Vibrational Electron Energy Loss Spectroscopy Research Articles

Introduction to Surface Spectra of Oxides II

This issue of Surface Science Spectra (SSS) is the second of two special issues dedicated to oxide surfaces. The Introduction contains a summary table listing all the submissions in both issues. Ultraviolet photoelectron spectroscopy (UPS), x-ray photoelectron spectroscopy (XPS), x-ray photoelectron diffraction (XPD), and high-resolution electron energy-loss spectroscopy (HREELS) for a wide range of oxides are included. Because HREELS (a vibrational electron energy loss spectroscopy) and XPD are new to SSS, a short introduction to these methods is provided.

Submonolayer coadsorption of chlorine and oxygen on Ag(110) studied by electron energy loss vibrational spectroscopy

We present a detailed investigation of the submonolayer coadsorption of Cl and O on Ag(110) at room temperature. Our main method is high-resolution electron energy loss spectroscopy (HREELS), supplemented by LEED, XPS and angle-resolved UPS. Preadsorbed chlorine at coverages below about 0.15 monolayer and subsequent exposure to causes the development of separate domains covered with either Cl or the O added row reconstruction. Above 0.2 monolayers of Cl precoverage, oxygen adsorption is inhibited completely. By contrast, oxygen preadsorbed in the O structure is almost completely removed by subsequent exposure to chlorine, and the added rows are dissolved.

Resonant vibrational excitation and electron induced dissociation processes in HREELS studies of trimethylindium chemisorbed on GaAs(100)

Vibrational high resolution electron energy loss spectroscopy (HREELS) has been used to monitor the wide variety of electron induced processes involving excited electronic states for trimethylindium ((CH 3 ) 3 In) chemisorbed on GaAs(100). The vibrational modes of the adsorbed CH 3 groups undergo strong resonant vibrational excitation at 10 eV. HREEL spectra of the adsorbed layer, recorded with beam energies greater than 8 eV, indicate the formation of a surface CH 2 species characterised by a single deformation mode (δ(CH 2 )) at 1340 cm −1 . The conversion of CH 3 to CH 2 rises dramatically at the resonance energy of 10 eV and reaches a broad maximum between 20 and 30 eV. The implication is that the incident HREELS beam leads to decomposition of adsorbed CH 3 groups. It is suggested that resonant dissociative electron attachment and dipolar dissociation processes are responsible for the conversion of CH 3 to CH 2 at the surface.

Resonant vibrational excitation and electron induced dissociation processes in HREELS studies of trimethylindium chemisorbed on GaAs(100)

Vibrational high resolution electron energy loss spectroscopy (HREELS) has been used to monitor the wide variety of electron induced processes involving excited electronic states for trimethylindium ((CH 3) 3In) chemisorbed on GaAs(100). The vibrational modes of the adsorbed CH 3 groups undergo strong resonant vibrational excitation at 10 eV. HREEL spectra of the adsorbed layer, recorded with beam energies greater than 8 eV, indicate the formation of a surface CH 2 species characterised by a single deformation mode (δ(CH 2)) at 1340 cm −1. The conversion of CH 3 to CH 2 rises dramatically at the resonance energy of 10 eV and reaches a broad maximum between 20 and 30 eV. The implication is that the incident HREELS beam leads to decomposition of adsorbed CH 3 groups. It is suggested that resonant dissociative electron attachment and dipolar dissociation processes are responsible for the conversion of CH 3 to CH 2 at the surface.

ChemInform Abstract: Vibrational Electron Energy Loss Spectroscopy (for Catalyst Characterization)

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The adsorption of sulfur on nickel: an electron energy loss spectroscopy investigation of Ni(110)/c(2×2)S and a LCGTO-LDF cluster model study

A c(2×2) sulfur overlayer on Ni(110), produced by adsorption and thermal dissociation of H 2S has been investigated with vibrational electron energy loss spectroscopy (EELS) and a feature at 331 cm −1 is assigned as a nickel-sulfur stretch. Also, cluster models of sulfur adsorption at Ni(001), Ni(110) and Ni(111) have been studied using the linear combination of Gaussian-type orbitals local density functional (LCGTO-LDF) method. The equilibrium bond distances and the absorbate vibrational stretching frequencies are found to be in good agreement with experiment, confirming the experimental assignment of the Ni(110)/S vibrational data.

