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.     

The influence of lateral interactions on the vibrational spectrum of adsorbed CO

Vibrational spectroscopy of adsorbed CO often reveals strong shifts of the CO stretch absorption band to higher frequencies as a function of increasing coverage. It is shown that such shifts are due essentially to dipole-dipole interactions. The extent of the experimentally observed effect can, however, only be reproduced when screening by the substrate is taken into account properly.     

Chemical effects on vibrational properties of adsorbed molecules on metal surfaces

Vibrational properties of chemisorbed molecules on metal surfaces are studied with a focus on the coverage (θ) dependent chemical shift of the frequencies. The electronic properties of an incomplete monolayer of adsorbates are calculated by means of the coherent potential approximation, in which the electron hopping between the adsorbates (band formation effect) and the depolarization effect due to the proximity of ionized adsorbed molecules are taken into account consideration. It is found that in a weakly chemisorbed system, such as CO/Cu, the negative shift in chemical origin amounts to the positive dipole shift at low coverage. It is also shown that the variation of the back-donated charge with θ gives rise to the coverage dependent polarizability, which in turn influences the frequency shift estimated by the previous dipole coupling theory. The coverage dependent back-donation also plays a significant role in the work function change Δϕ of the substrate. The polarity of a weakly chemisorbed CO remains unchanged compared to a free CO (⁻CO⁺) so that Δϕ exhibits the initial lowering in the presence of the positive dipoles. The increase in the back-donated charge with θ causes the decrease in the effective dipole moment towards a compensation of the positive hole due to 5σ donation. A simple explanation is offered to clarify the characteristic difference of the work function change between the strongly and weakly chemisorbed CO molecules on metal surfaces. In particular, a possible origin of the work function minimum observed for CO/Cu systems is discussed in terms of the coverage dependent back-donation.     

Coupled harmonic oscillator models of carbon monoxide adsorbed on stepped,platinum surfaces

Harmonic oscillator models are used to explain recent experimental data on infrared absorption by CO molecules adsorbed on two stepped platinum surfaces. These data reveal only a lower frequency band at low coverage and only a higher frequency band at high coverage. Both bands exist over a range of intermediate coverages. The data are explained by a coupled-dipole model which includes the effects of electronic polarizability, the tilted orientation of CO molecules at step sites, and the electric field enhancement at step sites. The lower-frequency band is associated with CO molecules adsorbed on step sites and the higher-frequency band is associated with two-dimensional islands consisting of both step and terrace CO. The model explains the observed variation of frequency and intensity with coverage for CO adsorption on Pt(533) and Pt(432) surfaces. The model calculations indicate that the wavenumber for a single, linearly bonded CO molecule is about 9 cm⁻¹ higher on a terrace site than on a step site.     

Infrared spectra for co isotopes chemisorbed on Pt “111”: Evidence for strong absorbate coupling interactions

Previous results for ¹²C¹⁶O chemisorbed on a Pt“111” recrystallised ribbon revealed that the infrared absorption band due to the CO stretch appears at low coverages at 2063 cm^?1 and shifts to ~2100 cm^?1 at saturation coverage at 300 K. The cause of this shift is studied in the present work, by investigating the vibrational spectra from a variety of mixtures of ¹²C¹⁶O and ¹²C¹⁶O. The results show that there is a strong dipole-dipole coupling interaction between adsorbate molecules in the overlayer, and provide conclusive evidence that the 35 cm^?1 frequency shift observed with increasing coverage for ¹²C¹⁶O is attributable to coupling.     

The reflection-absorption infrared spectrum of the dioxygen species adsorbed on platinum recorded by FT-IR spectroscopy

The reflection-absorption infrared spectrum of oxygen adsorbed on a recrystallized Pt foil at 80K shows a single band at 875 cm^?1 at saturation coverage. The spectrum produced by an equilibrated mixture of oxygen isotopes confirms that the adsorbate is molecular. The apparent effective charge, e*, calculated from the RAIR spectrum is in good agreement with that calculated from the electron energy loss (EEL) spectrum reported previously. The coverage dependent frequency shift can be accounted for by dipole-dipole coupling. The large inherent bandwidth is probably due to enhanced vibrational relaxation via electron-hole pair formation.     

