High Resolution Electron Energy Loss Spectroscopy Study Research Articles

HREELS study of vapor-deposited polyaniline on Ag(110)

Ultra-thin films of polyaniline on Ag(110) were characterized using high-resolution electron energy loss spectroscopy (HREELS) in the vibrational and electronic loss regimes. The films were grown in situ by chemical vapor deposition using an emeraldine source. Initially, short polyaniline oligomers adsorb on the surface followed by polymerization to form longer polyaniline chains. The loss spectra in the far-infrared region indicate that the films become highly conducting upon protonation. In addition, the role of the substrate in the initial stages of polymer film growth is demonstrated by comparison with polyaniline on Cu and Au surfaces.

Vibrational HREELS study of tris-neopentylgallium adsorbed on GaAs(100); identification of surface species and electron excitation mechanisms

High-resolution electron energy loss spectroscopy (HREELS) has been used to study the vibrational properties of tris-neopentylgallium (Np 3Ga) adsorbed on GaAs(100). Adsorption at room temperature leads to loss of one Np group and subsequent formation of a Np 2Ga surface species that is stable up to ∼250°C. For higher temperatures, decomposition leads to the formation of surface CH 2 species either via a β-Me elimination or a 1,5 H-atom abstraction mechanism. The CH 2 species is stable at high temperatures and is a possible cause of the high carbon incorporation levels found in GaAs layers grown by atomic layer epitaxy using this precursor. Spectra recorded for room temperature adsorption of Np 2Ga at different electron beam energies show that C–H and C–C vibrational modes of the molecule undergo strong resonant vibrational excitation at 8 and 12 eV. No beam-induced dissociation was evident in spectra recorded at the resonant and higher electron beam energies, even after prolonged exposure to the incident HREELS beam. This is in contrast to previous studies of Me 3X (X=Ga, Al, In) metalorganics which showed significant electron beam-induced conversion of CH 3 to CH 2. Dissociation does not occur with Np 3Ga because the CH 3 part of the neopentyl groups is not bonded directly to the surface, and any CH 2 groups that would be formed from beam-induced dissociation cannot be stabilised through interaction with the surface.

HREELS studies of ethylene oxide interaction with Pd(110)

Ethylene oxide (EO) adsorption and reaction on Pd(110) was investigated with high resolution electron energy loss spectroscopy (HREELS). The HREEL spectrum following EO adsorption on Pd(110) at 120K indicated molecular adsorption of EO, with the molecular ring plane tilted away from the surface normal. The suggestion of the complete absence of any symmetry of the metaladsorbate complex in the present study was based on the observation for adsorbed EO of vibrations which are infra-red forbidden for gaseous EO with C 2v symmetry. Further results from temperature programmed HREELS studies indicated that adsorbed EO underwent a ring opening reaction above 200 K, ultimately producing CO and methylene species on the metal surface.

HREELS study of the interaction of ethylene with Pt films supported on ZrO 2(100) and ZnO(0001)

The adsorption and reaction of ethylene on epitaxial Pt films supported on ZrO 2(100) and ZnO(0001) was studied using high-resolution electron energy-loss spectroscopy (HREELS). On the (111) oriented surfaces of the supported Pt films, ethylene was found to adsorb in both di-σ and π-bonded configurations at temperatures near 100 K. This result is in contrast to the interaction of ethylene with Pt(111) where only the di-σ species is stable at 100 K and suggests that the surfaces of the films contained a high concentration of steps and other defects. Both di-σ and π-bonded ethylene were also detected on Pt particles supported on ZrO 2(100) at 130 K. On the supported Pt films and particles, adsorbed ethylene underwent dehydrogenation and rearrangement to form ethylidyne upon heating to 300 K. The results of this study serve to further demonstrate the utility of HREELS in characterizing adsorbates on model supported metal catalysts.

