Infrared spectroscopy of CO on NaCl(100) IV. Bandshape analysis

The infrared bandshape of monolayer CO on NaCl(100) at 4 K is slightly asymmetric and its bandwidth of 0.09 cm⁻¹ is the narrowest observed for any surface molecule. From a variety of experiments we know that this band profile is determined by heterogeneous rather than homogeneous broadening. We offer an explanation for the experimentally observed bandshape by proposing that irregularities in the surface of the cleaved NaCl crystals give rise to many distinct two-dimensional domains of physisorbed CO. A first-order perturbation calculation, which generates the infrared cross-section as a function of domain size, has been used to estimate that on the order of 10⁵ molecules reside in an average size domain. The accuracy of the calculation, whose Hamiltonian represents the intermolecular potential among CO molecules by electric multipole coupling, will be discussed. We use the same coupling Hamiltonian to explore the effect of submonolayer coverages, isotopic dilution, and surface heterogeneities on the infrared spectroscopy of CO on NaCl(100). Our calculations can account for some, but not all, of the observed spectroscopic data.     

Cu(100)表面CO分子单层膜的原子结构

利用第一性原理研究了覆盖度分别为1.00, 0.50和0.25 ML时CO分子单层膜在Cu(100)表面的吸附系统. 计算表明CO分子对不稳定. 获得了CO分子单层膜在虚拟Cu(100)表面的原子结构, 以及CO分子单层膜在Cu(100)表面吸附系统的原子结构. 当CO分子单层膜在Cu(100)表面的三个吸附位吸附, 覆盖度为1.00 ML时, 顶位和桥位都稳定, 而空心位不稳定; 覆盖度为0.50和0.25 ML时, 三个吸附位都稳定.比较吸附前后CO分子单层膜的原子结构, 可知CO分子和Cu(100)表面的相互作用强于CO分子单层膜之间的相互作用. 关键词： CO分子单层膜 自组装 CASTEP Cu(100)     

Infrared spectroscopy of CO on NaCl(100) III. Submonolayers and isotopic mixtures

In this paper we explore lateral interactions within the CO adsorbate on NaCl(100) through its infrared spectroscopy.

The infrared absorption of a monolayer of CO on NaCl(100) at 55 K presents a symmetric band that can be fitted to a Lorentzian profile. As the coverage is reduced the integrated absorbance decreases, the band shifts to higher wavenumber, and its profile broadens and becomes less symmetric. These observations are consistent with a random arrangement of the molecules for submonolayer coverages. This spectroscopic behavior is likely due to myriad electric multipolar interactions plus dispersion and repulsion contributions that become diminished as coverage is reduced. Heterogeneities at the surface can also give rise to coverage dependent spectroscopic profiles. Finally, increased mobility of the adsorbate for decreasing coverages can effect the spectroscopic response. Because of these complications the quantitative behavior of the submonolayer absorption frequency and bandshape has not been successfully modeled. Isotopic composition of the monolayer also affects the spectroscopy. Dilution of ¹²C¹⁶O with ¹³C¹⁶O shifts the absorption to lower wavenumber and its band profile becomes broader and asymmetric. A similar behavior is exhibited by the ¹³C¹⁶O band on dilution with ¹²C¹⁶O. Models based on dynamic dipole coupling can account for the spectroscopic behavior of the isotope mixtures in the monolayer.     

Model calculation of infrared absorption by a monolayer of CO2 on NaCl(100)

We calculate the absorbance for a proposed model of CO₂ monolayer adsorption on the (100) surface of NaCl. The fitted results near the asymmetric stretch frequency agree quite well with recent experimental data. Implications for further measurements are discussed.     

Electronic structure of a cobalt monolayer on Cu(100)

Angular resolved photoemission spectra using synchrotron radiation have been measured for different amounts of cobalt evaporated on Cu(100). At room temperature cobalt grows layer-bylayer forming well-ordered layers in registry with the substrate, as judged by AES, LEED and UPS measurements. The energy position and linewidth of the Cu peaks remain unchanged when cobalt is deposited onto the surface, suggesting a rather weak interaction between the d-bands of Co and Cu. The two-dimensional band structure of the monolayer of cobalt has been determined. We have obtained a value for the magnetic exchange splitting of ΔE_exch = 0.80 ± 0.15 eV, which is nearly identical to the bulk value. A shift in the energy positions of the critical points for the monolayer versus bulk of cobalt is interpreted in terms of a narrower 2D density of states in the monolayer as compared to the bulk. A resonant valence-band two-electron satellite has been found. The correlation energy and screening effects of the two d-holes are very similar to the corresponding bulk values, while the decreased intensity of the satellite at resonance compared to the one for Co(0001) suggests that there are more d-states relative to s-states in the monolayer than in a bulk cobalt single crystal, in agreement with recent models of the valence band electronic structure at surfaces.     

