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).     

The adsorption sites of CO on Ni(111) as determined by infrared reflection-absorption spectroscopy

The adsorption of CO on Ni(111) has been studied using infrared reflection-absorption spectroscopy combined with LEED, Auger electron spectroscopy, thermal desorption spectroscopy and work function measurements. At low CO coverage (θ = 0.05) CO adsorbs on threefold sites with a strecthing frequency given by ω_CO = 1817 cm^?1. At θ = 0.30 all molecules have shifted to two-fold sites, and θ = 0.50, where a c(4 × 2) structure is observed, ω_CO = 1910 cm^?1. At θ = 0.57, with a (√7/2) × √7/2)R19.1° structure, one quarter of the molecules are adsorbed on top of the nickel atoms with the others in two-fold sites. Molecules bonded on the top sites give rise to a band at 2045 cm^?1. The frequency shift due to dipole-dipole interactions is small compared with the shift resulting from bonding to different crystallographic sites.     

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.     

Chemisorption of carbon monoxide on platinum {111}: Reflection-absorption infrared spectroscopy

Reflection-adsorption infrared spectroscopy has been combined with thermal desorption and surface stoichiometry measurements to study the structure of CO chemisorbed on a {111}- oriented platinum ribbon under uhv conditions. Desorption spectra show a single peak at coverages > 10¹⁴ molecules cm^?2, with the desorption energy decreasing with increasing coverage up to 0.4 of a monolayer, and then remaining constant at ≈135 kJ mol^?1 until saturation. The “saturation” coverage at 300 K is 7 × 10¹⁴ molecules cm^?2, and no new low temperatures state is formed after adsorption at 120 K. Infrared spectra show a single very intense, sharp band over the spectral range investigated (1500 to 2100 cm^?1), which first appears at low coverages at 2065 cm^?1 and shifts continuously with increasing coverage to 2101 cm^?1 at 7 × 10¹⁴ molecules cm^?2. The halfwidth of the band at 2101 cm^?1 is 9.0 cm^?1, independent of temperature and only slightly dependent on coverage. The band intensity does not increase uniformly with increasing coverage, and hysteresis is observed between adsorption and desorption sequences in the variation of both the band intensity and frequency as a function of coverage. The frequency shift and the virtual invariance of the absorption band halfwidt with increasing coverage (Jespite recent LEED evidence for overlayer compression in this system) are attributed to strong dipole-dipole coupling in the overlayer.     

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.     

Adsorption of oxygen on Pt(111)

Oxygen adsorbed on Pt(111) has been studied by means of temperature programmed thermal desorption spectroscopy (TPDS). high resolution electron energy loss spectroscopy (EELS) and LEED. At about 100 K oxygen is found to be adsorbed in a molecular form with the axis of the molecule parallel to the surface as a peroxo-like species, that is, the OO bond order is about 1. At saturation coverage (θ_mol= 0.44) a $\text{(}\text{3}\text{2}\text{×}\text{3}\text{2}\text{)R15°}$ diffraction pattern is observed. The sticking probability S at 100 K as a function of coverage passes through a maximum at θ = 0.11 with S = 0.68. The shape of the coverage dependence is characteristic for adsorption in islands. Two coexisting types of adsorbed oxygen molecules with different OO stretching vibrations are distinguished. At higher coverages units with v-OO = 875 cm^?1 are dominant. With decreasing oxygen coverages the concentration of a type with v-OO = 700 cm^?1 is increased. The dissociation energy of the OO bond in the speices with v-OO = 875 cm^?1 is estimated from the frequency shift of the first overtone to be ~ 0.5 eV. When the sample is annealed oxygen partially desorbs at ~ 160K, partially dissociates and orders into a p(2×2) overlayer. Below saturation coverage of molecular oxygen, dissociation takes place already at92 K. Atomically adsorbed oxygen occupies threefold hollow sites, with a fundamental stretching frequency of 480 cm^?1. In the non-fundamental spectrum of atomic oxygen the overtone of the E-type vibration is observed, which is “dipole forbidden” as a fundamental in EELS.     

