Vibrational bands, intensities

Some characteristics of, and comparisons between, surface-enhanced Raman spectroscopy (SERS) and infrared reflection-absorption spectroscopy (IRRAS) for examining reactive as well as stable electrochemical adsorbates are illustrated by means of selected recent results from our laboratory. The differences in vibrational selection rules for surface Raman and infrared spectroscopy are discussed for the case of azide adsorbed on silver, and used to distinguish between "flat" and "end-on" surface orientations. Vibrational band intensity-coverage relationships are briefly considered for some other systems that are unlikely to involve coverage-induced reorientation. 

Some theoretical considerations about vibrational band intensities. Spectrochim. Acta 14, 161—180 (1959). 

Table 6 gives the changes in vibrational band intensities due to melting 19) or dissolution. 

The principal information determined from the fluorescence spectra is the values of vibrational band intensities (energy s ). These are related to vibrational populations N - by... 

Perturbations between States of the Same Symmetry Vibrational-Band Intensity Anomalies.380... 

Pyridine certainly is the most extensively studied compound in the SERS literature. The potential dependence of its vibrational band intensities as a silver electrode is swept through its ORC at a rate of 5mVs-1 is shown [14] in Fig. 13. The intensity of the strongest band reaches a maximum at - 0.9 V (SCE), in keeping with the model described previously in this section and illustrated in Fig. 12. Also consistent with the model is the fact that the SERS-enhanced H20 band at ca. 3500 cm 1 becomes weaker as H20 is displaced from the electrode surface by Py. However, there is no increase in the H20 band intensity when the potential is swept beyond the PZC, as might be expected from the condition illustrated by Fig. 12(E). 

Spontaneous Raman scattering always occurs when the laser excitation frequency is less than the frequency associated with an allowed electronic transition of the molecule. As the probe laser frequency approaches that of an electronic transition in the molecule, certain vibrational modes that couple strongly to the transition increase in intensity (pre-resonance) with respect to other Raman allowed modes of the molecule. When the excitation frequency coincides with the electronic transition frequency (resonance), a dramatic increase in vibrational band intensities is observed. This effect has been observed in many molecules and especially in polymer films, such as polydiacetylene, that consist of extended regions of electron delocalization owing to the presence of conjugated double and triple carbon-carbon bonds in the linear network (40)(41). 

IR spectroscopy was used for quantitative analysis of the composition of three ethylene-acrylate copolymers, i.e. ethylene-methyl acrylate, ethylene-butyl acrylate and ethylene-2-ethylhexyl acrylate copolymers. Based on a simple model which explicitly considered vibrational band intensities characteristic for CH and for C 0, copolymer composition could be derived from the ratio of C 0 and CH integrated absorbances with a precision of + or -3 mol %. It was not necessary to know the optical path length of the copolymer samples which were subjected to IR analysis as pressed films. 15 refs. 

This chapter begins with a treatment of symmetry and electronic structure in diatomic molecules. The symmetry selection rules for electronic transitions are derived, and vibrational band intensities (cf. Fig. 4.1) are described in terms of... 

In Section 4.4 we worked out the El electronic and vibrational selection rules for electronic band spectra, and it remains for us to determine the selection rules that govern the rotational fine structure. We have seen that no symmetry selection rule exists for Ay, but that the vibrational band intensities are proportional to Franck-Condon factors in the Born-Oppenheimer approximation. To understand the selection rules for simultaneous changes in electronic and rotational state, we must find how l eiZrot) = transforms under... 

Many of the ideas that are essential to understanding polyatomic electronic spectra have already been developed in the three preceding chapters. As in diatomics, the Born-Oppenheimer separation between electronic and nuclear motions is a useful organizing principle for treating electronic transitions in polyatomics. Vibrational band intensities in polyatomic electronic spectra are frequently (but not always) governed by Franck-Condon factors in the vibrational modes. The rotational fine structure in gas-phase electronic transitions parallels that in polyatomic vibration-rotation spectra (Section 6.6), except that the rotational selection rules in symmetric and asymmetric tops now depend on the relative orientations of the electronic transition moment and the principal axes. Analyses of rotational contours in polyatomic band spectra thus provide valuable clues about the symmetry and assignment of the electronic states involved. 

The vibronically induced component, which is proportional to (dM dQ o, controls the vibrational band intensity according to HT theory when the intrinsic electronic transition is forbidden. Since the electronie dipole moment operator /igj depends only on the electronic coordinates (Eq. 4.47), we have... 

Here <0 n>p is the Franck-Condon factor for the 0- n vibrational band intensity in the electronic spectrum, and is the electronic transition... 

RusseU, R.A. and Thompson, H.W., Vibrational band intensities and the electrical anharmonicity of the NH group, Proc. R. Soc. London, A234, 318-326, 1956. 

Using these new DMSs a set of vibrational transition moments (3) was generated. To facilitate the comparison with experiment, these moments were used to compute vibrational band intensities as given by Brown et al. [127] and [128] ... 

The vibrational band intensities of the conformer bands, A a and Ab, are related to the concentration of each conformer as follows ... 

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