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.     

Vibration spectroscopy of benzene adsorbed on Pt(111) and Ni(111)

High resolution electron energy loss spectroscopy has been applied to study the adsorption of benzene (C₆H₆ and C₆D₆) on Pt(111) and Ni(111) single crystal surfaces between 140 and 320 K. The vibrational spectra provide evidence that benzene is chemisorbed with its ring parallel to the surface, predominantly π bonded to the platinum and nickel surface respectively. A significant frequency increase of the CH-out-of-plane bending mode, largest in the case of platinum, is observed compared to the free molecule. On both metals two phases of benzene exist simultaneously, characterized by a different frequency shift. The shifts are explained by electronic interaction between the metal d-orbitals and molecules adsorbed in on top and threefold hollow sites respectively. The vibrational spectra of the multilayer condensed phase of benzene exhibit the infrared active modes of the gasphase molecule as expected.     

Adsorption sites of CO on Pt (111)

The surface vibrations of CO adsorbed on Pt(111) single crystal surfaces at 320 K have been studied by electron-energy-loss spectroscopy. At low coverages two vibration modes at 58 and ∼260 meV are observed. For exposures >0.2 Langmuir two additional modes at 45 and 232 meV develop. Considering also the observed LEED structures these vibrations are attributed to CO molecules being adsorbed upright in on-top and bridge sites, respectively.     

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.     

Electronic excitations on clean and adsorbate-covered oxide surfaces

We have studied electronic excitations at the surfaces of NiO (100), Cr₂O₃(111), and Al₂O₃(111) thin films with Electron Energy Loss Spectroscopy (EELS). On NiO (100) we observe surface electronic excitations in the energy range of the band gap which shift upon adsorption of NO. Ab initio cluster calculations show that these excitations occur within the Ni ions at the oxide surface. The (111) surface of Cr₂O₃ is characterized by distinct excitations which are also strongly influenced by the interaction with adsorbates. Temperature-dependent measurements show that two different states of the surface exist which are separated by an activation energy of about 10 meV. For Al₂O₃(111) we present data for a CO adsorbate. The oxide is quite inert with respect to CO adsorption as indicated by desorption temperatures between 38 K and 67 K. Due to the weak interaction with the substrate the a³II valence excitation of CO shows a clearly detectable vibrational splitting which has not been observed previously for a CO adsorbate in the (sub)monolayer coverage range. For several different adsorption state the lifetimes of the a³II state could be estimated from the halfwidths of the loss peaks, yielding values between 10^–15 s for the most strongly bound species and 10^–14 s for the CO multilayer.     

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.     

Energetics and vibrational states for hydrogen on Pt(111)

We present a combination of theoretical calculations and experiments for the low-lying vibrational excitations of H and D atoms adsorbed on the Pt(111) surface. The vibrational band states are calculated based on the full three-dimensional adiabatic potential energy surface obtained from first-principles calculations. For coverages less than three quarters of a monolayer, the observed experimental high-resolution electron peaks at 31 and 68 meV are in excellent agreement with the theoretical transitions between selected bands. Our results convincingly demonstrate the need to go beyond the local harmonic oscillator picture to understand the dynamics of this system.     

Atomic and molecular adsorption on Pd(111)

The adsorption properties of a variety of atomic species (H, O, N, S, and C), molecular species (N₂, HCN, CO, NO, and NH₃) and molecular fragments (CN, NH₂, NH, CH₃, CH₂, CH, HNO, NOH, and OH) are calculated on the (111) facet of palladium using periodic self-consistent density functional theory (DFT–GGA) calculations at ¼ ML coverage. For each species, we determine the optimal binding geometry and corresponding binding energy. The vibrational frequencies of these adsorbed species are calculated and are found to be in good agreement with experimental values that have been reported in literature. From the binding energies, we calculate potential energy surfaces for the decomposition of NO, CO, N₂, NH₃, and CH₄ on Pd(111), showing that only the decomposition of NO is thermochemically preferred to its molecular desorption.     