Vibrational characterization of carbon monoxide adsorption on sulfur modified Ni(100) surfaces

Carbon monoxide adsorption has been studied on a series of presulfided Ni(100) surfaces using vibrational spectroscopy. The sulfided Ni(100) surfaces were characterized using Auger electron spectroscopy and low energy electron diffraction, binding states were isolated by heating CO-dosed surfaces to prescribed temperatures, corresponding to the desorption temperatures of the CO. Adsorption of CO on Ni(100) with a p(2 × 2) array of sulfur lead to CO stretching frequencies of 1740 and 1930 cm^?1 corresponding to desorption temperatures of 370 and 290 K, respectively. Adsorption of CO into the c(2 × 2)S structure resulted in a CO stretching frequency of 2115 cm^?1 and a desorption peak near 140 K. The binding sites on the p(2 × 2)S structure were interpreted as metal four-fold hollows and bridging sites. The high frequency state was interpreted as weak bonding into the four-fold hollow with back donation into the π¹ orbital on CO restricted by stearic hindrance due to adsorbed sulfur. Both the thermal desorption and vibrational results indicated that local CO-sulfur interactions are dominant on the presulfided Ni(100) surface in the coverage range studied.     

Evidence for long-range surface state mediated interaction between sodium atoms on copper (0 0 1)

Inelastic helium atom scattering from sodium atoms on the Cu(0 0 1) surface at 50 K reveals a remarkable 15% increase in the frequency of the frustrated translational vibrations (T-mode) from ?ω=5.56 to 6.34 meV with increasing coverage from Θ_Na=0.008 to 0.125. The coverage dependence and the negligible dispersion of the frequency cannot be explained by the direct dipole-dipole coupling but are well-understood in terms of the Lau-Kohn effective long-range interaction via intrinsic surfaces states.     

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.     

Electron stimulated desorption of CO from

The electron impact behavior of CO adsorbed on was investigated. The desorption products observed were neutral CO, CO⁺, and O⁺. After massive electron impact residual carbon, C/W = 0.15, but not oxygen was also found, suggesting that energetic neutral O, not detected in a mass analyzer must also have been formed. Formation of β-CO, i.e., dissociated CO with C and O on the surface was not seen. The total disappearance cross section varies only slightly with coverage, ranging from 9 × 10 ⁻¹⁸ cm² at low to 5 × 10⁻¹⁸ cm² at saturation (CO/W = 0.75). The cross section for CO⁺ formation varies from 4 × 10⁻²² cm² at satura to 2 × 10⁻²¹ cm² at low coverage. That for O⁺ formation is 1.4 × 10⁻²² cm² at saturation and 2 × 10⁻²¹ cm² Threshold energies are similar to those found previously [J.C. Lin and R. Gomer, Surf. Sci. 218 (1989) 406] for and CO/Cu₁/W(110) which suggests similar mechanisms for product formation, with the exception of β-CO on clean W(110). It is argued that the absence or presence of β-CO in ESD hinges on its formation or absence in thermal desorption, since electron impact is likely to present the surface with vibrationally and rotationally activated CO in all cases; β-CO formation only occurs on surfaces which can dissociate such CO. It was also found that ESD of CO led to a work function increase of the remaining Pd₁/W(110) surface of 500 meV, which could be annealed out only at 900 K. This is attributed to surface roughness, caused by recoil momentum of energetic desorbing entities.     

Adsorption of nitrogen on platinum

The adsorption and desorption of nitrogen on a platinum filament have been studied by thermal desorption techniques. Nitrogen adsorption becomes significant only after any carbon contamination is removed from the surface by heating the platinum filament in oxygen, and after the CO content in the background gas is reduced substantially. At room temperature nitrogen populates an atomic tightly bound β-state, E^≠ = 19 kcal mole^?1. The saturation coverage of the (3-state is 4.5 × 10¹⁴ atoms cm^?2. Formation of the (β-state is a zero order process in the pressure range studied. At 90 K two additional α₁- and α₂-desorption peaks are observed. The activation energy for desorption for the α₂-state is 7.4 kcal mole^?1 at low coverage decreasing to 3 kcal mole^?1 at saturation of this state, 6 × 10 molecules cm^?2. The maximum total coverage in the α-states was 1.2 × 10¹⁵ molecules cm^?2. A replacement process between the β- and α-states has been observed where each atom in the (β-state excludes two molecules from the α-state.     

The adsorption of CO on Cu(110)

The adsorption of CO on Cu(110) has been studied by LEED, surface potentials and infrared spectroscopy. With increasing surface coverage the s.p. passes through a maximum value of 0.29 V and than falls to 0.17 V at saturation. The heat of adsorption is nearly constant (~55 kJ mol^?1) up to the maximum s.p. but then falls rapidly. A ( 2× 1) structure is formed near the s.p. maximum, followed by a structure which is compressed in the [11?0] direction and poorly ordered in the [001] direction but tending towards c(1.3 × 2). At low coverage two infrared bands appear at 2088 and 2104 cm^?1; their relative intensity is similar at 77, 195 and 295 K. As the coverage increases, the bands shift in frequency and merge into a single band at 2094 cm^?1. The origin of the two bands is discussed in relation to the overlayer structure. Strong interaction between CO molecules is shown by the spectra of mixtures of ¹³CO and ¹²CO.     

