K), CO has been found to chemisorb into terminal, bridge, and three-fold sites on evaporated Ni films. The chemisorption takes place directly, rather than through a precursor state. At least two distinct terminal sites are occupied at high coverages. After the sample is warmed from 1.5K to 40K the infrared spectra change dramatically, showing substantial surface diffusion even at these low temperatures.     

Structure of CO adsorbed on Ni (100)

The c(2 × 2) configuration of CO chemisorbed on Ni(100) has been examined by the dynamical LEED method of surface structure analysis. Experimental LEED intensity spectra of the (00), ($\text{1}\text{2}\text{1}\text{2}$) (10) and (11) LEED beams measured at 175 K are compared with the corresponding calculated spectra for two different CO potential constructions and a number of trial structures. The best agreement was found for a structure where the CO molecules sit directly above the Ni atoms with vertical spacings between the Ni and C and the C and O layers of 1.80 ± 0.10 A and 0.95 ± 0.10 Å respectively. It is proposed that the CO molecule is tipped over at an angle of 34° ± 10° with respect to the surface normal so that the actual carbon-oxygen bond length is close to the figure 1.15 Å found in Ni(CO)₄.     

Electronic structure of graphene on Ni(111) and Ni(100) surfaces

Physics of the Solid State - A comparative investigation of graphene prepared by cracking of propylene (C3H6) on nickel surfaces with different orientations, Ni(111) and Ni(100), has been carried...     

The electronic structure of Cs adsorbed on Mo(111)

The main loss observed in an ELS study of Cs adsorbed on Mo(100) is described as an interband transition with initial and final states in the adsorbate. This interpretation is preferred to that involving the plasmon excitation mechanism observed for Cs on Cu(111) in the coverage range where the Cs has a metallic nature. The occurence of this transition is assumed to be related to the substrate effect.     

States of water molecule adsorbed on Si(111) surface

Calculated electronic energy structure of an overlayer of water molecules chemisorbed on Si(111), with a molecular plane perpendicular to the surface, reveals that 3a₁ molecular state is removed upon adsorption, and the resulting state densities cannot be reconciled with the UPS spectrum. These results, and state densities obtained from different atomic configurations are interpreted to rule out the molecular adsorption on Si(111) surface at room temperature.     

Angular distribution patterns of photoelectrons from orbitals of CO adsorbed on Ni(100), Pt(111) and Pt(110)

The synchrotron radiation from BESSY has been used to measure the photoemission from CO orbitals adsorbed as ordered overlayers on Ni(100) c(2 × 2), Pt(111) c(4 × 2) and Pt(110) (2 × 1)p2mg. Angular distribution patterns of photoelectrons from CO orbitals were recorded with a display-type analyzer. The data were compared with differential photoionization cross sections calculated for free and oriented molecules. The results demonstrate the upright orientation of CO on Ni(100) and Pt(111), while CO on Pt(110) shows a marked difference which can be explained by assuming that the CO molecules are tilted in the [001] directions of Pt(110), yielding a (2 × 1)p2mg superstructure observed in LEED. The tilt angle is estimated to about 20°. The structure model is supported by the shape resonances of the 4σ (5σ) orbitals of CO/Pt(110) as compared to CO/Pt(111).     

Proposed structure of hydrogen on Ni(111)

Recent observation of an order-disorder transition in H on Ni(111) is interpreted as indicating a transition to a (2 × 2) honeycomb state. Comparison with theoretical calculations leads us to infer that third neighbor attractions in addition to first and second-neighbor repulsions are present. The magnitude of the latter repulsion is estimated.     

The interaction of acetylene with Ni(111), chemisorbed oxygen on Ni(111), and NiO(111); the formation of CH species on chemically modified Ni(111) surfaces

Ultraviolet photoelectron spectroscopy, temperature programmed thermal desorption and low-energy electron diffraction have been used to study the interaction of acetylene with a clean Ni(111) surface, with a Ni(111) surface having co-adsorbed oxygen and with an epitaxially grown NiO(111) surface produced by room temperature oxidation ofNi(111). The adsorption of a (2 × 2) overiayer of π-bonded acetylene or oxygen on the Ni(111) surface markedly alters the subsequent interaction and reaction of the surface with incident acetylene. We find that in the presence of either a (2 × 2) overiayer of oxygen or π-bonded acetylene, a new more strongly bound hydrocarbon phase forms at room temperature. We identify this new phase from its ionization levels as a CH species, and for saturation coverages we find approximately twice as many of these species as the number of π-bonded acetylene molecules in the (2 × 2) structure. Preadsorption of oxygen limits the adsorption of π-bonded acetylene but does not affect the subsequent formation of this CH species. Exposure of acetylene to NiO at room temperature produces only CH species. Based upon these results we propose idealized models for the bonding geometry of π-bonded acetylene and CH species on the Ni(111) surface. The conditions for the formation of CH species and the significance of CH species to surface reactions on Ni are also discussed.     

Vibrational spectra of ammonia adsorbed on Fe(110)

EELS spectra of ammonia adsorbed on a Fe(110) surface at 120 K reveal three different adsorption states of molecular ammonia. Thermal processing of the ammonia covered Fe(110) surface to 315 K indicates fragmentation of the NH₃ molecules into atomic hydrogen and nitrogen. Formation of an NH₂ intermediate is not observed whereas the existence of NH_ad species cannot be excluded at present.