Adsorption and reaction of bromine with Ag(110)

The adsorption and reaction of Br₂ with Ag(110) was studied with Auger electron spectroscopy, LEED, work function measurements and thermal desorption spectroscopy in the temperature range of 130–1000 K. Depending on Br coverage and crystal temperature, four different adsorption and reaction states could be detected. For fractional monolayer coverages, chemisorbed Br(ad) is found to be the most stable species. This adsorption state saturates for θ(Br) ? 0.75. In the chemisorption stage, two LEED patterns, a p(2 × 1) with θ(Br) ? 0.5 and a c(4 × 2) with θ(Br) ? 0.75, were observed. For higher Br₂ exposures and T = 130 K a layer-by-layer growth of AgBr is detected. At higher temperature, T > 190 K, there is evidence for a transformation from a 2D growth mechanism of AgBr into a 3D agglomeration of larger AgBr cluster. Molecularly adsorbed.     

High-resolution photoemission from Ag(100), Ag(110), and Ag(111)

Energy distribution curves of photoelectrons emitted normal to (100), (110), and (111) faces of silver have been obtained at photon energies of 21.22, 16.85, and 11.83 eV. The results are compared with Christensen's relativistic band structure calculation of bulk silver yielding a close correspondence between experiment and theory. A surface state in the L gap immediately below the Fermi level is identified.     

Tetrathia- and tetraselenafulvalene adsorbed on Ag(110): A theoretical study

We have studied the adsorption properties of the π-donor molecule TXF, where X stands for the chalcogens sulfur and selenium [TTF = tetrathiafulvalene, TSF = tetraselenafulvalene], respectively, on the (110) surface of silver by means of periodic plane-waves based DFT (Density Functional Theory) calculations using slab models. We have determined and characterized the stable adsorption sites and have evaluated the charge transfer from TXF molecules into the surface. The simulation of the vibrational spectra for TXF and the fully deuterated TXF species has permitted to identify the fingerprints of both molecules on this surface.     

Adsorption geometry from angle-resolved photoemission: The case of Cl on Ag(001)

A recent LEED analysis of Ag(001)c(2×2)?Cl favors a simple overlayer model (SOM) for the Cl atoms and leads us to reject a mixed-layer model (MLM) with both Cl and Ag atoms alternating. Recent electronic structure calculations give agreement with published photoemission data only for the MLM. To study this discrepancy we performed a high resolution photoemission study of Ag(001)c(2×2)?Cl. Our results differ from previous measurements and are consistent only with the SOM.     

Adsorption of water and methanol on GaAs(110) surfaces studied by ultraviolet photoemission

UV photoemission spectroscopy (UPS) with He 1 radiation (hν = 21.2 eV) has been used to study the interaction of H₂O and CH₃OH with GaAs(110) surfaces prepared by cleavage in ultrahigh vacuum (UHV). For H₂O two molecularly adsorbed phases can be distinguished at 300 K: at low coverage H₂O is chemisorbed by its oxygen lone-pair orbital to the surface whereas for higher exposures a less perturbed species which resembles more a “physisorbed” or condensed H₂O layer is found. At 180 K only the less perturbed species can be identified. Also CH₃OH is chemisorbed molecularly at lower coverage with its oxygen end to the GaAs surface. For higher exposures two additional emission bands are observed which are interpreted as due to the methoxy radical CH₃O resulting from a partial decomposition of CH₃OH.     

Oxygen chemisorption on Cu(110): A combined study by second-harmonic spectroscopy and photoemission

We have studied the formation of the addedrow (2×1)O overlayer on Cu(110) using Second-Harmonic Generation (SHG). To characterize the electronic properties of the surface, simultaneous observations with LEED and angle-resolved photoemission were performed. We are able to interpret our results in terms of transitions between surface bands of Cu(110) and Cu(110)-(2×1)O, respectively.Paper presented at the 129th WE-Heraeus-Seminar on Surface Studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May. 30 to June 1, 1994     

Adsorption behavior of iron phthalocyanine on a Ag（110） surface

An investigation on the growth behavior of FePc on a Ag(110) surface is carried out by using scanning tunneling microscopy(STM).At an FePc coverage of 3.5 ML,an ordered superstructure(densely packed) with a lateral shift is observed.The densely packed superstructure can be attributed to the substrate commensuration and the intermolecular van der Waals attractive interaction.The in-plane lateral shift in the superphase is specifically along the direction of [10] azimuth.The results provide a new perspective to understanding the intermolecular and the molecule-substrate interactions.     

A photoemission study of the interaction of Ni(100), (110) and (111) surfaces with oxygen

Photoelectron spectroscopic studies of the oxidation of Ni(111), Ni(100) and Ni(110) surfaces show that the oxidation process proceeds at 295 and 485 K in two distinct steps: a fast dissociative chemisorption of oxygen followed by oxide nucleation and lateral oxide growth to a limiting coverage of 3 NiO layers. The oxygen concentration in the 295 K saturated oxygen layer on Ni(111) was confirmed by ¹⁶O(d,p) ¹⁷O nuclear microanalysis. At 295 and 485 K the oxide growth rates are in the order Ni(110) > Ni(111) > Ni(100). At 77 K the oxygen uptake proceeds at the same rate on all three surfaces and shows a continually decreasing sticking coefficient to saturation at ~2.1 layers (based upon NiO). An O 1sb.e. = 529.7 eV is associated with NiO, and O ls b.e.'s of ~531.5 and 531.3 eV can be associated, respectively, with defect oxide (Ni₂O₃) or (in the presence of H₂O) with an NiO(H) species. The binding energies (Ni 2p, O 1s) of this NiO(H) species are similar to those for Ni(OH)₂. Defect oxides are produced by oxidation at 485 K, or by oxidation of damaged films (e.g. from Ar⁺ sputtering) and evaporated films. Wet oxidation (or exposure to air) of clean nickel surfaces and oxides, and exposure of thick oxide to hydrogen at high temperature results in an O 1s b.e. ~531.3 eV species. Nuclear microanalysis ²H(³He,p) ⁴He indicates the presence of protonated species in the latter samples. Oxidation at 77 K yields O 1s b.e.'s of 529.7 and ~531 eV; the nature of the high b.e. species is not known. Both clean and oxidised nickel surfaces show a low reactivity towards H₂O; clean nickel surfaces are ~10³ times less reactive to H₂O than to oxygen.     

