Surface vibrations of CO on W(100)

High resolution electron-energy-loss spectroscopy has been used to study the surface vibrations of CO on a W(100) surface at 300 K. For small exposures (β-CO) two losses at ～68 meV and ～78 meV are observed. This vibrational spectrum of β-CO is a clear indication of dissociative adsorption with the carbon and oxygen atoms in fourfold coordination sites each. With further exposure to CO two additional losses at 45 meV and 258 meV are observed, which represent the vibration of undissociated α-CO in upright position on top of a W atom. Furtheron results of coadsorption of H₂/CO and O₂/CO on W(100) are reported.     

The low-energy thermal excitation spectrum of nitrogen molecules adsorbed on Ni(110): Implications for molecular adsorption sites

The adsorption of N₂ molecules on Ni(110) has been investigated by high-resolution He-atom energy-loss spectroscopy. Two external vibrations in the subthermal (< 30 meV) energy regime at 5.75 and 4.5 meV have been observed and are shown to imply the existence of two different adsorption sites, which are assigned to an on-top and a bridge-site. This finding is at variance with the interpretation of data from electron energy loss spectroscopy and InfraRed (IR) spectroscopy, where only one mode of the internal stretch vibration v₁ at frequencies is observed for the chemisorbed nitrogen molecules. A reanalysis of previously published high-resolution IR data reveals that the presence of the second unexpected adsorbate species correlates with a feature in the IR spectra, which previously has been assigned to N₂ molecules adsorbed at defect sites. These findings reveal that in this case the external vibrations are significantly more sensitive to different adsorption sites than the internal vibration, the latter exhibiting a difference between on-top and bridge sites of less than 6 cm^–1.     

Inelastic photoemission due to scattering by surface adsorbate vibrations

By measuring the very low energy photoemission spectra of the CO/Cu(001) surface with a high resolution, we have found the energy loss components due to inelastic scattering of electrons near the Fermi level by the CO vibrational modes. The main energy loss structure appears as a step at 254 meV below the Fermi edge for 12C16O. An isotope shift of the step to 240 meV was observed when 13C18O was adsorbed. This observation confirms that this step arises from the energy loss of photoelectrons near the Fermi level through the excitation of the C-O stretching mode.     

光致发光光谱研究金刚石光学中心的振动结构

金刚石中不同的缺陷中心具有不同的振动结构. 间隙原子相关中心在离开零声子线165 meV之外存在强且尖锐的局部振动模; 空位相关中心具有很强的振动耦合, 在其零声子线之后出现很强且宽的声子边带, 而在165 meV之外却观察不到局部振动模, 且对于仅涉及一个空位的缺陷来说, 其振动都与一个能量约为42 meV的声子或一个能量约为67 meV的声子或两声子有关.     

Evidence for two different adsorption sites of CO on Pt(111) from infrared reflection spectroscopy

The adsorption of CO on Pt(111) surfaces has been studied under clean conditions by a highly surface sensitive double-beam infrared reflection spectroscopy (IRS). In contrast to results of other authors two stretching vibrations of adsorbed CO rather than one are detected near 2100cm⁻¹ and 1870cm⁻¹. This is in agreement with recent findings in high-resolution electron energy loss spectroscopy (ELS). The results are discussed in terms of two adsorption sites: CO adsorbed in on-top positions and double coordinated on bridging sites, respectively. Furthermore, a precursor state and a preferential adsorption in islands at low coverage is taken into account.     

Electronic excitation of ice monomers on Au(111) by scanning tunneling microscopy

Scanning tunneling microscopy, inelastic tunneling spectroscopy, and electron induced manipulation are used to investigate electronic excitation of D₂O monomers and small clusters adsorbed at the elbows of the Au(111) reconstruction. Diffusion of molecules, dissociation of clusters, and rearrangement of the reconstruction is induced by electronic excitation. Threshold energies of between 200 and 250 meV and of 446 meV are explained by combined vibrational modes of D₂O molecules. External vibrational modes of D₂O molecules on Au(111) are identified by inelastic tunneling spectroscopy at ≈18, 30, and 41 meV.     