The adsorption of CO on evaporated iridium films,studied with infrared reflection-absorption spectroscopy

The adsorption of ^12CO on Ir films evaporated under ultrahigh vacuum (UHV) conditions was studied using infrared reflection-absorption spectroscopy (IRAS). Only a single absorption band was observed at 300 K, shifting continuously from the “singleton” value ~2010 cm^?1 at very low coverages to 2093 cm^?1 at saturation coverage. This band is attributed to CO adsorbed on top of the surface atoms. Synchronously with this shift the bandwidth at half maximum intensity $\text{Δv}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ decreases from ~30 to 8 cm^?1. The integrated peak area increases linearly with coverage up to a relative coverage (θ_r) of approximately 0.4, then the increase levels off and a maximum is observed. Upon continuing adsorption the intensity decreases slightly. In addition results are presented on adsorption at 300 K of ¹²CO?¹³CO isotopic mixtures. The coverage induced frequency shift is discussed in terms of a dipole-dipole coupling mechanism and it is concluded that intermolecular coupling can explain the shift (~83 cm^?1) observed. The decrease in intensity at coverages > 0.4 is attributed to the formation of a compressed overlayer with part of the CO molecules adsorbed in a multicentre position with different spectral properties. No infrared bands of nitrogen adsorbed at 78 K could be detected at pressures up to 6.7 kPa (1 Pa = 0.0075 Torr, 1 Torr = 133.32 Pa).     

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.     

Vibrational dynamics of fullerene molecules adsorbed on metal surfaces studied with synchrotron infrared radiation

Infrared (IR) spectroscopy of chemisorbed C₆₀ on Ag (111), Au (110) and Cu (100) reveals that a non-IR-active mode becomes active upon adsorption, and that its frequency shifts proportionally with the charge transferred from the metal to the molecule by about 5 cm^-1 per electron. The temperature dependence of the frequency and the width of this IR feature have also been followed for C₆₀/Cu (100) and were found to agree well with a weak anharmonic coupling (dephasing) to a low-frequency mode, which we suggest to be the frustrated translational mode of the adsorbed molecules. Additionally, the adsorption is accompanied by a broadband reflectance change, which is interpreted as due to the scattering of conduction electrons of the metal surface by the adsorbate. The reflectance change allows determination of the friction coefficient of the C₆₀ molecules, which results in rather small values (∼2×10⁹ s^-1 for Ag and Au, and ∼1.6×10⁹ s^-1for Cu), consistent with a marked metallic character of the adsorbed molecules. Pre-dosing of alkali atoms onto the metal substrates drastically changes the IR spectra recorded during subsequent C₆₀ deposition: anti-absorption bands, as well as an increase of the broadband reflectance, occur and are interpreted as due to strong electron–phonon coupling with induced surface states. Received: 6 June 2001 / Accepted: 23 October 2001 / Published online: 3 April 2002     

Infrared reflection absorption spectra of Fe(CO)5 adsorbed on Au surfaces

Infrared reflection absorption spectroscopy (IRAS) shows that the CO stretching bands of Fe(CO)₅ adsorbed on Au surfaces are significantly different in band shape as well as in frequency from the bands observed in a transmission mode. This difference has been observed for other metal substrates and explained in terms of the anomalous dispersion of the refractive index in the region of the observed bands. The refractive indices of Fe(CO)₅ are calculated using the Kramers-Kronig relation from the transmission spectra of Fe(CO)₅ adsorbed on a sapphire plate, an SiO₂-coated sapphire plate, and an Au film evaporated on a sapphire plate, and the IRA spectra of Fe(CO)₅ adsorbed on Au are calculated using Fresnel's formula. The results show that the ν₁₀ band of Fe(CO)₅ becomes very sharp and shifts to higher frequencies by more than 10 cm⁻¹, while the ν₆ band becomes a shoulder of the v₁₀ band, in good agreement with the observed IRA spectra. The IRAS calculation also shows that the weak band observed at 2114 cm⁻¹ for the Au film remains unchanged in position, in agreement with the observed IRAS.     

