EELS analysis of the low temperature phase of ethylene chemisorbed on Pd(111)

The chemisorption of C₂H₄ and C₂D₄ on Pd(111) at 150 K has been studied by high resolution electron energy loss spectroscopy. Analysis of the vibrational spectra indicates that (i) C₂H₄ is more weakly bound on Pd(111) than on Ni(111) and Pt(111) and (ii) softened and broadened CH stretching frequencies suggest hydrogen bond-like interactions between the molecule and the metal surface.     

Neutron inelastic spectroscopy of benzene chemisorbed on Raney platinum

The neutron inelastic scattering spectrum of benzene adsorbed at 300 K on Raney platinum has been measured between 350 and 2250 cm^?1. No deshydrogenation of the molecules is observed so that the benzene ring must be adsorbed parallel to the surface. Slight modifications of the force field of the model molecule (C₆H₆)Cr(CO)₃ were introduced to account for the vibrational frequency shifts. The benzene molecule is found less perturbed on platinum than on nickel. The calculated frequencies of adsorbed C₆H₆ and C₆D₆ are used to reassign some modes previously observed by electron loss spectroscopy.     

Benzene adsorption on nickel (100) and (111) faces studied by leed and high resolution electron energy loss spectroscopy

Studies of benzene (C₆H₆ and C₆D₆) adsorption have been performed by high resolution electron energy loss spectroscopy (HRELS) and LEED experiments on nickel (100) and (111) single crystal faces at room temperature. Chemisorption induces ordered structures, c(4 × 4) on Ni(100) and (2√3 × 2√3)R30° on Ni(111), and typical energy loss spectra with 4 loss peaks accurately identified with the strongest infrared vibration bands of the gazeous molecules. Benzene chemisorption preserves the aromatic character of the molecule and involves respectively 8 nickel surface atoms on the (100) face and 12 on the (111) face by adsorbed molecule. The interaction takes place via the π electrons of the ring. Significant shifts of the CHτ bending and CH stretching vibrations show a weakening of the CH bonds due to the formation of the chemisorption bond and a coupling of H atoms with the nickel substrate.     

A model of ethylene and acetylene adsorption on the (111) surfaces of platinum and nickel

Despite the application of a variety of surface sensitive techniques to the adsorption of simple hydrocarbons on well characterized metallic surfaces, no consistent picture has appeared. We review briefly the published spectroscopic results of ultraviolet photoelectron spectroscopy (UPS) and electron energy loss spectroscopy (EELS) which probe, respectively, the electronic and vibrational structure of the surface-molecular complex, and we consider appropriate free molecular analogues, not only in their ground state but also in their first excited states. A simplified approach to determine the chemisorption geometry from UPS level shifts and EELS is presented. The technique allows an isolation of distortion induced shifts from the total relaxation shift, and we find that the true relaxation shift is rather constant, approximately 2.1 eV for the cases considered. These shifts can then be used to estimate the distance of the molecule to the surface. We concentrate primarily on four systems, C₂H₂ and C₂H₄ on Ni(111) and Pt(111), adsorbed at low temperature (below the onset of dissociation). Depending on the metal, the hydrocarbon can adsorb in a di-σ arrangement or with a distortion resembling the lowest energy configuration of the first excited state of the free molecule. We also consider briefly C₂H₄ on Ag and Cu in which no distortion occurs. The distortions that resemble the first excited states might occur as a consequence of donation of bonding (backbonding) electrons from (to) the normally filled π (empty π¹) to (from) the empty (filled) d-band states of the metal. The net effect on the hydrocarbon to partially empty the π level and fill the π¹ level, is analogous to a low excitation of the free molecule, π → π¹. For C₂H₄ (planar in the ground state), the lowest excitation is the triplet T-state (3–4 eV) of minimal energy for a 90° twisted configuration with a lengthened C-C bond. Acetylene is a linear molecule in the ground state, but cis- or trans-bent for the triplet excitations, ~a (5.2 eV) or ~b (6.0 eV), respectively. Chemisorbed geometries derived from these configurations seem possible for C₂H₄ on Ni(111) and C₂H₂ on Pt(111), while interchanging the adsorbates and substrates gives di-σ bonding, (sp³ hybridization), as proposed previously in the literature. For C₂H₄ on Ni(111), two of the hydrogens are twisted into the surface which leads to a softening of the CH vibrational frequency. For the four systems considered, the data are consistent with the C-C bond essentially parallel to the surface, but tilted orientations are not ruled out. While the models are clearly oversimplified, they suggest an interesting point of departure for likely chemisorption geometries. Also, some intriguing correspondences to the (presumed) location of the normally empty π¹ level and the d-band are noted.     