The valence of Sm in a monolayer on Al(100)

Submonolayer and monolayer films of Sm deposited on an Al(100) crystal surface have been studied by LEED and X-ray photoemission spectroscopy. Layers deposited at — 100°C or RT and without subsequent annealing were disordered and composed of Sm in both divalent and trivalent forms. Annealed films showed a pseudo hexagonal structure and were predominantly trivalent. We conclude that a perfect monolayer of Sm on Al(100) is composed wholly of trivalent atoms.     

the determination of monolayer structure by infrared spectroscopy: CO2 on NaCl(100)

Infrared spectra of a ¹²C¹⁶O₂ monolayer on NaCl(100) are reported for the first time. The polarization and coverage dependence of spectroscopic signals indicate that there is only one kind of adsorption site, in which the molecular axis is tilted 68° from the surface normal. The adsorbate layer grows in the form of constant-density islands. Plane group symmetry analysis reveals a unique structure: there are two tilted molecules per layer unit cell, arranged in herringbone fashion. This structure is in quantitative agreement with the photometric observations. Splitting of the molecular v₃ band is interpreted by a vibration-vibration coupling mechanism.     

EELS of methane physisorbed on NaCl(100)

EELS spectra of CH₄, CD₄ and CH₂D₂ physisorbed on NaCl(100) at 40 K are presented. For the clean NaCl surface, increasing the incident beam energy to the 30 to 50 eV range is sufficient to overcome charging effects. However, when adsorbate is present charging prevents extended signal averaging. All methane modes appear to contribute to the EELS spectra. Loss peaks corresponding to adsorbate vibrations are very broad (400 to 600 cm⁻¹, FWHH), the low resolution being probably due to a combination of effects including surface disorder, scattering of electrons by gas phase molecules and charging effects.     

Adsorption of CO on a pseudomorphic palladium monolayer on Mo(110)

Adsorption of CO on a Pd monolayer (ML) supported on Mo(110) has been studied using low energy electron diffraction (LEED), temperature programmed desorption (TPD), and high resolution electron energy loss spectroscopy (HREELS). Three ordered CO substructures denoted as are observed with LEED. The binding energy of C0 on the 1.0 ML Pd/Mo(110) surface is reduced by 12 kcal/mol relative to the Pd(111) surface, consistent with previous results for supported palladium monolayers on other substrates. Two vibrational states of C0 are observed near 1950 and 2050 cm⁻¹, with the feature at the lower wavenumber having the smaller binding energy.     

Adsorbate-oxide interactions during the NO + CO reaction on MgO(100) supported Pd monolayer films

The potential energy diagram for the NO+CO reaction on 1, 2, and 3 monolayer (ML) Pd films supported by MgO(100) is calculated using density functional theory. Thin Pd films are generally found to be more reactive than thick films, with a notable exception for nitrogen adsorption on 2 ML Pd/MgO(100). For this system an attractive through-the-metal adsorbate-oxide interaction of 0.5 eV is identified. Nitrogen adsorption is consequently estimated to provide a thermodynamic driving force for the reconstruction of MgO(100) supported 3 ML (or thicker) Pd clusters into thinner Pd clusters.     