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.     

The coadsorption of potassium and co on the pt(111) crystal surface: A TDS,HREELS and UPS study

The interaction of CO with a potassium covered Pt(111) surface is investigated using thermal desorption (TDS), high resolution electron energy loss (HREELS) and ultraviolet photoelectron (UPS) spectroscopies. When submonolayer amounts of potassium are preadsorbed, the adsorption energy of CO increases from 25 to 36 kcal/mole, while substantial shifts in the site occupancy from the linear to the bridged site are observed. The CO stretching vibrational frequencies are shown to decrease continuously with either increasing potassium coverage or decreasing CO coverage. A minimum CO stretching frequency of 1400 cm^?1 is observed, indicative of a CO bond order of 1.5. The work function decreases by up to 4.5 eV at submonolayer potassium coverages, but then increases by 1.5 eV upon CO co-adsorption. The results indicate that the large adsorption energy, vibrational frequency and work function changes are due to molecular CO adsorption with a substantial charge donation from potassium through the platinum substrate and into the 2π¹_CO orbital.     

The effect of resonance interactions on the absorption spectra of (SF6)2 dimers in low-temperature matrices: Calculations and experiment

The IR absorption spectra of (SF₆)₂ dimers in Ar and N₂ matrices are investigated at 11 K. As a result of the resonance dipole-dipole interaction, the band of the triply degenerate vibration v ₃ is split into two components v _{X, Y} and v _Z. In comparison with the gaseous state, the splitting Δv = v _{X, Y} ? v _Z in the argon matrix decreases to 18.45 cm^?1, whereas, in the nitrogen matrix, the band v _{X, Y} is split into components v _X and v _Y, with the splitting being equal to δ ≈ 0.9 cm^?1. A model that takes into account the influence of the matrix on the spectra of dimers is developed. The model makes it possible to successively (i) calculate the resonance spectrum of an isolated dimer in terms of the model of local modes taking into account resonance interactions, (ii) determine with the help of the Monte Carlo method the structure of a matrix consisting of 512–1440 Ar (or N₂) atoms and a rigid (SF₆)₂ dimer, and (iii) take into account interactions of local dipole moments of a dimer with particles of the matrix in the approximation of dipole-induced dipole interactions. The model developed satisfactorily describes the experimental results. The calculated frequencies v _Z, v _X, and v _Y of a dimer in the matrix are shifted toward smaller frequencies as compared to the gaseous state, while the resonance splitting decreases virtually by 2 cm^?1. It is shown that, in an argon matrix with a symmetric arrangement of argon atoms nearest to a dimer, the splitting of v _{X, Y} proves to be smaller than 0.05 cm^?1. In a nitrogen matrix, this splitting increases virtually to 0.4 cm^?1.     

An estimation of surface vibration excitation cross sections of CO,O and H adsorbed on tungsten

A simple model is described for the evaluation of adatom loss spectra of dipole vibrations perpendicular to the substrate. It considers the inelastic electron scattering before and after specular reflection on the substrate. By comparing the loss to elastic peak area the total cross section σ of the inelastic electron scattering can be estimated. The model was used for evaluating experimental results published in the literature by Ibach, Froitzheim, Adnot, Backx and Barnes on the systems WCO, WO and WH. The main results are: for the CO 258 meV loss peak, σ = (11.6?18.3)×10^?18cm²; for the O 78 meV loss peak, σ = (8.4?16.2) × 10^?18cm²; and for the H 130 and 155 meV loss peaks, σ =(0.73?2.2) × 10^?18cm². They are close to the theoretical values. A reasonable agreement was found between σ values determined on WO and WβCO (dissociated) systems.     

Infrared spectra and stability of CO and H2O sorption over Ag-exchanged ZSM-5 zeolite: DFT study