HREELS study of triethylphosphine (TEP) and tertiarybutylphosphine (TBP) on a Si(001)-(2 × 1) surface

Adsorption and decomposition of TEP and TBP on a clean Si(001)-(2 × 1) surface have been studied using high-resolution electron energy loss spectroscopy (HREELS). TEP is adsorbed molecularly at 300 K and decomposed into ethylene, which is evolved from the surface, through β-hydride elimination, leading to formation of SiH. TBP is partially dissociated at 100 K and decomposed into isobutylene (2-methylpropene) through the same mechanism as TEP. It is suggested that ethyl ( t-butyl) groups bonded to phosphorus in TEP are not transferred to silicon and ethylene (isobutylene) as a decomposition product is not re-adsorbed on the surface. This would be the reason why carbon deposition does not occur on the surface upon annealing the sample exposed to TEP and TBP.

Resonant vibrational excitation in high-resolution electron energy-loss spectroscopy studies of trimethylamine chemisorbed on GaAs(100)

High-resolution electron energy-loss spectroscopy has been used to study the vibrational properties of trimethylamine [(CH3)3N] chemisorbed on GaAs(100) at room temperature. A number of vibrational modes of the adsorbate, involving both the C–H and C–N bonds, are preferentially excited via a resonance scattering mechanism involving the formation of a temporary negative ion due to capture of the incident electrons. The resonance, which is centered around 6 eV, is characterized by a large enhancement in the intensity of several fundamental vibrations and the appearance of a number of combination and overtone bands. Detailed energy-dependent studies and comparison with previous electron scattering studies of related gas phase molecules suggest that electron capture can occur in three molecular orbitals of the adsorbed molecule. By considering the electron density distributions obtained from semiempirical calculations of the isolated molecule, an assignment of the vibrational modes which are selectively excited due to occupation of a specific molecular orbital has been attempted.

High resolution electron energy loss spectroscopy study of the Si(001) 3 × 1 hydrogenated surface

Hydrogenation of the Si(001) 2 × 1 surface held at 400 K has been performed in a new experimental set-up for a characterization by high resolution electron energy loss spectroscopy (HREELS). Angular scan along the [110] direction recorded at 55 eV primary energy confirms the 3 × 1 surface reconstruction. HREELS performed at 6 eV demonstrates the existence of a dihydride phase, in particular through the observation of the scissor mode around 904 cm −1. The good quality of this surface, as shown by a high count rate on the elastic peak, allowed to obtain a resolution as good as 45 cm −1 at room temperature. Lineshape studies of the vibrational peaks related to hydrogen reveal an abnormal width of the 639 cm −1 mode. In fact, the resolution was improved by cooling down to ∼ 110 K, and a double peak was clearly resolved, separated by 35 cm −1. In view of previous HREELS studies of the Si(001) 1 × 1 and 2 × 1 hydrogenated surfaces, as well as of theoretical calculations of the vibrational modes of SiH 2 species, the assignment of these modes is given. For the first time, a clear monohydride signature within a dihydride phase is reported by HREELS.

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.

Characterization and chemical behavior of submonolayer coverages of titania on a Pt foil

Abstract The interaction between submonolayer titania coverages and Pt foil has been studied by Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). The submonolayer titania can be fully oxidized to TiO 2 at 923 K under 10 −8 Torr O 2 , and partially oxidized to TiO x at lower oxidation temperatures. The oxidized surface can be reduced by annealing to 1000 K or higher, or by heating in H 2 at 823 K, or by interacting with surface carbon formed from acetone decomposition. Under certain conditions (e.g., hydrogen reduction at 923 K), the surface titania can be fully reduced to metallic Ti which diffuses into bulk Pt readily. The reduced metallic Ti can resurface when the surface is oxidized at 923 K. Both XPS and HREELS data indicate the existence of subsurface oxygen, which plays an important role for the diffusion of Ti into and out of the Pt foil. Although no special interfacial active sites were revealed by HREELS studies of adsorbed acetone and CO, some TPD and XPS data suggest the presence of sites active for acetone decomposition.

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.