LEED,AES and thermal desorption studies of chemisorbed Hydrogen and hydrocarbons (C2H2, C2H4, C6H6, C6H12) on the (111) and stepped [6(111) × (100)] iridium crystal surfaces; comparison with platinum

The adsorption of hydrogen, ethylene, acetylene, cyclohexane and benzene was studied on both the (111) and stepped [6(111) × (100)] crystal surfaces of iridium. The techniques used were low energy electron diffraction, Auger electron spectroscopy, and thermal desorption mass spectrometry. At 30°C, acetylene, ethylene and benzene are adsorbed with a sticking probability near unity. The sticking probability of cyclohexane is less than 0.1 on both surfaces. Heating the (111) surface above 800°C, in the presence of the hydrocarbons, results in the formation of an ordered carbonaceous overlayer with a diffraction pattern corresponding to a (9 × 9) surface structure. No indication for ordering of the carbonaceous residue was found on the stepped iridium surface in these experimental conditions. The hydrocarbon molecules form only poorly ordered surface structures on both iridium surfaces when the adsorption is carried out at 30°C. Benzene is the only gas that can be desorbed from the surfaces in large amounts by heating. Ethylene remains largely on the surface, only a few percent is removed by heating while acetylene and cyclohexane cannot be desorbed at all. When adsorption is carried out at 30°C and the crystal is subsequently flashed to high temperature, hydrogen is liberated from the surface. The hydrogen desorption spectra from the iridium surfaces exposed to C₂H₄, C₂H₂, or C₆H₆ exhibit two hydrogen desorption peaks, one around 200°C and the second around 350°C. The temperatures where these peaks appear vary slightly with the type of hydrocarbon. The relative intensities of these two peaks depend strongly on the surface used. Arguments are presented that decomposition of the hydrocarbon molecules (C-H bond breaking nd possibly also C-C bond breaking) occurs easier on the stepped iridium surface than on the (111) surface. Hydrogen is desorbed at a higher temperature from an iridium surface possessing a high density of surface imperfections than from a perfect iridium (111) surface. The results are compared with those obtained previously on similar crystal surfaces of platinum. It appears that C-H bond breaking occurs more easily on iridium than on platinum.     

The interaction of CO and O2 with the (111) surface of Pt3Ti

The electronic properties of clean and partly oxidized Pt₃Ti(111) surfaces have been studied utilizing carbon monoxide both as a probe and as a reducing agent. Vibrational frequencies and desorption profiles of chemisorbed CO as well as ion scattering and angular resolved X-ray photoelectron spectroscopy (XPS) suggest that the first atomic layer of annealed Pt₃Ti(111) is quasi-pure platinum. Scarcely any (θ ≈ 0.01) dissociation of CO was observed. Minor shifts of vibrational frequencies and desorption temperatures compared to Pt(111) and a p(2 × 2) “reconstruction” of the clean surface reveal some influence of the bulk. Auger spectroscopy, XPS, and ion scattering all show an increased titanium signal as a result of oxidation. Surface bound atomic oxygen gives a vibrational band around 650 cm⁻¹ which coincides with infrared absorption spectra of TiO₂. Flashing with CO shifts the band to 500 cm⁻¹. Correlated with this shift we observe (i) CO₂ desorption at a temperature well above that observed for Pt(111)/O, (ii) an altered Ti XPS signal, and (iii) a reduced oxygen concentration. Subsequently adsorbed CO molecules vibrate at the same frequencies as on the bare surface, give the same c(4 × 2) LEED pattern, and desorb at the same temperatures but with reduced intensity, in all proving that the surface oxide only acts as a site-blocker with respect to the metal surface. Our current understanding of these observations is that oxygen creates “islands of TiO₂”, segregated to the surface but with no electronic influence on remaining areas of the platinum enriched metal surface. The hexacoordinated Ti⁴⁺ ions on the surface of these islands are reduced by CO to pentacoordinated Ti³⁺ species. The vibrational shift, 650 to 500 cm⁻¹, can be understood by the dipole active bands of a triatomic O−Ti⁴⁺ −O vibrator compared to a diatomic Ti³⁺−O vibrator.     

