Cluster model calculation and photoemission studies of molecularly adsorbed NO on Ni

Self-consistent scattered wave cluster model calculations for the NONi system are presented with NO bonded N-down in a fourfold coordination site. The results suggest that the charge is transferred from metal into the single occupied 2π¹ molecular orbital of NO which is thus significantly perturbed. The interaction involves the metal s, p, and d-derived states. We calculate total and local “density of states” defined within the framework of the cluster model and compare the results with Ultraviolet Photoemission experiments recorded at 80 K. At 300 K partial dissociation occurs as judged from UPS and XPS. Increased photoemission observed at about 2 eV below the Fermi level in the molecularly adsorbed state is attributed to the alteration of density of states due to the interaction of the 2π¹ molecular orbital of NO with the substrate. A detailed analysis of the nature of the relevant molecular orbitals is also given.     

“Chemical effects” of vibrational lifetime and frequency of CO ON Ag

The Raman band of the stretch vibration of CO adsorbed on cold deposited silver films shifts to lower frequency and broadens with increasing coverage. Isotopic mixture experiments show that this is not caused by dipolar interactions. A possible explanation of this pure “chemical effect” based on a finite density of the CO 2π¹ resonance at the Fermi energy is proposed and critically discussed.     

Influence of one CO molecule on structural and electronic properties of monatomic Cu chain

By using first-principles calculations, we have systematically investigated the structural and electronic properties of an infinite linear monatomic Cu chain with an adsorbed CO molecule. We find that the bridge geometry is energeticabsally favored not only when the Cu–Cu bond below the molecule is unstretched, but also for a wide range of d_Cu–Cu up to about 4.20 Å, while the substitutional geometry is favored only in the hyperstretched situation d_Cu–Cu>4.80 Å. Charge density differences point out the electron transfer is from the Cu atoms to the adsorbed CO molecule. The binding mechanism of CO to Cu chain can be described by the Blyholder’s model, in terms of σ-donation of electron density from the nonbonding CO-5σ orbital into empty metal orbitals and π-backdonation from the occupied metal d orbitals to empty CO-2π^⁎ orbital. The donation/backdonation process leads to the formation of bonding/antibonding pairs, 5σ_b/5σ_a and 2π_b^⁎/2π_a^⁎, with the 5σ_a lying above E_f and the 2π_b^⁎ below E_f.     

Penning ionization electron spectroscopy of molecular adsorbates on Pd and Cu surfaces

The electronic properties of CO, PF₃, NH₃, C₂H₂ and C₆H₆ adsorbed on Pd(111), Pd(110), and Cu(110) surfaces were studied by Auger deexcitation (AD) of metastable noble gas atoms (Penning ionization) and by ultraviolet photoelectron spectroscopy (UPS). Electron emission via AD is restricted to the outmost levels localized at the adsorbed particles. The AD process competes with reasonance ionization of the metastable atoms, and its probability depends on the geometry of the adsorbed species and on its adsorption sites as well as on coverage. The differences in kinetic energy of electrons emitted by AD or by photons reflect modifications of the electron affinities of the adsorbed molecules. The extreme surface sensitivity of AD spectroscopy allows in particular to probe the local density of states at the adsorbate in the energy range close to the Fermi level which arises from π “back donation” as well as σ antibonding contributions.     

Observation of occupation and dispersion of 2π1 associated states for CO adsorbed on Cu{100}

Angle-resolved photoemission observations of CO adsorbed on Cu{100} to form the (√2 × √2)R45° structure show emission from a CO 2π¹ related state away from the centre of the surface Brillouin zone. These observations are shown to be compatible with the anticipated dispersion of such states and with weak 2π¹ backbonding on this substrate.     

