Raman active vibrational modes

Figure 2. Selected infrared and Raman active vibrational modes of C12H14.

Another valuable advantage of Raman spectroscopy, which is unique, is its capability of being used to characterise carbon species, in particular graphitic and amorphous carbon this can be of value to many degradation and pyrolysis studies. Perfectly ordered graphite is characterised by a Raman-active vibrational mode that occurs at 1,575 cm-1 this band is usually referred to as the C7 band. With increasing disorder in the carbon, a new band, the D band, appears at... 

In CARS two ultrashort pulses of laser light (from femtoseconds to picoseconds in duration) arrive simultaneously at the sample of interest (Mukamel, 2000 Fourkas, 2001 and references herein). The difference between the frequencies (W) - w2) matches the frequency of a Raman active vibrational mode in the sample. A probe pulse (w3) emits a signal pulse of frequency Wj - w2 + w3 in a unique special direction. By scanning the delay time between the pump and probe pulses, the delay of the vibrational coherence can be measured. The distinct advantage of CARS is that it is a background free technique, since the signal propagates in a unique direction. 

We have seen that not all molecules are like water in having all vibrational modes both IR and Raman active. In fact, there is an extremely useful exclusion rule for molecules with a center of symmetry, i If a molecule has a center of symmetry. IR and Raman active vibrational modes are mutually exclusive if a vibration is IR active, it cannot be Raman active, and vice versaf ... 

Figure 18. Time-resolved fluorescence spectra of a polyatomic solute in ethanol at 247 K following a 1 ps excitation pulse.68 The model solute has the 29 Raman active vibrational modes of the retinal chromophore in bacteriorhodopsin and undergoes rapid vibrational relaxation, co, -a>ei = 1528 cm-1. The solvent and the broadening parameters are the same as used in Fig. 17.

Raman-active vibrational modes are frequently silent in the infrared and vice versa. This, of course, is true for the skeletal bands as well. 

Fig. 6. a) Resonance Raman active vibrational modes of Ni(OEP) in the high frequency region, b) Resonance Raman active vibrational modes of Ni(OEP) in the low frequency region, c) IR active in-piane modes of Ni(OEP) (from Ref. 28 and 73)... 

The main IR- and Raman-active vibrational modes may be assigned in the following way (Almeida 1987), where vj designates each particular peak as shown in figs. 18 and 19 v, is the symmetric stretch of nonbridging fluorines without Zr(Hf)... 

As a quick rule, if a vibrational mode has a linear function of x, y, or z associated with its irreducible representation, it is IR active (the IR spectrometer can see it). If it does not have one of these functions, then the mode can t be detected by IR. Similarly, Raman active vibrational modes exist only if the irreducible representation has a quadratic function (pcy, xz, yz, x, y, z or a combination of these terms (such as + z ). 

For the materials with fluorite structure, the only Raman active vibrational mode of T2g symmetry is observable. This mode arises from symmetric shift of the anions relatively to cations. Therefore the ions shifts owing to the thermal effects or impurities insertion influence Raman spectra. This is important for oxide materials and for Zr02iY203 in particular, where oxygen vacancies play an important role in ionic conductivity. 

SELECTION RULES FOR IR AND RAMAN-ACTIVE VIBRATIONAL MODES... 

Cottle et al. [15] reported in 1978 that the imposition of hydrostatic pressure caused an increase in frequency of the four Raman-active vibrational modes in... 

The three diagonal elements of the Cartesian polarization matrix can be simplified to two in spherical polar coordinates in a similar way to the d-orbital functions considered in Section 5.8. So, in the character tables, functions used for the d-orbitals can also be used to identify Raman-active vibrational modes. Thus, the Raman selection rule can be stated... 

The electronic structure of polyacetylene is very sensitive to the presence of conformational defects on the chains, and there are significant differences between the properties of polyacetylene prepared as unoriented and as stretch-oriented films. Stretch-oriented films show properties similar to those of good-quality Shirakawa polyacetylene, with the peak in the interband n-n optical absorption at around 1.9 eV [41-43]. In contrast, Ae band-gap in the unoriented material is raised, with the peak in the interband -absorption at about 2.3 eV [44-46], as seen in the optical absorption spectra shown in figure 4. There are concomitant increases in the frequencies of the Raman-active vibrational modes [44]. 

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