Vibrational spectra and thermal decomposition of methylamine and ethylamine on nickel(111)

The bonding and geometry of methylamine (CH{sub 3}NH{sub 2}) and ethylamine (CH{sub 3}CH{sub 2}NH{sub 2}) on Ni(111) have been investigated with high-resolution electron energy loss vibrational spectroscopy (HREELS). Both amines adsorb molecularly at 150 K through the nitrogen lone pair. Significant metal-hydrogen interactions in the alkyl chain were indicated by {open_quotes}softened{close_quotes} C-H stretching modes with frequencies shifted to 2660-2680 cm{sup {minus}1}. Temperature-programmed desorption (TPD) and HREELS were used to monitor their desorption and thermal decomposition on the Ni(111) surface. Both CH{sub 3}NH{sub 2} and CH{sub 3}CH{sub 2}NH{sub 2} are dehydrogenated in the temperature range 300-400 K. CH{sub 3}NH{sub 2} is dehydrogenated to HCN at about 330 K, which further decomposes above 360 K. CH{sub 3}CH{sub 2}NH{sub 2} is dehydrogenated to CH{sub 3}CN, initially by {alpha}-C-H bond scission, leading to desorption of that molecule at 350 K. On the basis of their spectra, the authors propose a mechanism for the dehydrogenation processes of CH{sub 3}NH{sub 2} and CH{sub 3}CH{sub 2}NH{sub 2} on Ni (111). 33 refs., 13 figs., 3 tabs.

Chemically modified semiconductor surfaces: 1,4-phenylenediamine on Si(100)

The formation of inorganic/organic hybrid systems in ultra-high-vacuum (UHV) is studied for the adsorption of 1,4-phenylenediamine (PDA) and aniline on Si(100)(2 × 1) single crystal surfaces. X-ray photoemission spectra (XPS) of PDA and aniline adsorbed on silicon are interpreted in terms of an adsorbate configuration involving two covalent bonds between one amino group per molecule and the dangling bonds of the clean silicon surface. Vibrational high resolution electron energy loss spectroscopy (HREELS) allows us to identify the remaining amino groups in the PDA-adsorbate. Ultraviolet photoemission spectroscopy (UPS) and electron energy loss (EELS) data in the valence band range indicate that the aromatic ring stays intact after the chemisorption of PDA. Changes in work function and energetic shifts of the electronic transitions characterize the charge transfer between adsorbate and substrate. The experimental results are finally discussed in the framework of semi-empirical cluster calculations using MNDO-PM3.

ChemInform Abstract: Surface Reactivity and Surface Spectroscopies

Abstract The formation of gaseous reaction products which result directly from surface reactions can be followed using multiplexed quadrupole mass spectrometry in ultrahigh vacuum by temperature-programmed reaction spectroscopy. Other methods such as electron energy loss vibrational spectroscopy and near-edge X-ray absorption fine structure analysis provide important information about the identity and structure of the intermediates. In this paper the combined use of these methods for understanding the oxidation of acetonitrile and formic acid on Ag(110) is described. For the -CH 2 CN intermediate formed in acetonitrile oxidation, interesting local azimuthal ordering of the -CN group along the [001] direction is observed. In another example, the combination of these methods shows that near-edge analysis is capable of detecting the dynamical moation of the adsorbed formate species on Ag(110).

Reactions of weak organic acids with oxygen atoms on Ag(100): Facile and selective conversion of cyclohexene to benzene

The reactions of cyclohexene with atomically adsorbed oxygen on Ag(100) have been examined by both temperature programmed reaction mass spectrometry and electron energy loss vibrational spectroscopy. Cyclohexene reacts with adsorbed oxygen atoms to form benzene and surface hydroxyl groups, with benzene formation complete by 200 K, as evidenced by the vibrational spectra. Benzene formed from cyclohexene dehydrogenation remains chemisorbed on the silver surface until 275 K, when it desorbs. Surface hydroxyls undergo disproportionation at 300 K to form gaseous water and chemisorbed oxygen atoms. Competing with benzene formation is another reaction pathway which leads to cyclohexene combustion. Vibrational spectroscopy indicates that a key intermediate on the pathway to cyclohexene contains an intact C-O bond.