原子氢与金刚石表面的相互作用

用高分辨率电子能量损失谱研究了氢原子吸附在金刚石表面的各种振动模式,其中主要是位于360meV处的C—H键的伸长振动和位于160meV处的“剪”振动。用氚原子代替氢原子吸附在金刚石表面,观测到上述振动模式所发生的同位素位移。将金刚石表面加热至900℃后,各种振动模式全部消失,这时吸附氢原子全部脱去,金刚石表面的悬键变成平躺在表面上,导致金刚石表面石墨化。由于石墨化后π-带的作用,在损失谱上出现一个非弹性的、连续的损失峰结构。我们用UPS和AES技术进一步证实了脱氢后金刚石表面的石墨化。 关键词：     

CO adsorption on Ni(1 0 0): Evidences for a weakly bound phase by HREELS measurements

The vibrational properties of CO on Ni(1 0 0) were investigated by high resolution electron energy loss spectroscopy. Loss measurements were performed as a function of temperature and CO exposures. At room temperature, regardless of CO coverage, we found stretching energies at 250 meV and 240 meV, assigned to CO at atop and bridge sites, respectively. At low temperature and for CO exposures lower than 0.6 L, the loss spectra showed a single stretching peak at 240 meV which is assigned to CO at bridge sites. For higher doses, a new intense peak at 260 meV appeared in the loss spectra. The rise of this new loss strongly influenced the intensity of the peak at 240 meV suggesting the occurrence of dipole couplings between adjacent CO molecules. This unusual high stretching frequency leads us to assign the peak at 260 meV to the stretching vibration of CO molecules which are weakly bonded to the Ni surface. We suggest that the formation of this phase is due to an electronic effect arising from a reduced back donation of electrons into the empty π orbital of CO.     

Normal modes of CO adsorbed on metal surfaces

A normal mode analysis of experimental data from helium atom scattering (HAS) and electron energy loss spectroscopy (EELS) for vibrations of CO molecules adsorbed in on-top and bridge sites on Ni(100) is presented, using a refined force-constant analysis. The similar case of CO adsorbed on Pt(111) is reinvestigated, revealing a misassignment of normal modes in previous work. Finally, available experimental data for CO on Pt(111) is used to construct a trial potential energy surface.     

HREELS investigation of hydrogenated and deuterated GaN{0001} surfaces

The vibrational properties of clean, H- and D-covered GaN{0001} surfaces were investigated by high-resolution electron energy-loss spectroscopy. Auger electron spectroscopy and low-energy electron diffraction were utilized to monitor the surface cleanliness and structure, respectively. At the clean surface the Fuchs-Kliewer surface phonon frequency was determined to 700 cm^-1 (86.8 meV). For the adsorbate-covered surfaces current structure models predict only Ga-H vibrations for surfaces of either polarity. Despite of this, the HREEL-spectra of the hydrogenated sample show a new loss structure at 3255 cm^-1 (403.6 meV) and a shoulder at 1900 cm^-1 (235.6 meV) which are attributed to N-H and Ga-H stretching vibrations, respectively. After deuterium exposure an isotope shift occurs. Again, a N-adsorbate vibration is clearly resolved. A Ga-D bending mode is observed at 390 cm^-1 (48.4 meV), indicating that vibrations of both species are present. Received 1 February 2000     

Adsorbate-substrate vibrational modes of benzene on Ag(110) resolved with scanning tunneling spectroscopy

Using a low temperature scanning tunneling microscope, we have detected low energy adsorbate-substrate (external or frustrated) vibrational modes of benzene molecules adsorbed on a Ag(110) surface. We demonstrate that such vibrations represent a fingerprint of the molecules' chemical state and environment; two different vibrational spectra are measured on molecules populating two different adsorption states. We also find that the distortion of the adsorption geometry of the molecules may give rise to the excitation of additional (initially hidden) modes. Important differences in the spatial distribution of the inelastic signal are also observed for these external modes.     

