Vibrational Electron Energy Loss Spectroscopy Research Articles

Absorbed states of hydrogen on the Ni(110) surface

Absorbed states of hydrogen on the Ni(110) surface have been studied using high-resolution vibrational electron energy loss spectroscopy and low-energy electron diffraction. Structural models for the (2×1)-H, (1×2)-H and streaky (1×2)-H surfaces are discussed.

Reactions of no with the Si(111) (7 × 7) surface: EELS, LEED and AES studies

High-resolution vibrational electron energy loss spectroscopy, low-energy electron diffraction and Auger electron spectroscopy are used to study the reactions of NO with the Si(lll) (7x7) surface. At 300 K, NO is adsorbed dissociatively on the (7 × 7) surface in the disordered structure. The N and O atoms are chemisorbed in the on-top sites forming the SiN and SiO single bonds which are characterized by the vibrational losses at 118–125 and 100–105 meV, respectively. The maximum fractional coverages of N and O atoms are both θ ≈ 0.5. By heating the NO-saturated surface at 1150–1300 K, the adsorbed oxygen is removed from the surface and the N-covered surface is reconstructed to form the “(8 × 8)”-N structure which is characterized by the losses at 91 and 120 meV. This structure is proposed to arise from the overlayer in which each N atom is bonded to three Si atoms with the sp 3-like bonds forming a hexagonal lattice with unit mesh vectors of 8 11 times those of the unreconstructed Si(111) surface. As this surface is further heated at 1300–1350 K, the “quadruplet” structure which is characterized by the losses at 61 and 120 meV is formed. It is proposed that this structure has planar geometry with each N atom bonded to three Si atoms forming the sp 2-like bonds. The SiN bond length is estimated to be ≈1.7 Å. The local structure of the quadruplet surface is very similar to that of the bulk silicon nitride (Si 3N 4).

Reactions of NO with the Si(7 × 7) surface: EELS, LEED and AES studies

High-resolution vibrational electron energy loss spectroscopy, low-energy electron diffraction and Auger electron spectroscopy are used to study the reactions of NO with the Si(ll1) (7 x7) surface. At 300 K, NO is adsorbed dissociatively on the (7 X 7) surface in the disordered structure. The N and 0 atoms are chemisorbed in the on-top sites forming the Si-N and Si-0 single bonds which are characterized by the vibrational losses at 118-125 and loo-105 meV, respectively. The maximum fractional coverages of N and 0 atoms are both 6’ - 0.5. By heating the NO-saturated surface at 1150-1300 K, the adsorbed oxygen is removed from the surface and the N-covered surface is reconstructed to form the “(8 X 8)“-N structure which is characterized by the losses at 91 and 120 meV. This structure is proposed to arise from the overlayer in which each N atom is bonded to three Si atoms with the sp3-like bonds forming a hexagonal lattice with unit mesh vectors of S/11 times those of the unreconstructed Si(ll1) surface. As this surface is further heated at 1300-1350 K, the “quadruplet” structure which is characterized by the losses at 61 and 120 meV is formed. It is proposed that this structure has planar geometry with each N atom bonded to three Si atoms forming the sp2-like bonds. The Si-N bond length is estimated to be - 1.7 A. The local structure of the quadruplet surface is very similar to that of the bulk silicon nitride (Si,N,).

Coordination of acetonitrile (CH 3CN) to platinum (111): Evidence for an η 2(C, N) species

Acetonitrile (CH 3CN) 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 CH 3CN 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 η 2(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 η 2(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 η 4(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 Fe 3(η 2-NCCH 3)(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 π ∗ 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 CH 3 CN Ni(111) is also an η 2(C, N) species, not an end-bonded species previously proposed by Friend, Muetterties and Gland.

