Shift and broadening of adsorbate vibrational modes

The shift and broadening of the vibrational frequency of the adsorbate on a metal surface are calculated on the basis of two different models. The first model is an anharmonic one which includes multiphonon processes to all order in first-order perturbation theory. We show that anharmonic damping due to multiphonon processes is substantial for CO adsorbed on a Ni(100) surface and small for H adsorbed on a W(100) surface. At room temperature multiphonon processes with n > 2 are very important even in cases when the frequency of vibration of the adsorbate lies below twice the maximum phonon frequency of the substrate (n is the order of the process). For the CNi stretching mode of top-bonded CO on Ni(100) our results are in accord with the experimental data. The second model is an electronic one which includes the electron-hole pair loss mechanism. We show that the observed shift and broadening of the CO stretching vibration mode for a CO molecule adsorbed on Cu(100) and for a H atom adsorbed on a W(100) surface can be explained on the basis of this model. Variation of the shift and broadening of the CO stretching mode with the distance from a CO molecule to Ni(111) surface are calculated.     

The role of the electrostatic interaction in shifting the vibrational frequencies for two adsorbed molecules

We use a simple model to compute the shift of the vibrational frequencies of two adsorbed molecules as a function of inter-molecular distance, and various molecular and metal parameters. We assume that the atoms posses static and dynamic charges and the molecule has an electronic polarizability. The chemical bonds are described by Morse potentials. The electrostatic interactions are computed by a density functional method. We conclude that the use of vibrating point dipoles and of the image theorem in such calculations is an oversimplification.     

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.     

金属电极与分子中非弹性电流的理论研究

考虑分子的振动能级,在简正反应坐标下研究了通过"金属/分子/金属"所组成系统的非弹性电流.研究发现非弹性电流传输过程中分子不同带电态的能量的相对位形对Franck-Condon阻滞有较强的影响,进而在理论上分析了分子的稳定态与Franck-Condon阻滞的关系,以及分子内部的振动能量分布、分子与金属电极的相对位置与系统的电流-电压曲线的关系.     

Heterogeneous relaxation of molecule vibrational energy on metals

A mechanism of heterogeneous relaxation of a molecule vibrational energy on metal via conduction electrons is proposed. The probability of vibrational transition W of a molecule adsorbed on metal surface is calculated. The not very sharp dependence of W on the distance between the molecule and the metal surface, on metal parameters and on the molecular vibrational quantum provides a satisfactory explanation of the experimental data available.     

Identification of a vibrational Stark shift within an adsorbate layer: NH3 on Ru(001)

Using high-resolution electron energy loss spectroscopy we investigated the coverage-dependent vibrational properties of ammonia chemisorbed on Ru(001). With increasing coverage we find a drastic red shift of 100 cm⁻¹ for the ammonia symmetric deformation mode. Based on results for ammonia layers containing different isotopic species, such as NH₃ as well as partially and totally deuterated ND_xH_3−x, this red shift can be identified as a dominantly static shift (no dynamical coupling). Based on DFT model calculations, the origin of this shift can be understood as a Stark shift due to the electric fields produced by the static NH₃ dipoles within the adlayer. Calculations for isolated ammonia and for ammonia adsorbed on small Ru clusters, both in the fields of neighboring electric dipoles, can well reproduce the sign and magnitude of the observed red shift and for this molecular system allow a clear distinction between pure electrostatic and adsorption-induced (chemical) effects. We show that as a consequence of the electric field, the ammonia molecule opens the H–N–H angle and a softening of the symmetric deformation mode occurs.     

Frequency shifts in the spectra of molecules adsorbed on metals,with emphasis on the infrared spectrum of adsorbed CO

Two processes are considered which cause the frequency of the band maximum in the infrared absorption spectrum of molecules adsorbed on metals to shift with increasing coverage. The first arises from the coverage-dependent modification of the local field acting on an adsorbed molecule while the second originates from vibrational coupling arising from a through-metal interaction between molecules adsorbed on different metal atoms. Infrared absorption strengths are discussed and the method used heretofore to obtain the extinction coefficients of adsorbed molecules is questioned. In particular, a factor of 2 which results from the constructive superposition of the incident and reflected electric fields is shown to be often ignored. We show that dipolar (through-space) coupling is too weak to account for the magnitudes of the shifts recently observed for CO on single-crystal Pt and Pd as coverage progressed from low to high values. Vibrational coupling is also shown to account for the disparity in the intensities of the two bands observed when ¹²CO and ¹³CO are coadsorbed on supported metals. Similar experiments on single-crystal surfaces are suggested to help determine to what extent the observed spectral frequency shifts are derived from adsorbate—adsorbate coupling as opposed to other mechanisms.     