N-doped graphene as a nanostructure adsorbent for carbon monoxide: DFT calculations

ABSTRACT

This work reports the physisorption of carbon monoxide (CO) on the surface of N-doped graphene. To study the adsorption of CO on N-doped graphene, some quantum chemical calculations were used through density functional theory. Based on our results, it can be found that the CO molecule could be adsorbed on the surface of N-doped graphene physically with the adsorption energies (E_ads) of ?2.9 and ?0.8 kcal mol^?1 (depends on the kind of configuration) while positive adsorption energies were calculated upon adsorption of CO on pristine graphene. We used the charge analysis for calculation of the net transferred charge of adsorbed CO on pristine and N-doped graphene sheets to evaluate the sensing ability of surface. The global indices of reactivity were calculated from the differences of the lowest unoccupied molecular orbital and highest occupied molecular orbital energies. Graphs for density of states point to some orbital hybridisation between CO molecule and N-doped graphene. Consequently, the N-doped graphene transforms the existence of CO molecules into electrical signal, and it may be potentially used as a sensor for CO.     

CO与K在Cu(111)面上共吸附的角分辨紫外光电子能谱

CO在有K沉积的Cu(111)面上吸附的角分辨紫外光电子能谱(ARUPS)测量结果表明:在室温下,有分子状态的CO吸附在该表面,CO分子的4σ态信号强度明显低于5σ/1π态的强度,这表明CO分子在该状态下是强烈倾斜的,低温下(～150K),有两种不同的CO吸附状态,一种只有在较高CO暴露量下才出现,而另一种则在任何暴露量下都存在,后者是一种类似于CO在室温时的吸附状态。 关键词：     

Surface segregation and surface electronic interactions in PtCu

Photoemission and Auger electron spectroscopy on Pt_0.98Cu_0.02 show that the (110) face has over twice as much Cu surface segregation as the (111) face. The Cu 3d-derived surface “density of states” differ strikingly in peak shape and in width (by 0.5 eV). The centroids, compared with bulk Cu d states, are shifted by more than 0.3 eV towards the Fermi level. This is the first experimental correlation between surface segregation and surface bonding.     

A density functional theory study on the adsorption of CO and O₂ on Cu-terminated Cu₂O（111） surface

The adsorptions of CO and O2 molecules individually on the stoichiometric Cu-terminated Cu2O(111) surface are investigated by first-principles calculations on the basis of the density functional theory.The calculated results indicate that the CO molecule preferably coordinates to the Cu2 site through its C atom with an adsorption energy of -1.69 eV,whereas the O2 molecule is most stably adsorbed in a tilt type with one O atom coordinating to the Cu2 site and the other O atom coordinating to the Cu1 site,and has an adsorption energy of -1.97 eV.From the analysis of density of states,it is observed that Cu 3d transfers electrons to 2π orbital of the CO molecule and the highest occupied 5σ orbital of the CO molecule transfers electrons to the substrate.The sharp band of Cu 4s is delocalized when compared to that before the CO molecule adsorption,and overlaps substantially with bands of the adsorbed CO molecule.There is a broadening of the 2π orbital of the O2 molecule because of its overlapping with the Cu 3d orbital,indicating that strong 3d-2π interactions are involved in the chemisorption of the O2 molecule on the surface.     

X-ray and UV photoelectron-spectroscopic studies of pyridine adsorbed on evaporated nickel and palladium in the temperature range 140–385 k

The states of pyridine adsorbed on evaporated nickel and palladium films have been investigated as a function of temperature in the range 140–385 K by means of X-ray and UV photoelectron spectroscopy. At ～ 140 K, pyridine “N-bonded” on the metal surfaces gives C 1s and N 1s peaks whose binding energies are very close to those for condensed pyridine and “N-bonded” pyridine on pre-oxidized nickel. The high-lying valence orbitals, 2b₁ (π) and 1a₂ (π) + 7a₁ (n), of pyridine show shifts similar to those for the “N-bonded” molecule on pre-oxidized nickel. At ～ 290 K, “π-bonded” pyridine shows large shifts in the C 1s and N 1s peaks and in the high-lying valence orbitals, as observed for “π-bonded” benzene on nickel. The assignments of the adsorbed states are supported by work-function change data. A large proportion of pyridine converts from the “N-bonded” to the “π-bonded” form between 220 and 290 K. Formation of “α-pyridyl” is suggested at ～ 375 K on nickel.     