Sulfur dioxide adsorption and reaction with atomic oxygen on the Ag(110) surface

The adsorption of SO₂ on Ag(110) and the reaction of SO₂ with oxygen adatoms have been studied under ultrahigh vacuum conditions using low energy electron diffraction, temperature programmed reaction spectroscopy and photoelectron spectroscopy. Below 300 K, SO₂ adsorbs molecularly giving p(1×2) and c(4×2) LEED patterns at coverages of one half and three quarter monolayers. respectively. At intermediate coverages, streaked diffraction patterns, similar to those reported for noble gas and alkali metal adsorption on the (110) face of face-centered cubic metals were observed, indicating adsorption out of registry with the surface. A feature at low binding energy in the ultraviolet photoemission spectrum appeared which was assigned to a weak chemisorption bond to the surface via the sulfur, analogous to bonding observed in SO₂-amine charge transfer complexes and in transition metal complexes. SO₂ exhibited three binding states on Ag(110) with binding energies of 41, 53 and 64 kJ mol^?1; no decomposition on clean Ag(110) was observed. On oxygen pretreated Ag(110), SO₂ reacted with oxygen adatoms to form SO_3(a), as determined by X-ray photoelectron spectroscopy. Reacting preadsorbed atomic oxygen in a p(2 × 1) structure with SO₂ resulted in a c(6 × 2) pattern for SO_3(a). The adsorbed SO_3(a) decomposed and disproportionated upon heating to 500 K to yield SO_2(g), SO_4(a) and subsurface oxygen.     

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

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 CO₂ geometry and favour a coordination site with C_2v symmetry. The bent CO₂ moiety is stable up to 230 K. Further heating to room temperature leads to dissociation of the bent CO₂ molecule into adsorbed CO and O. The CO molecule is oriented with its axis perpendicular to the surface. The bent CO₂ species appears to be a precursor to dissociation. Results on CO₂ adsorption on an oxygen precovered surface show that CO₂ 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.     

Angle-resolved photoemission study of no chemisorption on Ag(111) surface

An angle-resolved photoemission study using synchrotron radiation has been carried out to investigate the adsorption of NO on Ag(111). At 150 K, NO is adsorbed molecularly on the Ag(111) surface. The NO molecule is found to have an upright orientation from a study using incidence angle dependent measurements and symmetry selection rules. The upright orientation is also confirmed from the study of the shape resonance of the 5σ level. Photon induced dissociation of the chemisorbed NO was observed and is briefly discussed.     

A Raman spectroscopic study of the polyimide/Ag(110) interface

We have investigated the bonding of the monomers pyromellitic dianhydride (PMDA) and oxydianiline (ODA) and their subsequent polymerization to form polyimide on Ag(110) in ultrahigh vacuum. Unenhanced surface Raman spectroscopy was used to identify adsorbed species and to follow the polymerization reaction. ODA was unreactive on the cold (140 K) surface; spectral features were identical to those in the condensed multilayer indicating that the monolayer is physisorbed. PMDA, on the other hand, adsorbed dissociatively, the evidence being consistent with a bidentate surface carboxylate mode of bonding. Finally, heating a 30 Å film of codosed monomers to 473 K resulted in the formation of a polyimide film as shown by characteristic features in the carbonyl stretching region of the Raman spectrum.     

Adsorption of oxygen on Ag(110) studied by high resolution ELS and TPD

Adsorption of oxygen on Ag(110) has been studied by high resolution electron energy loss spectroscopy (ELS) and temperature programmed desorption (TPD) in the temperature range from ? 160°C to 310°C. At ? 160°C oxygen is absorbed as a diatomic species. The low vibrational frequency of the O-O stretch vibration is explained in terms of charge transfer from the metal into the π¹ antibonding orbital and donation from the π bonding orbital to the metal. A tentative model is presented, according to which the molecule is adsorbed in the grooves of the (110) surface with its axis parallel to the surface. It is explicitly shown that this diatomic species is the precursor for dissociative adsorption of oxygen at temperatures above ? 100°C. Upon dissociation part of the diatomic species is desorbed. Between ? 100°C and + 310°C a single type of adsorbed atomic oxygen is observed which is desorbed at 310°C. Above 150°C adsorbed atomic oxygen also diffuses to subsurface sites. Below 450°C subsurface oxygen neither desorbs nor diffuses into the bulk, although it does exchange with adsorbed atomic oxygen at a temperature below 310°C. Therefore, both forms of atomic oxygen coexist at temperatures at which ethylene epoxidation occurs.     

Spin-resolved photoemission from Ag(111): Theory and experiment

Using circularly polarized synchrotron radiation between 14 and 24 eV, spin-resolved normal photoemission spectra have been measured from an unreconstructed Ag(111) surface. Corresponding spectra were calculated by means of a fully relativistic one-step theory of photoemission together with the bulk band structures for real and complex potentials, using two different local approximations for the exchange-correlation potential. Experiment and theory employing anX potential agree well with regard to existence and positions of peaks. Relative peak heights match except for an observed enhancement at photon energies, at which two or more direct interband transitions may resonate.