Lock and key behaviours of an aromatic carboxylic acid molecule with differing conformations on an Au (111) surface

Tricarboxylic acid derivative with three-fold symmetry in physisorbed monolayers is an important organic molecule for applications in nanotechnology. In this paper, the behaviour of a single tricarboxylic acid derivative, 1,3,5-tris(carboxymethoxy)benzene (TCMB, C₆H₃(OCH₂COOH)₃) on an Au (111) substrate at 50 K is investigated by molecular dynamics simulation. Four possible conformations of the TCMB molecule adsorbed on the Au (111) substrate are found, the difference of which is the orientation of the CH2 chain. They also display different trajectories of movement and dynamical behaviours. As the molecule migrates across the Au (111) surface, the translational motion of TCMB is always accompanied by rotational motion. The lock-and-key (LAK) geometry between the TCMB molecule of different conformations and atomic arrangement of the Au (111) surface was also observed in this study, and the result has been verified by an ab initio calculation.     

Mo(CO)6在清洁的和氧化的Si(111)表面上吸附的对比

采用高分辨电子能量损失谱对比研究Mo(CO)₆在清洁的、预吸附氧的和深度氧化的Si(111)表面上的吸附行为. 吸附Mo(CO)₆的C-O伸缩振动模式向低频方向移动,说明Mo(CO)₆与清洁Si(111)和SiO₂/Si(111)表面发生了不同的相互作用,前者较弱而后者较强. 与SiO₂/Si(111)表面的强相互作用可能引起Mo(CO)₆部分解离,形成部分分解的羰基钼物种.     

Structure and electronic factors in benzene coordination to Cr(CO)3 and to cluster models of Ni,Pt, and Ag (111) surfaces

Bonding of benzene to a chromium tricarbonyl fragment and to cluster models of silver, nickel, and platinum (111) surfaces is found by means of molecular orbital calculations to be dominated by a benzene donation bond involving its π (e_1g) orbitals and metal d orbitals. Three-fold hollow sites on the clusters are calculated to be most stable for benzene coordination, a conclusion reached in a number of experimental studies of benzene on metal surfaces. On the Ag, Ni, and pt clusters, the benzene CH bonds are found to bend away from the surface by ?2°, 8° and 19°, respectively, a result of carbon atom hybridization to maximize overlap with metal orbitals. For benzene chromium tricarbonyl, the CH bonds are calculated to bend 3° toward the metal, compared to a 1.7° bend reported in a diffraction study. The direction and magnitude of the CH bending are shown to depend on the metal d orbital occupancy (an electronic factor) and the proximity of metal atoms in the adsorption site (a structure factor). Small Kekulé distortions are calculated for the chromium complex and for the C_3v sites on Pt(111). Finally, recent experimental studies showing a decrease in benzene adsorption energy when potassium is coadsorbed on Pt(111) may be understood to result from decreased π donation which accompanies the shift up of the metal d band with cathodic charging.     

H原子在Al(111)和Ag(111)面上的吸附

本文用电荷自洽的EHT方法,研究了H原子在Al(111)和Ag(111)面上的吸附,结果指出:在Al(111)面上,H以原子状态吸附在某些对称位置上,它也能渗透到表面层中去,成为填隙原子;H₂分子在表面处发生解离吸附。在Ag(111)表面上,H原子有可能以分子状态吸附,H—H键平行于表面,这与高分辨率电子能量损失谱所得到的实验结果一致;但H₂分子在Ag(111)表面也可能发生解离吸附。 关键词：     

The adsorption of N2: Chemisorbed on Ni(110) and physisorbed on Pd(111)

The bonding of molecular N₂ has been investigated with angle resolved photoelectron spectroscopy and inelastic electron scattering. The spectra obtained from N₂ chemisorbed onto a Ni(110) surface are compared to CO chemisorbed onto Ni(110) and to N₂ physisorbed onto Pd(111). The N₂ molecular axis was found to be normal to the crystal surface for the chemisorbed state on Ni(110) and random for the physisorbed state on Pd(111). The NN and NiN₂ stretching frequecies indicate that the N₂ molecule is terminally bonded to a single Ni atom on Ni(110). The binding energies of the two outer σ states and one π state of chemisorbed N₂ were measured, indicating that the bonding of N₂ to a metal surface is different than CO. Both σ states drop in energy compared to the π level due to the fact that both of them are involved in the N₂ substrate bond. The symmetry of the gas phase N₂ molecule is reduced upon adsorption. The consequences of this are seen in the dipole active NN vibrational mode, the large intensity of the Ni-N₂ vibrational mode and the coupling of the adsorbate 4σ(2σ_u) level to the final state resonance which is forbidden by symmetry in the gas phase. Many electron excitation satellite lines are observed in the valence spectra of both the chemisorbed and physisorbed N₂. The physisorbed satellite lines are nearly identical to those seen in gas phase N₂, while the chemisorbed N₂ spectra has new satellite structure, due to the interaction with the substrate.     