Influence of the metal nanofilm-semiconductor interface on surface properties of the nanofilm: The CO-Yb-Si(111) system

The influence of the “ytterbium nanofilm-single-crystal silicon substrate” interface on properties of the films has been investigated. It has been shown that, if the film thickness is less than 10 monolayers, the Friedel oscillations (standing waves of electron density) generated by the interface affect the work function of the films and the rate of adsorption of CO molecules on their surface. In turn, the CO molecules modify the electronic structure of ytterbium during adsorption on the surface of nanofilms by transforming ytterbium from the divalent to trivalent state. The completely filled layer of adsorbed CO molecules consists of two phases. The first phase is a two-dimensional gas whose molecules weakly interact with each other, but their lone electron pairs form a donor-acceptor bond with the Yb 5d level; as a result, this level is located below the Fermi level and the metal transforms into the trivalent state. After filling the two-dimensional phase, the second (island) phase, in which the CO molecule are bound by horizontal π-bonds, begins to grow. The formation of these bonds becomes possible due to the filling of 2π states in the molecules upon compaction of the adsorbed layer. The considered the adsorbed two-phase layer is responsible for the complete transition of ytterbium into the trivalent state.     

Contribution of the surface state to the observation of the surface Kondo resonance

Tunneling spectra obtained on and near Co atoms adsorbed on Ag(111) show at 5 to 6 K a Kondo resonance that appears as a characteristic dip around the Fermi energy. The feature is present up to 1.5 nm around Co atoms adsorbed on terraces with the surface state onset in the occupied region of the density of states. On a narrow terrace, where the surface state onset lies in the unoccupied region of the density of states, it is only present up to 0.5 nm. This difference demonstrates directly the importance of the surface state electrons in the observation of the surface Kondo resonance.     

Angle-resolved photoemission from molecular N2 adsorbed on Ni(100)

Angle-resolved photoemission has been used to study the band structure of the ordered c(2 × 2) molecular nitrogen overlayer chemisorbed on Ni(100). The molecular nitrogen chemisorbs to the Ni(100) with the molecular axis perpendicular to the surface. The 1π_u, 3σ_g and 2σ_u (1π, 5σ and 4σ of adsorbed N₂) orbitals of N₂ are identified at 8.1 ± 0.05, 8.3 ± 0.1 and 12.8 ± 0.2 eV below the nickel Fermi energy at Г̄gG respectively. Angle-resolved photoemission of adsorbed molecular N₂ exhibits several satellite lines with less kinetic energy than the primary molecular orbital emission. These satellite lines are a final state effect. A k_∥ (parallel momentum vector) dependence of the satellite line intensity was observed.     

Carbon monoxide adsorption on the metastable fcc-Co/Cu(1 0 0) surface

The adsorption properties of CO on the epitaxial five-monolayer Co/Cu(1 0 0) system, where the Co overlayer has stabilized in the metastable fcc-phase, are reported. This system is known to exhibit metallic quantum well (MQW) states at energies 1 eV or greater above the Fermi level, which may influence CO adsorption. The CO/fcc-Co/Cu(1 0 0) system was explored with low energy electron diffraction (LEED), inverse photoemission (IPE), reflection-absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD). Upon CO adsorption, a new feature is observed in IPE at 4.4 eV above E_F and is interpreted as the CO 2π^∗ level. When adsorbed at room temperature, TPD exhibits a CO desorption peak at ∼355 K, while low temperature adsorption reveals additional binding configurations with TPD features at ∼220 K and ∼265 K. These TPD peak temperatures are correlated with different C-O stretch vibrational frequencies observed in the IR spectra. The adsorption properties of this surface are compared to those of the surfaces of single crystal hcp-Co, as well as other metastable thin film systems.     