Effect of side by side interactions on the thermodynamic properties of adsorbed CO molecules on the Ni(111) surface: a cluster model study

The effect of electrostatic interactions on vibrational frequencies and thermodynamic properties of CO adsorbate on the Ni(111) surface is calculated by taking the first and second nearest-neighbour interactions into account. In order to obtain reasonable results, the cluster model of various surface adsorption sites with CO adsorbate is partially optimized, using Density Functional Theory and also the MP2 method for the hcp site. Comparison between DFT and MP2 results shows that DFT results are more reliable for this system. The stretching and bending frequencies of CO adsorbate are calculated using both Partial Hessian Analysis and Cluster–Adsorbate Coupling methods. Stretching and bending frequencies are both shifted by the side by side interactions. The coupling of surface phonons and adsorbate vibrations reduces the side effects. The largest side effects on the vibrational internal energy, isochoric heat capacity, entropy and total Helmholtz free energy of adsorbed CO molecule calculated using the CAC method are found for 0.5 ML coverage. The results of the CAC method are better, but the PHA method can be used as a simple upper bound estimation. The adsorptive phase acts as an intelligent material in such a way that it changes its configuration in order to reduce the side effects.     

Summary abstract: Vibrational excitations of hydrogen and oxygen on Pd(100)

EMIRS spectra for ¹²CO-¹³CO mixtures on platinum, HSO₄^? and acrylonitrile on gold and water on silver are discussed. These examples illustrate the adsorbate identification, bonding, and orientation information which EMIRS data offer to complement electrochemical Raman data and infrared reflection absorption and EELS results. The advantages of experimental fine control of the electronegativity of the metal are shown with application to studies of coupling mechanisms between adsorbed species.     

Reflection-absorption infrared and thermal desorption studies of carbon monoxide and hydrogen adsorbed on rhodium

Reflection-absorption infrared spectroscopic and thermal desorption techniques have been used to study the interaction of mixtures of carbon monoxide and hydrogen with evaporated rhodium films. For equimolar mixtures near 10^?9 Torr, hydrogen adsorbed much more rapidly, but long exposure times or increases in CO pressures to 10^?6 Torr led to its partial, but never complete, displacement by adsorbed carbon monoxide. Hydrogen desorption spectra taken during the displacement process showed two peaks which was consistent with a cooperative interaction between adsorbed CO and H species. In contrast to previous transmission studies of CO adsorption on small rhodium particles, the present reflection—absorption infrared study of the film system showed a single absorption band at 2075 ±10 cm^?1. While explanations for the discrepancy in terms of particle size effects are possible it is considered more likely that all CO molecules are linearly bound to individual Rh atoms in the present situation. In our work, increases in CO pressure (especially above 10^?6 Torr) were accompanied by an upward frequency shift (from 2065 cm^?1 to 2085 cm^?1) and a narrowing in half width (from 25 to 17 cm^?1). Several possible explanations for the latter unusual effect are discussed.     

Collective vibrational modes in isotopic mixtures of CO adsorbed on Cu (100)

Isotopic mixtures of CO adsorbed on Cu(100) have been studied by infrared spectroscopy. For mixtures of ¹²C¹⁶O/¹²C¹⁸O at a constant coverage of 0.5 it was found that the molecules constitute a collective vibrational system. The C-O stretching vibrations were studied as a function of composition of the overlayer and the data are successfully interpreted by a dipole — dipole coupling theory.     

Highly sintered nickel oxide: surface morphology and FTIR investigation of CO adsorbed at low temperature

The monolayer of CO molecules adsorbed at low temperature on highly sintered nickel oxide, gives rise to a very symmetric IR absorption band at 2136 cm⁻¹ (θ_max) with a full-width at half-maximum (FWHM) of 3.7 cm⁻¹. This band shifts to higher frequenc upon decreasing the coverage, reaching the 2152 cm⁻¹ value for θ→0. The observed shift is due to changes in the lateral interactions (dynamic and static) among the adsorbed molecules. The observed spectral simplicity implies that most of the adsorbed CO molecules occupy crystallographically identical sites with a similar environment. Moreover, the remarkably small half-width indicates that inhomogeneous broadening effects, due to surface defects, are very small and that NiO microcrystals behave as single crystals. The morphology of microcrystals has been studied by SEM, AFM and HRTEM techniques: it was concluded that the surface termination of the sample is mainly represented by the (100) and (111) faces.     