Low temperature electron energy loss spectra of acetylene chemisorbed on metal single-crystal surfaces; Cu(111), Ni(110) and Pd(110)

Vibrational spectra of acetylene chemisorbed on Cu(111), Ni(110) and Pd(110) at 110–120 K were measured using electron energy loss spectroscopy. Loss peaks were assigned to vibrational modes of the non-dissociatively adsorbed molecules with the aid of the corresponding C₂D₂ spectra. The spectra show that the molecules undergo significant rehybridisation on adsorption. Comparisons are made with the spectra of acetylene adsorbed on a range of other transition metal surfaces at low temperature. Taking into account these and earlier literature results, two distinct patterns of spectra are observed (Type A and Type B) for specular spectra. The Cu(111) spectrum is classified as Type A while the Ni(110) and Pd(110) spectra are classified as Type B. Suggestions are made for the structures of the surface species corresponding to the two spectral types.     

Raman spectra of molecules considered to be surface enhanced

High resolution electron energy loss spectroscopy (HREELS) and low energy electron diffraction (LEED) have been used to study the structure of adsorbed benzene (C₆H₆ and C₆D₆) monolayers on the Rh(111) surface at 300K. A surface bonding geometry is proposed for benzene adsorbed to give a c(2?3×4) rectangular structure, which involves very little perturbation of the molecular structure with the ring plane parallel to the surface. Only one chemical environment for adsorbed benzene is indicated by a single frequency shift of the symmetric CH out-of-plane bending mode. The adsorption site is tentatively assigned to benzene centered over a single Rh atom.     

Coordination of acetonitrile (CH3CN) to platinum (111): Evidence for an η2(C,N) species

Acetonitrile (CH₃CN) coordination to a Pt(111) surface has been studied with electron energy loss vibrational spectroscopy (EELS), XPS, thermal desorption and work function measurements. We compare data for the surface states with known acetonitrile coordination complexes. For CH₃CN adsorbed on Pt(111) at 100 K, the molecule is rehybridized and adsorbs with the CN bond parallel or slightly inclined to the surface plane in an η²(C, N) configuration. The ν(CN) frequency is 1615 cm^?1 and the C ls and N ls binding energies are 284.6 eV and 397.2 eV respectively. By contrast, weakly adsorbed multilayer acetonitrile exhibits a ν(CN) vibrational frequency of 2270 cm^?1, and C ls and N ls binding energies of 286.9 eV and 400.1 eV respectively. Both the EELS and XPS results are consistent with rehybridization of the CN triple bond to a double bond with both C and N atoms of the CN group attached to the surface. In addition to this majority η²(C, N) monolayer state, evidence is found for a second, more strongly bound minority molecular state in thermal desorption spectra. As a result of the low coverage of this state, EELS was unable to spectroscopically identify it and we tentatively assign it as an η⁴(C, N) species associated with accidental step sites. By contrast to the surface complexes, almost all of the known platinum-nitrile coordination complexes are end-bonded via the N lone-pair orbital. Several cases of side-on bonding are known, however, and we compare the results with the known complex Fe₃(η²-NCCH₃)(CO)₉. The difference in the coordinative properties of a Pt(111) surface versus a single Pt atom must be due to the increased ability of multi-atom arrays to back-donate electrons into the π¹ system of acetonitrile. Previously published EELS and XPS results for monolayer acetonitrile on Ni(111) and polycrystalline films are almost identical to the present results on Pt(111). We believe that the monolayer of $\text{CH}{}_3\text{CN}\text{Ni}\text{(111)}$ is also an η²(C, N) species, not an end-bonded species previously proposed by Friend, Muetterties and Gland.     

Benzene and thiophene formation from acetylene on sulfided Pd(111) studied by LITD/FTMS

We report the first laser-induced thermal desorption (LITD) studies of acetylene on Pd(111). LITD coupled with Fourier transform mass spectrometry (FTMS) probes the surface molecular composition. Our results show simultaneous formation of thiophene and benzene at 120 K after a 6 L dose of acetylene at 80 K on a Pd(111) surface with 0.06 ML of sulfur. Simultaneous formation implies that formation of the C₄H₄ intermediate is the slow step in the formation of both cyclic products. The relative amounts of thiophene and benzene observed with LITD/FTMS are comparable, while thermal desorption spectroscopy (TDS) yields integrated thiophene signals that are < 2% as intense as for benzene. This indicates that thiophene primarily decomposes upon heating. Low coverage (0.5 L) results confirm reports that the presence of sulfur enhances benzene production.     