HREELS study of the interaction of aniline with Cu(110)

The adsorption and reaction of aniline on Cu(110) was studied using a combination of high resolution electron energy loss spectroscopy (HREELS) and temperature programmed desorption (TPD). HREEL spectra with an energy resolution of ≈ 13 cm −1 were obtained for both multilayer and monolayer aniline-derived surface species permitting a comprehensive comparison with the infrared spectra of liquid and gaseous aniline. The experimental results indicate that aniline forms randomly-oriented multilayers upon adsorption at 100 K, while at 300 K aniline adsorbs dissociatively forming stable C 6H 5NH surface species. This species decomposes between 525 and 875 K producing gaseous H 2 and N 2 and adsorbed carbon.

Photoemission and HREELS study of K adsorption on TiO2(100)

The adsorption of K and coadsorption of O2 on TiO2(100) have been studied using photoemission and high resolution electron energy-loss spectroscopy (HREELS). We compare our results with those of earlier studies of potassium oxides. The energy separations of peaks appearing in photoemission spectra indicate the formation of a peroxide or superoxide at the surface following adsorption of > 1 monolayer of potassium. HREELS results are consistent with this conclusion.

Interaction of water with potassium on graphite: A HREELS study

Water and coadsorbed water + potassium on the basal plane of graphite were studied with high resolution electron energy loss spectroscopy (HREELS) at 85 K and after stepwise annealing up to 500 K. Water adsorbs non-dissociatively on both clean and potassium precovered surface at 85 K. The vibration spectra, together with thermal desorption spectroscopy (TDS) reveal a series of reactions within the coadsorbed layer leading to the formation of KOH, KH, and KO x and volatile products. Eventually a precursor to CO 2 formation, of yet unidentified composition and observed also in K + O 2 coadsorption studies on graphite, is observed at ≈ 27 meV.

Negative ion resonances and surface vibrations

This paper reviews some recent developments in high resolution electron energy loss spectroscopy (HREELS) studies of adsorbed molecules. The principal theme is the negative ion resonance scattering mechanism and its usefulness in the investigation of surface vibrational states. Several examples are considered: first, the excitation of low frequency molecule-surface modes, including overtones, as illustrated by the case of physisorbed O 2/Pt(111); secondly, the relationship between the σ resonance energy and the bond length in HREELS studies of chemisorbed molecules, and illustrated by the examples of chemisorbed O 2/Pt(111) and chemisorbed CO; finally, the utility of the resonance scattering mechanism in unravelling the behaviour of a complex adsorption system, coadsorbed K and O 2 on graphite.

HREELS study of CH 3I and CH 3 adsorbed on Rh(111) surface

The thermal and photochemistry of methyl iodide on Rh(111) surface has been investigated by high resolution electron energy loss spectroscopy (HREELS) supplemented by Auger electron (AES) and temperature programmed desorption (TPD) spectoscopies. CH 3I adsorbs dissociatively at submonolayer coverage at 90 K and molecularly at higher coverages. The dissociation of a monolayer starts above 148 K and completes below 200 K. Illumination of adsorbed CH 3I enhanced the extent of the dissociation which is ascribed to the generation of photoelectrons. The primary products of thermal and photoinduced dissociation is adsorbed CH 3 and I. A fraction of CH 3 is hydrogenated to CH 4 above 150 K, another fraction exists on the surface even at 300–400 K and decomposes through the formation of CH X species to hydrogen and surface carbon. No dimerization of CH 3 was observed.

HREELS study of subsurface oxygen on the Ni(100) surface

New high frequency vibrational loss features have been measured on O/Ni(100) for low oxygen coverages, using high-resolution electron-energy-loss spectroscopy (HREELS). These high frequency losses only appear after thermal annealing. Oxygen atoms are migrating underneath the surface with increasing temperature, giving rise to the high frequency modes. The modes are shown to result from subsurface oxygen atoms, squeezed in interstitial sites. We have detected five different high frequency loss signals, while five different interstitial sites are precisely available below the Ni(100) surface. Model calculations have been performed in order to relate frequencies to interstitial sites. The matching technique approach has been applied to analyze the case of periodic structures, whereas a cluster approach has been used to obtain the localized vibrational states associated with interstitial oxygen atoms.