Helium scattering and work function investigation of co adsorption on Pt(111) and vicinal surfaces

CO adsorption on Pt(111) and vicinal Pt(111) surfaces has been studied by means of work function variation and He scattering measurements. AES and LEED were used mainly for correlations with other work. Special attention has been paid to the low coverage regime (θ_co < 0.1) with emphasis on surface structural dependencies. The minimum of the work function versus CO exposure curve occurs at a coverage less than 11% on “kink-free” surfaces. This is much lower than the hitherto commonly accepted value of 33%, and does not relate to any observed LEED superstructure. The value of Δφ_min depends strongly on the surface structure. For an “ideal” Pt(111) surface with a step density less than 10^?3 at a temperature of 300 K, Δφ_min = ?240 meV. The scattering cross section Σ of CO adsorbed on Pt(111) for 63 meV He is typically > 250 Ų, i.e. much larger than expected from the Van der Waals radii of He and CO. For two nominal Pt(111) surfaces with step densities of 10^?2 and less than 10^?3, respectively, the measured Σ values varied by a factor of three. This can be explained by preferential CO occupation of defect sites, which are already not “seen” by thermal helium. By comparing results on a stepped (997) and a kinked (12 11 9) Pt surface with similar defect densities, the kinks are proven to play a decisive role. They probably form saddles in the recently proposed activation barrier for migration between terrace and step sites.     

Infrared absorption of aligned H−Na+H− complexes in Kcl crystals

The localized vibrations of a complex in KCl consisting of a triangular array of two H^? ions and one Na⁺ ion in substitutional, neighboring positions have been observed in infrared absorption. As one would expect for such a molecule, five modes are observed: two longitudinal modes at 365 and 508 cm^-1, two complanar transverse modes along <110> at 459 and 562 cm^?1, and one transverse mode along <100> at 551 cm^?1.     

Raman scattering studies of spin‐waves in hexagonal BaFe12O19

We present the results of polarized Raman spectroscopy of hexagonal BaFe₁₂O₁₉ single crystal. The spectra, recorded from 200 to 800 cm^–1 and 1100 to 1700 cm^–1 in the 20–250 K temperature range, are analyzed on the basis of both crystal vibrations and spin‐waves. In the low wavenumber range, the Γ‐point phonons are observed. In the high wavenumber range, phonon mixings are observed; more interestingly, four modes of spin‐waves are identified in hexagonal BaFe₁₂O₁₉. Both have not been studied previously. Our analyses of the spin‐waves provide an optical method for quantitatively estimating the spin exchange interactions in hexagonal BaFe₁₂O₁₉. The four strong exchange integrals are found to have the values of J_ce = 1.31 meV, J_ae = 1.36 meV, J_cd = 1.46 meV, and J_bd = 1.71 meV. Our results also indicate that at ~200 and ~80 K, there would be additional spin‐ordering transitions in hexagonal BaFe₁₂O₁₉. Copyright © 2012 John Wiley & Sons, Ltd.     

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.     

High-resolution electron energy-loss spectroscopy of CO on an Ni(110) surface

High-resolution vibrational electron energy-loss spectra of CO on an Ni(110) surface were studied at 300 K with the in-situ combination of LEED, Auger electron spectroscopy and work-function change measurement. The observed peaks are at 436 cm^?1, 1855 cm^?1 (shifting to 1944 cm^?1 with increasing coverage) and at 1960 cm^?1 (shifting to 2016 cm^?1 with increasing coverage). The experimental results indicate that CO is adsorbed non-dissociatively at all coverages. Three adsorbed states of CO have been found. At fractional CO coverages less than θ ~ 0.9 where the disordered adsorbed structure dominates, CO is adsorbed in two inequivalent sites (short- and long-bridge sites) at random with its axis oriented perpendicular to the surface. At high coverages (θ > 0.9) where the (2 × 1) structure develops, our results indicate that the adsorbed CO molecules may occupy the distorted long-bridge sites forming zig-zag chains which lie essentially in the troughs of the (110) surface.     