Vibrational EELS,XPS and UPS study of CO chemisorbed on Pt10Ni90(111): Evidence for the existence of different sites

CO adsorption on the (111) face of a Pt₁₀Ni₉₀ alloy single crystal has been investigated at room temperature by vibrational electron energy loss spectroscopy (EELS) and photoelectron spectroscopy (XPS and UPS). Two well separated CO stretching modes develop at 2070 and 1820 ± 10 cm^?1, with their intensities reaching 64 and 36% respectively of the total intensity at saturation coverage. They are attributed to CO adspecies in terminal and bridge bonded configuration respectively. The UPS spectra of 4σ, 5σ and 1π molecular orbitais of adsorbed CO show complex features which may be resolved into two components having the main characteristics of CO adsorbed on pure Pt(111) and Ni(111) respectively. Such behaviour is also observed by XPS on C 1s on O 1s peaks. Their respective contributions, in both XPS and UPS spectra are about 64 and 36% of the whole spectrum. Finally compared to Ni(111) — on which CO adsorbs mainly in bridge configuration — the alloying with 10% Pt has generated the appearance of a large number of new sites for CO chemisorption associated with the presence of Pt atoms at the surface. The large amount of terminal CO adspecies is interpreted in terms of considerable surface enrichment of the alloy in platinum.     

Thermal evolution of acetylene and ethylene on Pd(111)

The thermal evolution of acetylene and ethylene and their deuterated counterparts on a palladium (111) surface has been studied by high-resolution electron energy loss spectroscopy in the temperature range 150–500 K. Analysis of the vibrational spectra indicates that chemisorbed acetylene evolves at 300 K in the presence of surface hydrogen to mainly ethylidyne, CCH₃, and a small amount of residual acetylene. Spectra obtained with and without preadsorbed hydrogen provide evidence for a 〉C CH₂ intermediate in the reaction. Chemisorbed ethylene also evolves to ethylidyne after heating from 150 to 300 K but much of the ethylene desorbs. The high temperature (400–500 K) behavior of C₂H₂ and C₂H₄ involves formation of a CH species. Although a small amount of the CH species may be formed from the dehydrogenation of ethylidyne, it is found that carbon-carbon bond scission of acetylene near 400 K is the dominant mechanism in CH formation.     

Experimental study of the vibrations of acetylene chemisorbed on Ni(111)

High resolution electron energy-loss measurements of normal and deuterated acetylene chemisorbed on Ni(111) have been obtained. Observed vibrational modes are identified using the frequency shifts for the deuterated species and comparisons to the free molecule and a di-cobalt compound of acetylene. These vibrational frequencies indicate that chemisorbed acetylene is strongly rehybridized having a state of hybridization between ~sp^2.5 and sp³. Consideration of the types of modes observed, their assignments and the surface selection rule suggests a molecular orientation with the C-C bond axis slightly skewed relative to the surface and with the plane of the distorted molecule normal to the surface. A bonding geometry is proposed which has the carbon atoms residing above two adjacent 3 fold hollow sites of the Ni surface. This molecular geometry differs from that deduced previously by electron energy-loss spectroscopy for molecularly adsorbed acetylene on Pt(111).     

High resolution electron-energy-loss spectroscopy of vibrational states of absorbates on (111) tungsten

Electron-energy-loss spectra are reported for adsorbed species on a (111) tungsten surface following exposure to gaseous H₂, D₂ and C₂H₂ at 300 K. Vibrational frequencies are resolved which are characteristic of the chemical and surface structural nature of the adsorbates. Dissociative chemisorption occurs in all cases at low coverage (?1 L exposure). At high coverages, acetylene shows complex coverage-dependent chemisorption behaviour. A transition from dissociative to molecular chemisorption occurs in the range 2 to 4 L exposure with corresponding spectral and work function changes. Fundamental, overtone and combination frequencies are resolved, which serve to identify a single surface molecular complex at high coverage. A comparison of the results with the vibrational excitation energies of free molecules indicates a high-coverage surface molecular complex in which significant rehybridization of the C atom has occurred.     

Scanning tunneling spectroscopy and manipulation of C<Subscript>60</Subscript> on Cu(111)

The properties of C₆₀ adsorbed on Cu(111) have been studied using low temperature scanning tunnelling microscopy and spectroscopy. In the electronic spectrum of the molecule, we observe features that can be assigned to molecular orbitals. The LUMO level is split into two states, as a consequence of the charge transfer from the substrate to the carbon cage. The data from the inelastic electron tunnelling spectroscopy reveal two peaks that can be assigned to the intramolecular vibrational modes of the C₆₀ cage. We demonstrate also controlled manipulation of single molecules. The plot of the tip height, recorded during the manipulation process, indicates that the C₆₀ is pushed along the surface. PACS 68.37.Ef; 73.61.Wp; 68.43.Pq; 82.37.GK; 68.43.-h