Photoemission from physisorbed Co on clean and Xe-covered Al(111)

Molecular-orbital energy shifts are observed in photoemission from weakly physisorbed CO on clean and Xe-covered Al(111) surfaces. These shifts in ionization potentials are mainly due to final-state relaxation effects, which can be described approximately by a point-charge image-potential model. Differential distance- and orbital-dependent energy shifts suggest that CO molecules lie flat on the substrates. CO is adsorbed on Al(111) with a heat of formation of 0.21 eV/molecule.     

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.     

Valence and core excitations in rare gas mono- and multilayers: Production,decay, and desorption of neutrals and ions

The aim of this survey is the understanding of the dynamics of medium to high energy excitations in simple condensed systems on very short time scales. For this purpose we examine the modifications of the electronic excitations and their evolution in rare gases (mainly Ar and Kr) due to a nearby metal surface (monolayer case) or by embedding into a rare gas condensate (multilayers). Ionic excitations are shifted to lower energies compared to the gas phase by polarization of the surroundings, while neutral excitations stay constant or are shifted to somewhat higher energies. This decreases the spacing between excitonic and ionic states from both sides. Deexcitation events can be analysed by linewidths for valence excitations, and by comparison of autoionisation and Auger spectra for core excitations. For monolayers, we conclude that excitonic states are unstable relative to ionic states but nevertheless are quite long-lived. For multilayers, only minor modifications relative to the gas phase are usually found. All electronic excitations in Ar and Kr mono- or multilayers lead to desorption of neutrals; core excitations in multilayers also lead to ions and cluster ions. The probable mechanisms in all cases are discussed, and open questions are pointed out.     

Co stretching vibration of carbon monoxide adsorbed on nickel(111) studied by high resolution electron loss spectroscopy

The frequency of the v-CO stretching vibration measured by HRELS has been followed as a function of the CO coverage and in the presence of coadsorbed hydrocarbons on the Ni(111) face. The v-CO frequency shifts continuously from 225 meV (1814 cm^?1) to 237 meV (1911 cm^?1) when the CO coverage increases from 0 to 0.41. Coadsorption of electron donor molecules, such as ethylene and benzene, generates a significant lowering of the v-CO frequency. Results are discussed in terms of the back donation of metallic electrons into the 2π ¹ antibonding orbitals of CO, the dipole-dipole coupling and the coordination number of the CO adsorbed molecules. The back donation is found to play the major role in the range of coverage explored but we cannot exclude some contribution of a dipole-dipole coupling effect.     

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.     

Interactions between NO and CO on Pt(111)

Temperature programmed desorption (TPD) of coadsorbed NO and CO on Pt(111) shows that no reaction occurs (less than 2%) up to the desorption temperature of NO. At 100 K, adsorption is competitive, but neither gas displaces the other from the surface. Coadsorbed CO causes the NO desorption temperature to be lowered by as much as 100 K, but NO does not affect the CO desorption temperature. TPD spectra for NO depend on which gas is adsorbed first, indicating that equilibrium between species is not established on the surface during desorption. Electron energy loss spectra show that the vibrational spectrum of each gas is only weakly affected by the other. When NO is adsorbed first, CO does not affect the ratio of bridged and terminal NO but lowers the frequencies of the bridged NO by approximately 50 cm^?1 and lowers the intensities of vibrational peaks of both species by a factor of about four. When CO is adsorbed first, the ratio of terminal to bridged NO increases for given coverage of NO, and the frequency of the bridged NO remains at the pure NO value. These results are explained in terms of CO island formation, repulsive interactions between NO and CO, and low adsorbate mobilities.     