Carbon monoxide adsorption on Re(0001) between 120 and 600 K studied by vibrational electron energy loss spectroscopy

Between 120 K and room temperature carbon monoxide is molecularly adsorbed on top of the metallic atoms. A shift of the v C-O vibration, mainly due to dipole interactions, is observed as coverage increases. At 600 K a new species is observed on the surface: the β-CO which corresponds to a molecule totally or partially dissociated.

Apparatus for high-resolution vibrational electron energy-loss spectroscopy of solid surfaces: M Nishijima et al, Rev Sci Instrum, 53 (6), 1982, 790–796

Apparatus for high-resolution vibrational electron energy-loss spectroscopy of solid surfaces: M Nishijima et al, Rev Sci Instrum, 53 (6), 1982, 790–796

Chemisorptive properties of Pt-Ni(111) single crystal alloys with high platinum content: C 2H 4 and C 6H 6 adsorption

The chemisorption of ethylene and benzene has been investigated on Pt 78 Ni 22 and Pt 50Ni 50(111) single crystal alloys by vibrational electron energy loss spectroscopy and thermal desorption spectroscopy. These alloys exhibit a strong platinum segregation leading to a quasicomplete platinum layer on the surface. On Pt 78Ni 22(111), the hydrocarbonated surface complexes formed after C 2H 4 and c 6h 6 adsorption show the same main features as these recorded on Pt(111). However, evidence for more weak interactions with the metal surface is obtained. On Pt 50Ni 50(111), no C 2H 4 and c 6h 6 adsorption has been obtained, even at 180 K. These results evidence the specific behaviour of the superficial layers of quasi-pure platinum supported on the bulk alloys with respect to pure platinum: a decrease of chemisorption strengths is observed after alloying, which is much more pronounced on the Pt 50Ni 50 sample.

Identification of the intermediates in the dehydration of formic acid on Ni(110) by high resolution electron energy loss vibrational spectroscopy

High resolution electron energy loss vibrational spectroscopy was used to study the intermediate formed in the dehydration reaction for formic acid on Ni(110) and Ni(110)(4 × 5)C. On the carbided surface only the formate was observed. The frequencies of the asymmetric and symmetric OCO sketch indicated a monodentate configuration. On the clean surface a mixed adlayer of CO and HCOO formed. No losses expected for formic anhydride were observed. Lateral interactions between CO and HCOO appear to be responsible for the autocatalytic decomposition of the formate.

Spatial intensity distributions from electron impact scattering modes: W{100}(1 × 1)H

The detailed mechanism of electron energy loss vibrational spectroscopy has been examined in the present study by recording a large data base of spectra with the detector moved both in and out of the incidence plane, for saturation coverage of atomic H adsorbed on W{1OO} at 300 K. In agreement with earlier work we find only a single electron loss (130 meV) in the specular direction, while away from the specular direction additional losses at 80 meV and 160 meV appear. These losses correspond to the symmetric stretching, the wagging and the asymmetric stretching modes respectively of H atoms occupying a C 2V bridge site. Large intensity variations of all these modes are found in backscattering directions and out of the incidence plane, in general agreement with recent theoretical predictions. The usefulness of these spatial intensity variations in adsorbate structure determination will be discussed. The signal enhancement with electron collection in the plane orthogonal to the incidence plane may allow the observation of otherwise weak impact modes.

A study of nitric oxide adsorption on copper (100) and (110)

The adsorption of nitric oxide on single crystal copper (100) and (110) surfaces has been studied with electron energy loss vibrational spectroscopy, low energy electron diffraction and thermal desorption spectroscopy. In the case of Cu(110), vibrational spectra confirm that molecular adsorption of NO occurs for very low exposures with a crystal temperature of 80 K. For exposures exceeding 2.2 L, dissociation occurs with atomic oxygen and nitrogen retained on the surface and located at the long bridge site. Vibrational frequencies obtained using two isotopic forms of NO, 14NO and 15NO, are assigned to the stretching and bending modes of NO and indicate a molecule strongly tilted relative to the surface normal in a predissociative state. Dissociation occurs at temperatures above 113 K with the atomic species again retained on the surface. Adsorption of nitric oxide on Cu(100) was found to be more complicated. Vibrational spectra indicate a difference in molecular species between low exposure (∼1 L) and high exposure (∼10 L) cases for T<100 K. Depending on exposure and temperature, evidence is found for the presence of N 2O, two forms of NO, O, and probably N 2. In contrast to adsorption of NO on Cu(110), a large fraction of the NO dissociates upon initial adsorption on Cu(100). All species containing nitrogen are found to desorb or dissociate below room temperature.