Cleavage of NH bonds by active oxygen on Ag(110): II. Selective oxidation of ethylamine to acetonitrile

The adsorption of ethylamine on Ag(110) and its reaction with adsorbed oxygen was studied with temperature programmed reaction spectroscopy (TPRS), X-ray photoelectron spectroscopy (XPS), and electron energy loss vibrational spectroscopy (EELS). Adsorption of ethylamine on the clean Ag(110) surface proceeds at 110 K without dissociation. Desorption from the multilayer occurs at 160 K, and a series of broader desorption peaks from monolayer states are observed from 150 to 400 K. The chemical shift of the N(1s) X-ray photoelectron spectrum indicates that ethylamine bonds through the nitrogen lone pair in the monolayer. The reaction of ethylamine with adsorbed oxygen starts with the formation of adsorbed CH 3CH 2NH and adsorbed hydroxyl groups upon adsorption at 110 K. These species undergo further reaction to yield water from 280 to 370 K, and hydrogen, acetonitrile, and regenerated ethylamine at 370 K. Acetonitrile is the only carbon-containing product observed (other than ethylamine), indicative of highly selective bond breaking processes. Deuterium labeling experiments showed that preadsorbed oxygen activates the NH bonds, leading to water formation, while the hydrogen product evolved at higher temperatures originates from the carbon skeleton via CH bond metallation by the silver surface. Disproportionation of CH 3CH 2NH groups was identified. Ethylamine is formed by rate-limiting CH bond cleavage of adsorbed CH 3CH 2N. No evidence was found for the activation of either CC or CN bonds, and the surface metallation of NH bonds does not occur.

The combined use of spectroscopies in the study of reaction intermediates on surfaces

Vibrational Spectroscopy plays a key role in the identification of intermediates formed during chemical transformations on surfaces. In combination with temperature programmed methods and photoelectron spectroscopies a great deal has been learned about the identity and structure of such reaction intermediates. In this paper attention is focused on the combined use of temperature programmed reaction spectroscopy (TPRS), electron energy loss vibrational spectroscopy (EELS) and near edge X-ray absorption fine structure (NEXAFS) analysis. Two systems are considered: (1) carbonate anion on Ag(110) and (2) -CH 2CN on Ag(110). These systems illustrate, in turn, the combination needed to determine the orientation of the carbonate anion and the corrrect assignment of vibrational modes, the orientation and mode of bonding of the nitrile to silver.

The shape resonance for CO chemisorbed on Ni(110) studied by vibrational electron energy loss spectroscopy

Vibrational high-resolution electron energy loss spectroscopy has been used to identify the shape resonance for CO chemisorbed on Ni(110) at room temperature. The intensity of the CO stretching vibration is found to be enhanced at an incident electron energy E i = 18 eV. Angular-dependent measurements were recorded over a range of incident electron energies ( E i = 9–35 eV). For E i = 18 eV, the intensity of the CO stretch is peaked dominantly along the surface normal. This correlates with enhanced resonance scattering along the CO internuclear bond axis. We compare the energy of the shape resonance with previous electron scattering studies of gas-phase and condensed layers of CO, and also with recent inverse photoemission studies of CO chemisorbed on Ni(100).

Surface reactivity and surface spectroscopies

The formation of gaseous reaction products which result directly from surface reactions can be followed using multiplexed quadrupole mass spectrometry in ultrahigh vacuum by temperature-programmed reaction spectroscopy. Other methods such as electron energy loss vibrational spectroscopy and near-edge X-ray absorption fine structure analysis provide important information about the identity and structure of the intermediates. In this paper the combined use of these methods for understanding the oxidation of acetonitrile and formic acid on Ag(110) is described. For the -CH 2CN intermediate formed in acetonitrile oxidation, interesting local azimuthal ordering of the -CN group along the [001] direction is observed. In another example, the combination of these methods shows that near-edge analysis is capable of detecting the dynamical moation of the adsorbed formate species on Ag(110).

Nitric oxide adsorption on the Si(100)(2 × 1) surface — a vibrational study

High-resolution vibrational electron energy loss spectroscopy has been used to study the adsorbed state of NO on the Si(100)(2×1) surface. At 300 K, NO is adsorbed dissociatively on the Si(100) surface in the disordered structure, and the Si 3N and SiOSi species are formed. By heating at 1200–1300 K, the O adatoms are removed from the surface and the silicon nitride is formed; the (2×1) structure is recovered, which is interpreted to indicate that the nitride is formed mainly in the subsurface region.