Methanol decomposition on platinum (111)

The interaction of methanol with clean and oxygen-covered Pt(111) surfaces has been examined with high resolution electron loss spectroscopy (EELS) and thermal desorption spectroscopy (TDS). On the clean Pt(111) surface, methanol dehydrogenated above 140 K to form adsorbed carbon monoxide and hydrogen. On a Pt(111)-p(2 × 2)O surface, methanol formed a methoxy species (CH₃O) and adsorbed water. The methoxy species was unstable above 170 K and decomposed to form adsorbed CO and hydrogen. Above room temperature, hydrogen and carbon monoxide desorbed near 360 and 470 K, respectively. The instability of methanol and methoxy groups on the Pt surface is in agreement with the dehydrogenation reaction observed on W, Ru, Pd and Ni surfaces at low pressures. This is in contrast with the higher stability of methoxy groups on silver and copper surfaces, where decomposition to formaldehyde and hydrogen occurs. The hypothesis is proposed that metals with low heats of adsorption of CO and H₂ (Ag, Cu) may selectively form formaldehyde via the methoxy intermediate, whereas other metals with high CO and H₂ chemisorption heats rapidly dehydrogenate methoxy species below room temperature.     

Structure and vibrations of CO adsorbed on the Ni(100)p(2 × 2)O and c(2 × 2)O surfaces

Chemisorption of CO on the Ni(100)p(2 × 2)O and c(2 × 2)O surfaces has been investigated by high-resolution electron energy loss spectroscopy (EELS) and low-energy electron diffraction (LEED). At 175 K CO adsorption on Ni(100)p(2 × 2)O saturates at about 1 L exposure in a structure interpreted to be Ni(100)p(2 × 2)O—p(2 × 2)CO. The CO layer is stable at 175 K but desorbs readily around 300 K. The EEL spectrum for p(2 × 2)CO shows vibrational losses at 46 meV and 245 meV interpreted to be due to excitations of the Ni-C and C-O stretching vibrations of CO molecules bridge bonded to two nearest neighbour Ni atoms. This interpretation is also supported by the LEED observations. For the preceeding dilute CO layer the vibrational loss spectrum reveals CO adsorption both to Ni bridge sites and hollow sites. At 175 K CO does only adsorb stationary on p(2 × 2)O defects in the Ni(100)c(2 × 2)O surface and not at all on epitaxially grown NiO(111) and (100) surfaces.     

CO dynamics induced by tunneling electrons: differences on Cu(110) and Ag(110)

The electronic current originating in a scanning tunneling microscope (STM) can be used to induce motion and desorption of adsorbates on surfaces. The manipulation of CO molecules on noble metal surfaces is an academic case that has received little theoretical attention. Here, we do thorough density functional theory calculations that explore the chemisorption of CO on Cu(110) and Ag(110) surface and its vibrational properties. The STM induced dynamics are explored after excitation of the highest lying mode, the C–O stretch. In order to give a complete account of this dynamics, the lifetime of the different CO modes is evaluated (by only including the mode decay into electronic excitations of the host surface) as well as the intermode coupling. Hence, after excitation of the stretch mode, the lower-energy modes are populated via intermode coupling and depopulated by electron-hole excitations. This study reveals the intrinsic features of the STM induced motion of CO on Cu(110) and Ag(110).     

UV photoemission from physisorbed atoms and molecules: Electronic binding energies of valence levels in mono- and multilayers

Angle-resolved UV photoemission spectra were measured for Ar, Kr, Xe, CO, O₂ and N₂ adsorbed on a Ni(110) surface at 20 K. The different gases were adsorbed also on the Ni(110) surface which had been precovered by mono- and multilayers of the same gases. Upon physisorbing one of these species onto the bare and precovered Ni surface, binding energy shifts up to 3 eV were found. These shifts will be explained by work function changes of the substrate onto which the gas is physisorbed. It will be shown that for the investigated gases the binding energy referred to the vacuum level is an atomic or molecular property which is independent of the substrate, to a first approximation. By physisorption of a known gas the work function of any substrate can be evaluated by UPS. The density of valence states for bulk Ar, Kr and Xe will be discussed. There is evidence that the conduction band can be seen in the secondary electron background of the UP spectra.     

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.     

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.