Electronic structure and magnetism of a Pd monolayer on Fe(100)

Spin-polarized self-consistent localized-orbital (SCLO) calculations have been performed for a five-plane slab simulating Pd/Fe(100). Pd monolayers were placed in registry with both faces of a three-plane Fe(100) slab. We chose a PdFe bond length equal to the sum of metallic radii, 2.62 Å, and an FeFe bond length equal to the bulk value, 2.49 A. The computed energy bands for the Pd/Fe₃/Pd slab resemble those calculated for a five-plane Fe(100) slab, except for a positive work-function shift of 0.5 eV. The Pd monolayer has a magnetic moment of 0.37μ_B/atom. The magnetic moment of an adjacent iron atom is 2.74μ_B, slightly smaller than the value 2.89μ_B at the surface of Fe₅, but still significantly larger than the central-plane value 2.37μ_B. The d bands of the two metals are strongly hybridized, but very little charge transfer takes place across the interface. Compared with the isolated Pd(100) monolayer, or the clean Fe(100) slab, the surface density of states of Pd/Fe(100) is rather weak near the Fermi level, suggesting a reduced chemical reactivity for this surface.     

Esdiad from CO on W(100): Evidence for non-vertically adsorbed CO

Mass-discriminating measurements of the angular distributions of electron-stimulated ion desorption (ESDIAD) from CO adsorbed on W(100) and coadsorbed with C and O have been performed. The O⁺ beams indicate normal and off-normal (by 5 to 13°) desorption which is interpreted as due to CO molecules bound in symmetric and in two types of unsymmetric bridges. Preadsorption of C suppresses the vertical state, while oxidation of the surface suppresses the off-normal states.     

Dissociation of CO on Mo(100)

LEED and Auger spectroscopy have been used to obtain the superficial carbon density on a carburized Mo(100) surface. Using this calibration procedure, comparison is made between the carburization and the adsorption of CO. The results are consistent with the dissociation of the (β-CO phases; using this assumption the adsorption kinetics of CO on Mo(100) is reinterpreted.     

Explosive production of CO2 from (NO + CO)/Pd(100)

Explosive CO₂ production followed by the gradual N₂ production has been observed by TDS as a result of the NO-CO reaction on the Pd(100) surface which does not reconstruct. The location and width of the CO₂ desorption peak have been found to be almost independent of the total coverage. For this autocatalytic reaction, the vacancy requirement model is considered to be valid. Change in the CO₂ production by varying the NO coverage (with the CO coverage fixed) has been also studied, from which information on the island formation is derived.     

The structure of ammonia molecules on MgO(100) surfaces from monolayer to bulk condensation

The structure of ND₃ molecules adsorbed on MgO (100) surfaces has been studied by neutron diffraction within the 10–80 K temperature range and at 0.7, 1 and 2.3 monolayer coverage. The neutron spectra suggest that the monolayer presents a short range order with a hcp packing of ammonia molecules, a coherence length of 25 ± 2 Å and a nearest neighbour distance of 3.61 ± 0.04 Å The molecular spacing remains the same between 10 and 80K that we interpret as small higher order commensurate islands. Above one monolayer coverage, bulk crystallites form on top of the first monolayer.     

Adsorption of O and CO on Ir(100) from first principles

The adsorption of O and CO on Iridium (100) surface with different coverages (Θ = 1.0, 0.5, 0.25 monolayer (ML)) is studied using density functional theory (DFT). The most energetically preferred site of adsorption for O is found to be the bridge site. However, the top site is the preferred one for CO at coverages of 0.25 ML and 0.5 ML. Oxygen adsorbed on the bridge site at 0.25 ML and 0.5 ML coverages causes a row pairing. A missing row reconstruction appears in the case of 0.25 ML coverage. We find that the adsorption of O (CO) on Ir(100) surface causes disruptions of Ir–Ir bonds in the metal, which reduces (increases) the Ir–Ir bond length.     

Structure of CO adsorbed on Ni (100)

The c(2 × 2) configuration of CO chemisorbed on Ni(100) has been examined by the dynamical LEED method of surface structure analysis. Experimental LEED intensity spectra of the (00), ($\text{1}\text{2}\text{1}\text{2}$) (10) and (11) LEED beams measured at 175 K are compared with the corresponding calculated spectra for two different CO potential constructions and a number of trial structures. The best agreement was found for a structure where the CO molecules sit directly above the Ni atoms with vertical spacings between the Ni and C and the C and O layers of 1.80 ± 0.10 A and 0.95 ± 0.10 Å respectively. It is proposed that the CO molecule is tipped over at an angle of 34° ± 10° with respect to the surface normal so that the actual carbon-oxygen bond length is close to the figure 1.15 Å found in Ni(CO)₄.