Molecules adsorbed. vibrations

Yu N-T, Nie S and Lipscomb L 1990 Surface-enhanced hyper-Raman spectrosocpy with a picosecond laser. New vibrational information for non-centrosymmetric carbocyanine molecules adsorbed on colloidal silver J. Raman Spectrosc. 21 797-802... 

RAIRS IR photons (Adsorbed molecules) Chemical (vibrational) 1-2 1 mm Monolayers 10" n.a. No No No... 

If the molecule adsorbs via a physisorbed precursor state in which it is free to move across the surface, while rotating and vibrating (with possibly modified frequencies and rotational modes), we obtain ... 

The polarlsablllty of a molecule will vary during vibrations which change the Internuclear separations. Thus the vibrations of a molecule sitting In an electrical field will be coupled to the field via the polarlsablllty. This should be particularly noticeable for a molecule adsorbed on an electrode surface where the field strength Is typically In the range 10 -10 V cm, The dipole, perpendicular to the surface, Induced In the molecule by the static electric field will fluctuate In step with the normal mode vibrations of the molecule. 

Persson BNJ, Ryberg R. 1981. Vibrational interaction between molecules adsorbed on a metal surface The dipole-dipole interaction. Phys Rev B 24 6954-6970. 

The vibrational modes of molecules adsorbed on a surface provide one with direct information on the nature of the chemical bonds between a molecule and its... 

Maximum enhancement will be obtained by molecules adsorbed in such a way as to maximise vibrational dipole changes perpendicular to the particle surface. 

Vibrational dynamics of small molecules adsorbed on cation sites in zeolite channel systems IR and DFT investigation... 

Adsorption enthalpies and vibrational frequencies of small molecules adsorbed on cation sites in zeolites are often related to acidity (either Bronsted or Lewis acidity of H+ and alkali metal cations, respectively) of particular sites. It is now well accepted that the local environment of the cation (the way it is coordinated with the framework oxygen atoms) affects both, vibrational dynamics and adsorption enthalpies of adsorbed molecules. Only recently it has been demonstrated that in addition to the interaction of one end of the molecule with the cation (effect from the bottom) also the interaction of the other end of the molecule with a second cation or with the zeolite framework (effect from the top) has a substantial effect on vibrational frequencies of the adsorbed molecule [1,2]. The effect from bottom mainly reflects the coordination of the metal cation with the framework - the tighter is the cation-framework coordination the lower is the ability of that cation to bind molecules and the smaller is the effect on the vibrational frequencies of adsorbed molecules. This effect is most prominent for Li+ cations [3-6], In this contribution we focus on the discussion of the effect from top. The interaction of acetonitrile (AN) and carbon monoxide with sodium exchanged zeolites Na-A (Si/AM) andNa-FER (Si/Al= 8.5 and 27) is investigated. 

Now we pass on the analysis of the relations derived focusing on several particular cases of importance which enable us of correlating the calculated values with the available experimental data. CO molecules adsorbed on the (100) face of a NaCl crystal reside at the sites of a square lattice (a = b/2 = 3.988 A) at sufficiently low temperatures (T < 25 K), they have inclined orientations (B = 25°) with alternating dipole moment projections onto the axes of the neighboring chains (

x = 180°).28 For this system, the Davydov splitting of vibration spectral lines is determined as ... 

This wide range of questions is to be elucidated in the present chapter. The bulk of attention is given to the effects induced by the collectivization of adsorbate vibrational modes whose low-frequency components are coupled to the phonon thermostat of the substrate. This coupling gives rise to the resonant nature of low-frequency collective excitations of adsorbed molecules (see Sec. 4.1). A mechanism underlying the occurrence of resonance (quasilocal) vibrations is most readily... 

Intermolecular lateral interactions and resulting collectivized vibrations of individual adsorbed molecules greatly add to the complexity of description for local vibrational excitations in adsorbates. Fig. 4.5 schematically demonstrates that these interactions on a simple planar lattice of adsorbed molecules which vibrate with high (toh) and low (co/) frequencies lead to the emergence of the corresponding energy bands, with energy levels classified by the wave vector K. 

Fig. 4.8. Temperature dependences of shifts (a) and widths (b) for Davydov-split spectral lines of local vibrations in the 2x1 phase of CO molecules adsorbed on the NaCI(lOO) surface.

In this section we give a simple and qualitative description of chemisorption in terms of molecular orbital theory. It should provide a feeling for why some atoms such as potassium or chlorine acquire positive or negative charge upon adsorption, while other atoms remain more or less neutral. We explain qualitatively why a molecule adsorbs associatively or dissociatively, and we discuss the role of the work function in dissociation. The text is meant to provide some elementary background for the chapters on photoemission, thermal desorption and vibrational spectroscopy. We avoid theoretical formulae and refer for thorough treatments of chemisorption to the literature [2,6-8],... 

Insight into chemisorption bonds is important for the interpretation of photoemission and vibrational spectra of adsorbed molecules, as well as for the understanding of catalytic reactions. If we have a feeling for how electrons rearrange over orbitals when a molecule adsorbs, we may be able to understand why molecules dissociate or not, to what extent they are activated, or also how a promoter influences the reactions on the surface. 

Figure 7 shows SNIFTIRS spectra for isoquinoline molecules adsorbed on mercury. The reference spectrum in each case was obtained at 0.0V vs. a SCE reference electrode at this potential the molecules are believed to be oriented flat on the metal surface. The vibrational frequencies of the band structure (positive values of absorbance) are easily assigned since they are essentially the same as those reported by Wait et al. (22) for pure isoquinoline. The differences in the spectra are that the bands for the adsorbed species exhibit blue shifting of 3-4 cm" relative to those of the neat material, and the relative intensities of the bands for the adsorbed species are markedly changed. 

Subtractively normalized interfacial Fourier transform infrared spectroscopy has been used to follow the reorientations of isoquinoline molecules adsorbed at a mercury electrode. Field induced infrared absorption is a major contribution to the intensities of the vibrational band structure of aromatic organic molecules adsorbed on mercury. Adsorbed isoquinoline was observed to go through an abrupt reorientation at potentials more negative than about -0.73 V vs SCE (the actual transition potential being dependent on the bulk solution concentration) to the vertical 6,7 position. 

Inelastic electron tunneling spectroscopy (lETS) takes advantage of the general applicability of vibrational spectroscopy by measuring the vibrational spectrum of molecules adsorbed on the insulation of a metal-insulator-metal junction (Figure 1). 

Every example of a vibration we have introduced so far has dealt with a localized set of atoms, either as a gas-phase molecule or a molecule adsorbed on a surface. Hopefully, you have come to appreciate from the earlier chapters that one of the strengths of plane-wave DFT calculations is that they apply in a natural way to spatially extended materials such as bulk solids. The vibrational states that characterize bulk materials are called phonons. Like the normal modes of localized systems, phonons can be thought of as special solutions to the classical description of a vibrating set of atoms that can be used in linear combinations with other phonons to describe the vibrations resulting from any possible initial state of the atoms. Unlike normal modes in molecules, phonons are spatially delocalized and involve simultaneous vibrations in an infinite collection of atoms with well-defined spatial periodicity. While a molecule s normal modes are defined by a discrete set of vibrations, the phonons of a material are defined by a continuous spectrum of phonons with a continuous range of frequencies. A central quantity of interest when describing phonons is the number of phonons with a specified vibrational frequency, that is, the vibrational density of states. Just as molecular vibrations play a central role in describing molecular structure and properties, the phonon density of states is central to many physical properties of solids. This topic is covered in essentially all textbooks on solid-state physics—some of which are listed at the end of the chapter. 

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