Low temperature adsorption and site-conversion process of CO on the Ni(111) surface

Low-temperature (25 K) adsorption states and the site conversion of adsorbed CO between the ontop and the hollow sites on Ni(111) were studied by means of temperature programmed desorption and infrared reflection absorption spectroscopy. The activation energy and pre-exponential factor of desorption were estimated to be 1.2 eV and 2.6 × 10¹³ s^? 1, respectively, in the limit of zero coverage. At low coverage, CO molecules preferentially adsorbed at the hollow sites below 100 K. With increasing temperature, the ontop sites were also occupied. Using a van't Hoff plot, the enthalpy and the entropy differences between the hollow and ontop CO were estimated to be 36 meV and 0.043 meV K^? 1, respectively, and the vibrational entropy difference was estimated to be 0.085 meV K^? 1. The positive entropy difference was the result of the low-energy frustrated translational mode of the ontop CO, which was estimated to be 4.6 ± 0.3 meV. With the harmonic approximation, the upper limit of the activation energy of site hopping from ontop sites to hollow sites was estimated to be 61 meV. In addition, it was suggested that the activation energy of hollow-to-hollow site hopping via a bridge site was less than 37 meV.     

Electronic transitions of Ar,Xe, N2, CO physisorbed on Ag(111) and Al(111)

High Resolution Electron Energy Loss Spectroscopy has been extended to study also the excitonic (low lying electronic) transitions of physisorbed rare gas atoms (Ar, Xe) and diatomic molecules (N₂, CO) on Ag(111) and Al(111) surfaces at ～20K. Electron Loss Spectra were performed using a pair of hemispherical analyzers mounted at a fixed scattering angle (90°). This spectrometer allowed high transmission in the range of 0–15eV loss energies and incident beam energies up to 2OeV. AES, LEED and UV Photoemission (HeI) were also used in situ to characterize these surfaces and to identify the adsorbed gases and delineate their absolute coverage regimes.In contrast to optical absorption experiments, we observe both, optical (dipole) forbidden and allowed electronic transitions which show vibrational line structure for condensed multilayers. By comparison to gas phase data we find only weak perturbations in the condensed state. The observed electronic excitations show changes in intensity and FWHM depending on the coverage of the adsorbed gases.The FWHM of the electronic excitations of CO and N₂ adsorbed in the monolayer regime is larger than in multilayers. Nitrogen, on both surfaces exhibits an increase from 60meV to 120meV (FWHM) whereas for CO the vibronic features are broadened out leaving peaks with FWHM of ～1eV.The intensities of the electronic losses for all gases are smaller in the first monolayer than in the second or in multilayers. At submonolayer coverage the loss intensifies due to electronic excitations are strongly reduced and no longer observable although vibrational bands and photoelectron spectra show the presence of physisorbed adsorbates.Our results will be compared to optical absorption experiments (ref.1) on similar systems and to atom-on-jellium calculations (ref.2).     

Vibration properties of low-fraction hydrogen in deuterium ices

Inelastic incoherent neutron scattering spectra of D2O high-density amorphous （hda） ice, ice-Ⅷ and ice-Ⅱ mixed with small amount of H2O （（5%） have been measured recently on high-energy transfer spectrometer at Rutherford Appleton Laboratory （UK）. The hydrogen atom on D2O ice lattices has three distinguished vibrational modes, two bending at low frequencies and one stretching at high frequencies, and their frequencies are slightly different for different phases of ice. It was found that the lower one of the bending modes is located at -95 meⅤ for hda-ice, at -95 meⅤ for ice-Ⅷ and at -96 meⅤ for ice-Ⅱ and they are all lower than the value of 104 meⅤ for ice-Ih. It was also measured that the O-D and O-H covalent bond stretching modes of ice-Ⅷ are at -315 and -425 meⅤ, ice-Ⅱ at 307 and -415 meⅤ, hda-ice at 312 and -418 meⅤ, respectively. They are significantly higher than the values of ice-Ih at -299 and -406 meⅤ, respectively.     

Vibrational modes of oxygen on W(110)

Electron energy loss measurements of the vibrational modes of oxygen on W(110) as a function of coverage up to 0.5 monolayer are presented and analyzed. A single loss at 67 meV is observed initially; with increasing exposure this loss shifts to 72 meV and another loss appears at 47 meV. These data indicate coexistence of two species on the surface with a coverage-dependent conversion. Angular profiles of the specular elastic beam show a dramatic increase in width with initial oxygen coverage; this is possibly due to an oxygen-induced static disordering of the W surface layer.