Interpretation of the IR absorption band of CO adsorbed on Ru(001)

The qualitative variation of the IR absorption spectrum of CO adsorbed at 200 K on Ru(001) in the range of coverage $\text{0 < θ ?}\text{1}\text{3}$ is reproduced by a calculation based on a random configuration of adsorbed molecules and nearest neighbour interactions.     

Site occupation of CO adsorbed on Ni(100) at high CO pressures

The adsorption of CO on Ni(100) has been investigated by infrared reflection-absorption spectroscopy in the temperature range of 85 to 300 K. At 300 K we achieved CO coverages up to 0.8 ML by applying CO pressures up to 1 mbar. The fraction of CO molecules adsorbed at terminal and bridge sites on Ni(100) at a fixed temperature depends strongly on the total CO coverage. The variation of the adsorption site occupancy with coverage is well reproduced by a lattice-gas model which takes into account the lateral interaction of the adsorbed molecules.     

Size-selected, supported clusters: the interaction of carbon monoxide with nickel clusters

We report on the size-dependent chemical reactivity of nickel clusters with up to 30 atoms. Monodispersed Ni₃₀ clusters show a higher reactivity for CO dissociation than Ni₁₁ and Ni₂₀. Under our experimental conditions the smallest nickel clusters (Ni_x, x<4) produce nickelcarbonyl complexes. These results demonstrate that such small clusters are unique for catalytic reactions not only due to their high surface-to-volume ratio but also essentially because of the distinctive properties of different cluster sizes. In addition thermal desorption spectroscopy of CO shows that on average four molecules are weakly adsorbed per Ni₁₁ at saturation coverage. Using an isotopic mixture of ¹²CO and ¹³CO, infrared spectroscopy reveals the existence of a vibrational coupling interaction between the four COs. A semi-classical model of interacting dipoles is applied to correlate the observed vibrational frequency shifts with the arrangement of the COs on the cluster. This simple analysis favors a three-dimensional structure for the deposited clusters. Received: 23 March 1998/Accepted: 25 August 1998     

Adsorption of carbon monoxide on tungsten (210)

Thermal desorption spectra taken after adsorption of carbon monoxide at room temperature on W(210) show sequential formation with increasing coverage of strongly bound β₂ and β₁ binding states, correlated to the sequential formation of P(2 × 1) and (1 × 1) adsorbate structures as observed by LEED. Adsorption at room temperature gives a poorly ordered arrangement of adsorbed CO molecules, but well-ordered structures are produced by subsequent anneal. For adsorption without anneal the work function increases monotonically with coverage to a maximum of Δφ = + 0.70 eV at saturation coverage of 1 monolayer. For adsorption followed by anneal the work function dependence upon coverage is less simple, with even a decrease of work function at coverages less than a quarter monolayer. LEED intensity-voltage measurements from P(2 × 1)CO and P(2 × 1)N structures suggest that CO molecules occupy the sites of 4-fold symmetry upon which nitrogen is believed to be adsorbed. The distinction between the β₂ and β₁ states of adsorbed CO is attributed to heterogeneity induced by the reduction in binding energy of a CO molecule when its nearest-neighbor sites are occupied.     

Neutron inelastic spectroscopy of benzene chemisorbed on Raney platinum

The neutron inelastic scattering spectrum of benzene adsorbed at 300 K on Raney platinum has been measured between 350 and 2250 cm^?1. No deshydrogenation of the molecules is observed so that the benzene ring must be adsorbed parallel to the surface. Slight modifications of the force field of the model molecule (C₆H₆)Cr(CO)₃ were introduced to account for the vibrational frequency shifts. The benzene molecule is found less perturbed on platinum than on nickel. The calculated frequencies of adsorbed C₆H₆ and C₆D₆ are used to reassign some modes previously observed by electron loss spectroscopy.