The infrared spectra and stability of CO and H₂O sorption over Ag-exchanged ZSM-5 zeolite were investigated by using density function theory (DFT). The changes of NBO charge show that the electron transfers from CO molecule to the Ag⁺ cation to form an σ-bond, and it accompanies by the back donation of d-electrons from Ag⁺ cation to the CO (π*) orbital as one and two CO molecules are adsorbed on Ag-ZSM-5. The free energy changes ΔG, −5.55 kcal/mol and 6.52 kcal/mol for one and two CO molecules, illustrate that the Ag⁺(CO)₂ complex is unstable at the room temperature. The vibration frequency of C-O stretching of one CO molecule bonded to Ag⁺ ion at 2211 cm⁻¹ is in good agreement with the experimental results. The calculated C-O symmetric and antisymmetric stretching frequencies in the Ag⁺(CO)₂ complex shift to 2231 cm⁻¹ and 2205 cm⁻¹ when the second CO molecule is adsorbed. The calculated C-O stretching frequency in CO-Ag-ZSM-5-H₂O complex shifts to 2199 cm⁻¹, the symmetric and antisymmetric O-H stretching frequencies are 3390 cm⁻¹ and 3869 cm⁻¹, respectively. The Gibbs free energy change (ΔGH₂O) is −6.58 kcal/mol as a H₂O molecule is adsorbed on CO-Ag-ZSM-5 complex at 298 K. The results show that CO-Ag-ZSM-5-H₂O complex is more stable at room temperature.     

Dynamic and static interactions in CO overlayers on sintered NaCl films

Sublimation of NaCl onto a cold (77 K) NaCl plate followed by sintering at 300 K gives an IR transparent film formed by individual crystallites having cubic shape. CO adsorbed on {100} faces of the crystallites gives an IR peak at 2161-2156 cm^?1, while CO adsorbed on edges (steps) gives a weaker peak at ～2175 cm^?1. The CO molecules are adsorbed on Na⁺ ions and form a bidimensional array of parallel oscillators. Lateral adsorbate-adsorbate interactions (dynamic and static) are responsible for the shift of the IR frequency. The relative weight of the static and dynamic effects is estimated by using ¹²CO?¹³CO mixtures. The dynamic effect is well described by the modified Hammaker equation, with $\text{α}{}_{\mathrm{<mtext>v</mtext>}}\text{=0.0235}\text{A}\text{?}$^?3. The unusually low value of the dynamic polarizability agrees very well with literature data concerning the intensity of the stretching band of CO adsorbed on ions. The static effect, causing a consistent negative shift of the CO frequency, is related to through-solid interactions of inductive nature.     

Surface-enhanced Raman spectroscopy of electrochemically characterized interfaces; potential dependence of Raman spectra for thiocyanate at silver electrodes

Surface-Enhanced Raman Spectra have been obtained for thiocyanate anions at silver electrodes following an oxidation-reduction cycle (ORC) as a function of electrode potential and electrolyte composition and compared with the extent of thiocyanate adsorption determined under the same conditions from differential capacitance-potential data. A spectrograph equipped with an optical multichannel analyzer (OMA) detector and a scanning spectrometer were used to make the Raman measurements. Spectra were obtained over the frequency range 100–2200 cm^?1, where all three normal vibrations, CN stretching (v_CN, 2090–2120 cm^?1), CS stretching (v_CN, 735 cm^?1), NCS angle bending (δ_NCS, 450 cm^?1) occur, along with a low frequency vibration attributed to a metal-ligand stretching mode (v_ml, 200–215 cm^?1) arising from a silver-sulfur surface bond. Both v_CN and v_ML decreased in frequency as well as intensity as the potential was made more negative in the region ?100 to ?700 mV versus Ag/AgCl for bulk thiocyanate concentrations of one millimolar and above, even though the thiocyanate surface concentration remained close to a monolayer throughout. By means of rapid time-resolved spectral measurements following potential steps using the OMA, the decrease of the intensity and frequency of the v_CN mode with increasing negative electrode potential was separated into a rapid “reversible” component and a slower “irreversible” decay. The latter component is attributed to the decay of Raman-active sites associated with the dissipation of metastable silver clusters formed during the ORC, that are prevented from rearranging at more positive potentials due to the presence of surrounding anionic adsorbate.     

Electron beam interactions with CO on W {100} studied by Auger electron spectroscopy

The interaction of 2500 eV electrons with carbon monoxide chemisorbed on tungsten {100} was investigated by rapid-scan Auger electron spectroscopy. When no α state was present the O and C signals from the β state of CO were invariant during electron bombardment, giving an upper limit estimate for the electron stimulated desorption cross section, Q_β of 2 × 10^?21 cm². With the crystal at room temperature and saturated with CO, however, electron-beam induced accumulation of carbon was observed and characterised, the rate of the process being independent of CO pressure at pressures above 2 × 10^?8 Torr. At 450 K the rate was found to be pressure dependent up to at least 6 × 10^?7 Torr. A model is proposed for the accumulation process, which is based on electron beam dissociation of α₂-CO to form adsorbed carbon and gaseous O and the creation of new sites for further α₂-CO adsorption; it is in quantitative agreement with the results and yields a cross section for ESD of α₂-CO (Q_α2 = 1.55×10^?18cm²) in close agreement with direct measurements.