Nitric oxide adsorption on the Si(100)(2×1) surface — a vibrational study

High-resolution vibrational electron energy loss spectroscopy has been used to study the adsorbed state of NO on the Si(100)(2×1) surface. At 300 K, NO is adsorbed dissociatively on the Si(100) surface in the disordered structure, and the Si3N and SiOSi species are formed. By heating at 1200–1300 K, the O adatoms are removed from the surface and the silicon nitride is formed; the (2×1) structure is recovered, which is interpreted to indicate that the nitride is formed mainly in the subsurface region.

Oxygen-hydrogen and carbon-hydrogen bond activation in ethylene glycol by atomic oxygen on silver(110): heterometallacycle formation and selective dehydrogenation to glyoxal

The absorption and reaction of ethylene glycol [(CH 2 OH) 2 ] on clean and oxygen-covered Ag(110) surfaces have been studied with temperature-programmed reaction spectroscopy (TPRS) and high resolution electron energy loss vibrational spectroscopy (EELS). On the clean surface (CH 2 OH) 2 was reversibly adsorbed, desorbing with an activation energy of 15 kcal/mol at 250 K from a monolayer state and a 225 K from a multilayer state with a heat of sublimation of 16 kcal/mol. Vibrational spectra of both the multilayer and monolayer were in good agreement with that of liquid (CH 2 OH) 2 , indicating that (CH 2 OH) 2 is only weakly bonded to the Ag(110)

Carbon-carbon bond activation in the 1,2-ethanedioxy heterometallacycle by atomic oxygen on Ag(110)

The effects of coadsorbed oxygen atoms on the decomposition of the 1,2-ethanedioxy surface species (OCH 2CH 2O (a)) formed during the partial oxidation of ethylene glycol (CH 2OH) 2 on the Ag(110) surface has been studied using temperature programmed reaction spectroscopy (TPRS), isotope labelling and high resolution electron energy loss vibrational spectroscopy (EELS). Surface oxygen atoms react with OCH 2CH 2O (a) at 300 K to form gaseous water and formaldehyde, and surface formate (HCOO (a)), which decomposes at 415 K to yield CO 2 and H 2. Through the use of isotopic labelling it was found that the oxygen in the formaldehyde originates solely from the 1,2-ethanedioxy surface species, while in the surface formate one comes from OCH 2CH 2O (a) and the other from coadsorbed oxygen atoms. The kinetic isotope effect for the reaction of adsorbed d 4-1,2-ethanedioxy with surface oxygen indicates that the rate limiting step is the abstraction of a proton from a methylene carbon in OCH 2CH 2O (a) to form OH a and OCHCH 2O (a). Nucleophilic attack by surface oxygen at the carbonyl carbon of OCHCH 2O (a), followed by C-C bond cleavage then releases H 2CO and forms HCOO (a). This study underscores the importance of looking at a wide range of concentrations of the pertinent surface species in order to probe a wider variety of reaction channels.

Interactions of nitrogen with Ni(110) — EELS, AES and LEED studies

The interactions of nitrogen with an Ni(110) surface at both 90 and 300 K have been investigated by means of high-resolution vibrational electron energy loss spectroscopy (EELS). Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED). N 2 molecules are chemisorbed nondissociatively at 90 K, and are desorbed at ~170 K. N 2 molecules are not adsorbed on Ni(110) at 300 K, however, as the Ni(110) surface is exposed to a mixture of N 2 molecules and N atoms at 300 K, N 2 admolecules (double- and triple-bonded N 2) are formed in addition to N adatoms (which are located, most probably, in the long-bridge, short-bridge and on-top sites, and also in the “subsurface” and “bulk”). It is suggested that N 2 admolecules are formed in the “active sites” on the Ni(110) surface which is distorted by the adsorption of N atoms. Thermal treatment of the (N 2 + N)-exposed Ni(110) surface is discussed. The N 2 admolecules ormed by the (N 2 + N) exposure are desorbed at ~400–450 K. The (2 × 3)-N structure is formed by heating at ~650–780 K. The Ni(110)-(N 2 + N) interaction is compared with the Pd(110)-(N 2 + N) interaction.

ChemInform Abstract: Formation of Adsorbed Sulfate from the Oxidation of Sulfur Dioxide on Pd(100).

AbstractThe formation of adsorbed SO4 from SO2 on clean and oxygen‐precovered Pd(100) surfaces is studied by electron energy loss vibrational spectroscopy (EELS), temp.‐programmed desorption/reaction spectroscopy (TPD/TPRS), LEED, and AES.