Vibrational spectroscopy metal-surface selection rule

In addition to the Raman selection rules described above there are surface selection rules that apply for SERS because the process occurs close to metal surfaces [40—42]. The SERS surface selection rule predicts that the vibrational bands that have contributions from the Raman polarizability tensor component where z is the surface normal, will be most intense with weaker contributions from vibrational bands which have contributions from and o. This is essentially because tlic electric field of the exciting hght is enhanced in the direction of the surface normal (Figure 6.2). The surface selection rule for Raman spectroscopy is more complex than that for infrared spectroscopy. Modes with the bond axis paraUel to... 

The use of a surface selection rule in infrared spectroscopy and in EELS has been widespread and has yielded much valuable information on surface orientation of adsorbed species. The rule is a very simple one for adsorbates at metal surfaces, only vibrational modes which modulate a component of the molecular dipole perpendicular to the surface are active in these spectroscopies. This rule arises from the fact that metals have high electrical conductivities at vibrational mode frequencies and this results in the parallel (but not the perpendicular) component of the radiation field going to zero at the surface. 

The phenomenon of surface-enhanced infrared absorption (SEIRA) spectroscopy involves the intensity enhancement of vibrational bands of adsorbates that usually bond through contain carboxylic acid or thiol groups onto thin nanoparticulate metallic films that have been deposited on an appropriate substrate. SEIRA spectra obey the surface selection rule in the same way as reflection-absorption spectra of thin films on smooth metal substrates. When the metal nanoparticles become in close contact, i.e., start to exceed the percolation limit, the bands in the adsorbate spectra start to assume a dispersive shape. Unlike surface-enhanced Raman scattering, which is usually only observed with silver, gold and, albeit less frequently, copper, SEIRA is observed with most metals, including platinum and even zinc. The mechanism of SEIRA is still being discussed but the enhancement and shape of the bands is best modeled by the Bruggeman representation of effective medium theory with plasmonic mechanism pla dng a relatively minor role. At the end of this report, three applications of SEIRA, namely spectroelectrochemical measurements, the fabrication of sensors, and biochemical applications, are discussed. 

The standard technique for probing the vibrational excitation specfrum of molecular materials in the bulk is infrared absorption spectroscopy. Photons in the infrared regime (4(X) - 4000 cm ) can be absorbed by excitation of vibrations with appropriate frequencies. Since photons in the IR-regime are not intrinsically surface sensitive, the application of the technique for problems related to surface science is hampered by the small absorbance of an adsorbed monolayer. For a saturated monolayer of CO molecules, a rather favorable case, the extinction has been found to vary between 1.7 x 10 for CO on Pt(l 11) [92HOL] and 1.4 X 10 for CO on Cu(lll)[88RAV]. In addition, on metal surfaces electric fields are rather effectively screened. At the surface of a metal, this screening is very strong parallel to, but less effective normal to the surface. This effect is the basis of the so-called IR surface selection rule, which states that in... 

The high sensitivity of tunneling spectroscopy and absence of strong selection rules allows infrared and Raman active modes to be observed for a monolayer or less of adsorbed molecules on metal supported alumina. Because tunneling spectroscopy includes problems with the top metal electrode, cryogenic temperatures and low intensity of some vibrations, model catalysts of evaporated metals have been studied with CO and acetylene as the reactive small molecules. Reactions of these molecules on rhodium and palladium have been studied and illustrate the potential of tunneling spectroscopy for modeling reactions on catalyst surfaces,... 

An important consequence of the presence of the metal surface is the so-called infrared selection rule. If the metal is a good conductor the electric field parallel to the surface is screened out and hence it is only the p-component (normal to the surface) of the external field that is able to excite vibrational modes. In other words, it is only possible to excite a vibrational mode that has a nonvanishing component of its dynamical dipole moment normal to the surface. This has the important implication that one can obtain information by infrared spectroscopy about the orientation of a molecule and definitely decide if a mode has its dynamical dipole moment parallel with the surface (and hence is undetectable in the infrared spectra) or not. This strong polarization dependence must also be considered if one wishes to use Eq. (1) as an independent way of determining ft. It is necessary to put a polarizer in the incident beam and use optically passive components (which means polycrystalline windows and mirror optics) to avoid serious errors. With these precautions we have obtained pretty good agreement for the value of n determined from Eq. (1) and by independent means as will be discussed in section 3.2. 

The cross-section in Eq. (1 illustrates another distinguishing feature of inelastic neutron scattering for vibrational spectroscopy, i.e., the absence of dipole and polarizability selection rules. In contrast, it is believed that in optical and inelastic electron surface spectroscopies that a vibrating molecule must possess a net component of a static or induced dipole moment perpendicular to a metal surface in order for the vibrational transition to be observed ( 7,8). This is because dipole moment changes of the vibrating molecule parallel to the surface are canceled by an equal image moment induced in the metal. 

The various ethene adsorbate species can be identified by vibrational spectroscopy (cf. Fig. 43) (46,138,448,470 75). Calibration SFG spectra recorded under UHV include three vibrational features, at 2880, 2910, and 3000 cm (138), which are similar to those characterizing the adsorbates on Pd(l 11). The peak at 2880 cm is attributed to the Vs(CH3) stretch vibration of ethylidyne (MSC-CH3), the feature at 2910 cm results from the Vs(CH2) of chemisorbed di-a-bonded ethene, and the very weak peak at 3000 cm represents the Vs(CH2) of physisorbed 7i-bonded ethene. As has been stated, the Vs(CH2) signal characterizing 7i-bonded molecules on single-crystal surfaces is very weak and explained by the surface-dipole selection rule for metal surfaces (17). 

On metal surfaces, two additional selection rules apply. The first is that only vibrations perpendicular to the surface are HREELS active. This rule follows from two phenomena unique at metal surfaces (i) Electromagnetic waves polarized perpendicularly to the plane of incidence (parallel to the plane of the surface) undergo a 180° phase shift upon reflection. That is, at the metal surface, the out-of-phase electric-field vectors of the incident and reflected waves cancel each other as a result, no field exists that can couple with dipoles that oscillate parallel to the surface, (ii) The dynamic dipole moment generated by an oscillator that vibrates in the surface-parallel direction is cancelled by that of its image dipole (Figure 1) hence, there the net dynamic dipole moment is zero. On the other hand, if the real dipole is oriented perpendicularly to the surface, its dynamic dipole moment is reinforced by that of its image dipole. This selection rule is the same as that for infrared reflection-absorption spectroscopy (IRAS). =... 

Inelastic electron tunnelling spectroscopy (lETS) has been used to study some silanes on aluminium oxide. The technique records vibrational spectra of an absorbed monolayer. Silanes can be applied to the oxidised metal from solution or vapour, and devices are completed by evaporation of a top electrode which is usually of lead, because of its superconductivity. The device is cooled to the temperature of liquid helium (4.2 K) to minimise thermal broadening. Most electrons (>99%) pass through the device elastically, but a small number excite vibrational modes. It is these that are detected and displayed as a spectrum. Both IR and Raman modes can be observed the selection rule for lET spectroscopy is one of orientation, in that bonds which are aligned perpendicular to the surface give the most intense peaks. 

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