Els and leed study of CO adsorption on Pd(001) and Pd(001) 6° [110]

LEED studies indicate that both CO and carbon adsorb differently on Pd(001) surfaces with and without steps. However, electron energy loss spectra for Pd(001) samples with and without ordered steps show no detectible surface structural sensitivity to CO adsorption. For both samples an additional loss feature at 13.5 eV appears upon CO adsorption. This feature is identified as a (1π?5σ) to 2π^★ type intramolecular electronic excitation, and it appears to be a universal feature of molecular CO adsorption on metals. A second loss expected in the 6–7 eV range associated with a d to 2π^★ type charge-transfer excitation was not detected. Either the transition probability is low for this excitation or hybridization with surface plasmons may obscure its identification.     

Adsorption of CO on clean and oxygen covered Ni(111) surfaces

Adsorption of CO on Ni(111) surfaces was studied by means of LEED, UPS and thermal desorption spectroscopy. On an initially clean surface adsorbed CO forms a √3 × $\text{√3}\text{R30°}$ structure at θ = 0.33 whose unit cell is continuously compressed with increasing coverage leading to a c4 × 2-structure at θ = 0.5. Beyond this coverage a more weakly bound phase characterized by a $\text{√7}\text{2}$ × $\text{√7}\text{2R19°}$ LEED pattern is formed which is interpreted with a hexagonal close-packed arrangement (θ = 0.57) where all CO molecules are either in “bridge” or in single-site positions with a mutual distance of 3.3 Å. If CO is adsorbed on a surface precovered by oxygen (exhibiting an O 2 × 2 structure) a partially disordered coadsorbate 2 × 2 structure with θ_o = θ_co = 0.25 is formed where the CO adsorption energy is lowered by about 4 kcal/mole due to repulsive interactions. In this case the photoemission spectrum exhibits not a simple superposition of the features arising from the single-component adsorbates (i.e. maxima at 5.5 eV below the Fermi level with O_ad, and at 7.8 (5σ + 1π) and 10.6 eV (4σ) with CO_ad, respectively), but the peak derived from the CO 4σ level is shifted by about 0.3 eV towards higher ionization energies.     

Optical reflectance studies of chemisorption on a clean metal surface

Changes in optical reflectance from a Mo (100) surface due to chemisorption of O₂ and CO in amounts up to a monolayer show that adsorbate-dependent surface states are present near the Fermi energy and are sensitive to the adatom binding configuration.     

The theoretical and experimental study of the ionization processes during the low energy ion sputtering

The process by which atoms are ionized as they are sputtered from a metal surface has been analyzed both theoretically and experimentally. In the theoretical part the expressions for ionization coefficient R⁺ of atoms having the ionization energy much larger than the metal work function have been derived using a molecular orbital method. The effect of the level crossing was estimated in an approximate way. In the experimental part the SIMS experiments on clean Ni and Al surfaces and on Ni surface covered with a submonolayer of adsorbed K, Na and Al are reported. It has been found and it is for the first time reported that the energy distribution of ions sputtered from a submonolayer of adatoms is independent of energy (200–2500 eV) and mass (Ar⁺ Xe⁺ of incident ions and depends only upon the adsorption energy of the adatom. The energy distribution of ions sputtered from bulk samples has been found dependent on the primary ion energy. The measurement of the absolute value of R⁺ has shown that there is a strong correlation between the number of the adatom valence d-electrons and the value of R⁺, the value of R⁺ being smaller for atoms with more d-electrons. These experimental data have been compared with the theoretical expressions and the important role of the mechanism which takes into account the bending of the adatom energy level has been assessed.     

A comparison of the electronic properties of CO adsorbed on a thick copper film,and on a copper monolayer deposited on Ru(0001), as determined by metastable quenching spectroscopy

We use thermal desorption and metastable quenching spectroscopy to study the properties of CO chemisorbed on Ru(0001); on a Cu monolayer deposited on Ru(0001); and on a Cu film. We find that CO binds more strongly to the Cu monolayer than the Cu film; that the Penning ionization peaks of CO are more prominent on the film; and that addition of CO causes a greater lowering of the work function of the Cu film, than that of the Cu monolayer. We also observe that the 2π¹ emission of CO adsorbed on Ru(0001) has a double peak; that the saturation of Ru(0001) with CO seems to lead to CO tilting; and that the Penning spectrum of CO on Cu has two small features which have not been observed for CO adsorbed on other metals.