Tunneling spectroscopy as a probe of adsorbate-surface interactions

Tunneling spectroscopy is a sensitive probe of two classes of adsorbate-surface interactions: interactions of the adsorbate with the substrate on which it is adsorbed and adsorbate interactions with the top metal electrode that is evaporated on top of it. The talk by Professor Hipps focuses on the first of these classes. This talk focuses on the second. In general, the interaction of the adsorbed molecules with the top metal electrode produces a down-shift in the vibrational mode position ranging in size from 0.1% to 10% depending on the dipole derivative of the mode and the type of top metal electrode.     

Influence of intramolecular vibrations in charge redistribution at the pentacene-graphite interface

We have investigated the relation between the intramolecular vibrational modes of pentacene and the charge redistribution at the pentacene-graphite interface by using high-resolution electron-energy-loss-spectroscopy. The three main vibrational peaks shift to lower energies as the pentacene film thickness decreases. In order to discuss this energy shift, we have calculated the vibrational energies of a free pentacene molecule by changing its charge state. We have also calculated the vibrational energies of a pentacene molecule adsorbed on a graphite sheet by changing the pentacene-graphite distance. Taking the experimental and calculation results into account, we conclude that the observed energy shifts result from an intramolecular charge redistribution. The present results indicate that the effect of an intramolecular charge redistribution is essential to discuss the origin of an energy shift observed in a vibrational study of an organic molecule/substrate interface.     

Frequency dependence of the dissociation of polyatomic molecules by radiation

A procedure is described for calculating the dependence of the collisionless dissociation probability of a polyatomic molecule on the frequency of the laser pulse impinging upon it. Sample calculations performed on a simple molecular model show how coherent multiphoton transitions in the vibrational excitation spectrum can be observed in the curve of dissociation probability versus laser frequency.     

A methane molecule adsorbed on a graphite surface

In this paper we present a molecular dynamics study of a methane molecule adsorbed on a graphite surface. The methane is modelled using five interaction sites and the surface is an infinite hexagonal array of carbon atoms. The potential parameters are fitted using static calculations to the rotational barrier heights and using molecular dynamics to the isoteric enthalpy at zero coverage. The effective pairwise potential predicts the height of the monolayer above the surface and the vibrational frequency against the surface. We have examined the translational and rotational behaviour of a single methane molecule across the surface.     

Measurement of Au(111) surface phonons by low energy helium scattering

The vibrational state of a chemisorbed molecule on a metal surface is studied by focusing an attention on the role of electron-hole pair excitations of the coupled molecule-metal system. The vibrational line shapes thus calculated are characterized by both large red shift in the frequency and broad width (short life time) compared with those of free molecules.     

in situ FTIR Spectroscopic Characterization of Electroactive Species

A spectroelectrochemical technique based on the Fourier Transform Infrared Reflection Absorption Spectroscopic (FTIR-RAS) method is described, whereby high resolution in situ vibrational spectra of electrogenerated reaction intermediates (including radicals) and species adsorbed on electrode surfaces are obtained. Theoretical and experimental aspects of the method in a single reflection mode are discussed. Details of the cell design and the optimization of the operational parameters for running the FT-IR spectrometer in the RAS mode are also described. Apart from the potential of this technique to identify adsorbed species through their vibrational spectra, it can also be used to determine the orientation of the adsorbed species on the electrode surface by making use of surface selection rules. Using this technique, it has been possible to record the spectra of molecules adsorbed from aqueous and non aqueous media on metal and carbon electrodes. It has been possible to distinguish the vibrational spectra of adsorbed molecules from those in the bulk. Various systems covering a wide range of molecules have been studied and the potential of the technique as a method for the characterization of the electrode-electrolyte interface has been demonstrated.     

Inelastic electron tunnelling from a metal tip

A simple model for inelastic electron tunnelling between a metal probe tip and a surface is presented. The inelastic tunnelling cross section resulting from vibrational or electronic excitation of an adsorbed molecule is estimated. The role thermal and, more important, non-thermal processes have on the measurement process is also discussed.     