Apparatus for high-resolution vibrational electron energy-loss spectroscopy of solid surfaces

A high-resolution vibrational electron energy-loss spectrometer of 127°-cylindrical electrostatic deflection type has been constructed for surface studies. In the direct mode, an electron current at the collector entrance of 2.3×10−11 A is obtained with the energy spread (full width at half-maximum) of 60 cm−1 for electrons whose energy at the sample position is 5 eV. A ghost reduction slit is used to suppress spurious peaks due to the electron reflection from electrodes of the analyzer. Electron energy-loss spectroscopy can be combined in situ with low-energy electron diffraction, Auger electron spectroscopy, and thermal-desorption mass spectroscopy. Design and performance of the apparatus are reported. The vibrational spectrum of the Ni(110) (1×2)-H surface formed at room temperature is presented, and the location of hydrogen atoms on the surface is discussed.

The chemisorption of oxygen on Cu(110) studied by EELS and LEED

Abstract The chemisorption of oxygen on Cu(110) has been studied with electron energy loss vibrational spectroscopy and low energy electron diffraction. Depending on crystal temperatures during oxygen exposure and subsequent heat treatments, (1 × 1), c(6 × 2), and (2 × 1) LEED patterns are obtained. However, in all cases and independent of exposure, a single vibrational energy was observed at 49 meV for 16 O and at 46 meV for 18 O. This invariance of energy suggests a shielded site location in or beneath the surface in support of the result of a recent ion scattering study. The (2 × 1) structure is shown to form in islands at less than saturation coverage on the basis of observed vibrational intensities.

The chemisorption of oxygen on Cu(110) studied by EELS and LEED

The chemisorption of oxygen on Cu(110) has been studied with electron energy loss vibrational spectroscopy and low energy electron diffraction. Depending on crystal temperatures during oxygen exposure and subsequent heat treatments, (1 × 1), c(6 × 2), and (2 × 1) LEED patterns are obtained. However, in all cases and independent of exposure, a single vibrational energy was observed at 49 meV for 16O and at 46 meV for 18O. This invariance of energy suggests a shielded site location in or beneath the surface in support of the result of a recent ion scattering study. The (2 × 1) structure is shown to form in islands at less than saturation coverage on the basis of observed vibrational intensities.

Vibrational EELS, XPS and UPS study of CO chemisorbed on Pt 10-Ni 90: Evidence for the existence of different sites

CO adsorption on the (111) face of a Pt 10-Ni 90 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 4σ, 5σ and 1π molecular orbitals 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.

Vibrational EELS, XPS and UPS study of CO chemisorbed on Pt 10Ni 90(111): Evidence for the existence of different sites

CO adsorption on the (111) face of a Pt 10Ni 90 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.

Vibrational EELS studies of CO chemisorption on clean and carbided (111), (100) and (110) nickel surfaces

Room temperature adsorption of CO on bare and carbided (111), (100) and (110) nickel surfaces has been studied by vibrational electron energy loss spectroscopy (EELS) and thermal desorption. On the clean (100) and (110) surfaces two configurations of CO adsorbed species, namely “terminal” and bridge bonded CO, are observed simultaneously. On Ni(111), only two-fold sites are involved. The presence of superficial carbon lowers markedly the bond strength of CO on Ni(111)C and Ni(110)C surfaces, while no adsorption has been detected on the Ni(100)C surface. Moreover, on the carbided Ni(110)C surface, the adsorption mode for adsorbed CO is changed with respect to the clean surface; only “terminal” CO is then observed.