A LEED, TDS, and HREELS study of CO adsorbed on the Rh(311) stepped surface

The bonding of CO on the stepped Rh(311) surface has been investigated by high resolution electron energy loss spectroscopy (HREELS), low-energy electron diffraction (LEED), and thermal desorption spectroscopy (TDS). At saturation coverage (estimated to be 1 monolayer) an ordered (2 × 2) surface structure is formed in the range of 120 to 270 K. Thermal desorption spectroscopy measurements (23 K s −1) show first order desorption with a shift in peak temperature from 528 K at low coverage to 503 K at high coverage, accompanied by an additional peak at 388 K. The heat of desorption at low coverage is found to be 31.1 kcal/mol which is nearly identical to the heat of desorption of CO from Rh(110), (100), and (111) showing that CO bonding at low coverages is structure insensitive. From the HREELS spectra both the Rh-C and CO stretching frequencies are observed to shift from 524 to 443 cm −1 and 1935 to 2056 cm −1 respectively as a function of CO coverage. HREELS spectra of the (2 × 2) structure find a CO stretch at a frequency of 2056 cm −1 and a Rh-C stretch at 443 cm −1, which indicates that the CO molecule is bonded to the top site of the step edges. Based on these results a structural model with CO top site adsorption is proposed.

HREELS study on CO adsorbed on clean, nitrided and oxidized [formula omitted] surfaces

CO adsorption on as-deposited, nitrided and oxidized Fe Ru(001) surfaces was investigated using high resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD), Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). The behaviour of CO on Fe Ru(001) was compared to that on low Miller index planes of an Fe single crystal. It was found that the as-deposited Fe(1 ML) Ru(001) surface resembles the Fe(110) surface. A single CO species with a ν(C−O) of 1860–1990 cm −1 was found on the as-deposited surface. A new molecular adsorption state appeared on the annealed Fe Ru(001) surface. This species has a higher C−O stretching frequency and a higher desorption temperature than the other species. The effects of nitridation and oxidation of Fe Ru(001) on CO adsorption were also investigated.

HREELS study on CO adsorbed on clean, nitrided and oxidized Fe/Ru(001) surfaces

CO adsorption on as-deposited, nitrided and oxidized FeRu(001) surfaces was investigated using high resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD), Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). The behaviour of CO on FeRu(001) was compared to that on low Miller index planes of an Fe single crystal. It was found that the as-deposited Fe(1ML)Ru(001) surface resembles the Fe(110) surface. A single CO species with a ν(C−O) of 1860–1990 cm−1 was found on the as-deposited surface. A new molecular adsorption state appeared on the annealed FeRu(001) surface. This species has a higher C−O stretching frequency and a higher desorption temperature than the other species. The effects of nitridation and oxidation of FeRu(001) on CO adsorption were also investigated.

The chemisorption of water onto Ni(100) and Pd(110): a HREELS study

The adsorption of H 2O and D 2O on Ni(100) and Pd(110) has been studied using high resolution electron energy loss spectroscopy (HREELS). A series of spectra were measured as a function of coverage, and each series was fit to a set of Gaussian functions to determine the individual loss features contributing to the spectra. By examining the effects of water coverage and the metal surface on these loss features, details about the geometry and bonding of the adsorbed water can be obtained. The analysis indicated that the PdOH 2 bond is substantially weaker than the NiOH 2 bond. This difference, along with the structural difference between the (110) and (100) surfaces, greatly influences the adsorption process. The Pd(110) data are consistent with the formation of a bilayer structure on the surface, while the Ni(100) data indicate a more disordered arrangement. These results are also discussed in relation to the rather broad base of work done on the adsorption of water on various single crystal metal surfaces.