A model for a stochastic multiphoton absorption process of an anharmonic oscillator

A model of multiphoton absorption of i.r. laser radiation by an anharmonic, molecular, vibrational mode is discussed. The multiphoton absorption is described as a stochastic process for which the reservoir, including rotation and other vibrational modes, modulates the frequency of the active oscillator stochastically. The treatment of the nonlinear oscillator is based on quantization of a classical, driven anharmonic oscillator.     

To the quantum theory of chemical activity of the surface of transition metals

The dissociation of a gas molecule and the formation of a new chemical bond upon adsorption of this molecule on the surface of a transition metal are studied using the method of equations of motion. It is shown that both processes involve the formation of a mixed intermediate state during the adsorbate-substrate interaction. The dissociation is caused by a resonance growth of the vibrational mode, whereby the dissociation barrier is determined by the hybridization energy and by the frequency of electron transitions between molecular levels and the d electron energy levels of the metal in the mixed intermediate state. The resonance conditions for the formation of new surface structures are established.     

Low energy adsorbate vibrational modes observed with inelastic helium atom scattering: CO on Pt(111)

We report on new low energy loss and gain features which appear in the time-of-flight distributions of He atoms scattered from CO adsorbed on Pt(111). On the basis of calculated normal mode energies we assign the 6 meV features to the excitation of the CO vibrational mode corresponding to a hindered translation of the upright molecule parallel to the surface. The energy and intensity of the 6 meV mode as a function of coverage, sample temperature and scattering conditions are investigated. At high coverages, concurrent with the appearance of a well ordered (4 × 2) overlayer structure, the 6 meV mode is observed to broaden and shift in energy to about 7.4 meV. The dispersion curve measured at high coverages is flat suggesting that there is no direct coupling between hindered translations of adjacent CO molecules.     

Infrared-visible sum-frequency generation spectra of CO on Ni(100)

Infrared-visible sum-frequency generation (SFG) spectra of the stretching region of carbon monoxide adsorbed on Ni(100) are measured as a function of overlayer coverage and substrate temperature. The frequency and linewidth of the CO vibrational resonance in SFG have a similar dependence on coverage and temperature to that observed in linear absorption and emission spectroscopies. The lineshape of the SFG resonance, however, is influenced by the nonlinear response of the underlying metal surface. A simple model indicates that the nonresonant response of the substrate is out of phase with respect to that of the resonant CO response.     

Experimental study of the vibrations of acetylene chemisorbed on Ni(111)

High resolution electron energy-loss measurements of normal and deuterated acetylene chemisorbed on Ni(111) have been obtained. Observed vibrational modes are identified using the frequency shifts for the deuterated species and comparisons to the free molecule and a di-cobalt compound of acetylene. These vibrational frequencies indicate that chemisorbed acetylene is strongly rehybridized having a state of hybridization between ~sp^2.5 and sp³. Consideration of the types of modes observed, their assignments and the surface selection rule suggests a molecular orientation with the C-C bond axis slightly skewed relative to the surface and with the plane of the distorted molecule normal to the surface. A bonding geometry is proposed which has the carbon atoms residing above two adjacent 3 fold hollow sites of the Ni surface. This molecular geometry differs from that deduced previously by electron energy-loss spectroscopy for molecularly adsorbed acetylene on Pt(111).     

Low frequency surface resonance modes in electron energy loss spectroscopy

Electron energy loss spectroscopy has proved a powerful probe of vibrational modes of a wide variety of adsorbed species. Here the primary focus has been on modes with frequency well above the maximum phonon frequency of the substrate. Examples are internal vibration modes of adsorbed molecules, possibly shifted significantly in frequency from their gas phase analogues, and high frequency vibrations of an adsorbed molecule or atom against the substrate. Recent experiments explore features in the energy loss spectrum with frequency below the maximum phonon frequency of the substrate, for ordered overlayers of atoms adsorbed on low index metal surfaces. We shall summarize our theoretical studies of such spectra for several adsorbate/substrate combinations, with emphasis oh the physical origin of the features which appear in the calculations. We obtain a good account of the existing data, within the framework of a rather simple lattice dynamical model, and the calculations show that the features which appear are quite sensitive to the symmetry of the adsorption sits, and other details of the surface geometry. We shall illustrate this with several specific examples.