Vibrational spectra of molecular and atomic oxygen on Ag(11O)

Atomic and molecular states of bonding of oxygen have been resolved on Ag(11O) by electron energy loss vibrational spectroscopy. The O-O bond order is reduced below unity, indicating appreciable charge transfer into the antibonding orbitals of O2 by analogy with O2 on Pt(111). Dissociation of the molecular oxygen into the atomic form was observed above 200 K.

C 2H 4 adsorption on the Ni(111) face at room temperature: Vibrations of surface complexes studied by electron energy loss spectroscopy

Ethylene adsorption at room temperature on the Ni(111) surface has been followed by vibrational high resolution electron energy loss spectroscopy coupled with LEED, work function changes and thermodesorption measurements. The vibrational spectra of adsorbed species exhibit loss peaks at 365, 151, 85, 60 and 39 meV (274, 147, 79 and 39 meV with C 2D 4). The similarity of these spectra with those obtained after short exposure to acetylene suggests a dehydrogenating adsorption well confirmed by thermodesorption data: C 2H 4 gas→ C 2H 2ads + 2 H ads. From the observed v C-C frequencies (151 and 147 meV for the respective C 2H 2 and C 2D 2 adsorbed species) one can deduce a residual force constant ƒ C-C ≅ 5.25 mdyn/Å. By comparison to gaseous molecules, this value suggests a near sp 3 hybridization state of the C atoms confirmed by the position of the v C-H stretching mode (365 meV). Taking in account these results, a triangular surface chemisorption site for the C 2H 2 species with two σ bonds and one π bond to three Ni atoms is suggested. Frequency analysis of this model allows to fit the experimental loss energies for reasonable force constant values, the C 2H 2 residue lying flat on the triangular site with a bent H-C-C-H arrangement (C-C-H angle ≅ 155°).

An ELS vibrational study of carbon monoxide adsorbed on the Cu(110) surface

The adsorbate site location for CO on Cu(110) has been investigated with electron energy loss vibrational spectroscopy. With low coverage at 80 K vibrational energies of 85 and 258 meV are observed. At high coverages and/or near the thermal desorption temperature of 170 K, only a single peak at 260 meV is observed. A simple spring model analysis of the vibrational energies indicates that a bridge position is occupied by CO in the first case and the on-top position in the second case although LEED results indicate that CO cannot be highly localized at any one site in the high coverage case.

Coadsorption of carbon monoxide and hydrogen on a Ni(111) surface: Influence of the “surface carbide”

Vibrational electron energy loss spectroscopy (HRELS) combined with LEED, work function changes and thermodesorption measurements have been used to study H 2 CO interactions on a Ni(111) surface. Coadsorption of H 2 and CO on a Ni(111) surface, maintained at 140°C and then cooled down to room temperature, under the reactional mixture, leads to the appearance of three surface species: (a) a weakly bonded CO thermally desorbed around 75°C and exhibiting a ν-C0 frequency loss peak near 221 meV; (b) an oxyhydrocarbonated compound characterized by vibrations at 362, 178 and 168 meV and by simultaneous desorption of H 2 and CO at 180°C; (c) a hydrocarbonated species which dehydrogenates at 280°C and exhibits vibrations at 376 and 95 meV. Since disproportionation of CO occurs at 140°C and leaves the so called “surface carbide” behind, it is possible that this carbide plays an important role in the formation of the observed surface species. In fact, coadsorption of CO and H 2, at room temperature on a carburized Ni(111) surface, allows the appearance of the weakly bonded CO in addition to the oxyhydrocarbonated species. Nevertheless, the third species (hydrocarbonated) is then not observed. Tentative identification of the different surface complexes is developed by reference to their vibrational spectra.