Pre-resonance Raman effect

The Resonance Raman Effect (RRE) ca be observed when a molecule is excited by light with a frequency which falls under an obsorption band of the molecule. Whereas an excitation of this type commonly produces fluorescence for the gas phase, the fluorescence is usually suppressed for solutions, pure liquids, and sohd state samples. The Pre-Resonance Raman Effect (PRRE) is observed if the exciting line comes close to, but is not overlapping with an absorption band. 

The intense lignin peak in Fig. 4.6.8 is also due to a pre-resonance Raman effect. The pre-resonance Raman effect observed with some chemical compounds (Long 1977) has also been seen when Raman studies of wood samples are carried out using a number of different excitation frequencies (Atalla, unpubl. 1989). The 1595 cm-1 band is reduced to less than half its intensity when excitation is changed from 514.4 to 647.1 nm. The pre-resonance Raman effect in wood, when it is excited at 514.5 nm, may be attributed to phenoxy radicals and other lignin structures capable of light absorption at that wavelength (Tripathi and Schuler 1984, Schmid and Brosa 1971). 

Ordinary Raman scattering is an inefficient process, and in general it is less sensitive than IR absorption. However, in certain lignin samples, conjugation, resonance, or pre-resonance Raman effects can arise, and for particular vibrational modes a higher level of sensitivity can be achieved (Long 1977, Schmid and Brosa 1971). In lignins, the latter effect can be induced by proper laser frequency selection. 

The term on the right-hand side of this equation represents the microscopic parameters of the sample. According to Placzek s theory (1934), this expression should be independent of the frequency of the exciting radiation. In the absence of a resonance or pre-resonance Raman effect, it would be equal for all exciting lines. The term on the left-hand side normalizes the observed Raman intensity by including the P factor. These values have the dimension cm - sr. ... 

There are several techniques to avoid or overcome one or the other problem. Resonance Raman scattering, for instance, is able to enhance the Raman scattering cross section by orders of magnitude. The resonance Raman effect can be observed when the exciting frequency is close to (pre-resonance Raman effect) or even coincides (rigorous resonance Raman effect) with dipole allowed electronic transitions in the molecule un-... 

Comparing the Raman spectra with different excitations, it may be seen that the relative intensities of the different Raman lines change with excitation wavelength (Gaft and NagU 2009). For example, the relative intensity of 03 and 2d2 vibrations is much higher in spectra with excitation at 266 nm compared to excitation at 785 nm. This selective enhancement of Raman lines becomes even more pronounced under excitation at 248 nm (Table 6.2). It is evidently connected with resonance or pre-resonance Raman effects, which are known to provide an additional increase in Raman cross section above the inverse fourth power dependence on excitation... 

Figure 1. Diagrams of potential energy, V, versus Internuclear separation, q, for a molecule undergoing vibrational excitation by (a) the Raman effect or (b) a resonance Raman effect (hVfj-hvg) or a pre-resonance effect h >

The molecular cross section of the ordinary Raman effect can be considerably enhanced. If the exciting radiation has a higher frequency, the intensity increases basically by the fourth power of the frequency. Moreover, there is a further increase as electronic absorption bands are approached the pre-resonance and resonance Raman effect (Sections 3.6 and 6.1). Further, the so-called surface-enhanced Raman effect (SERS) increases the molecular cross section. Both effects produce an enhancement of several orders of magnitude (Gerrard, 1991) (see Sec. 6.1). However, these two effects have to be carefully adapted to the specific properties of the investigated molecules. Photochemical decomposition and excitation of fluorescence may make it impossible to record a Raman spectrum. The described techniques may thus be of considerable importance for the solution of special problems, but they are by no means routine techniques to be generally used. 

Bolis et al (43) reported volumetric data characterizing NH3 adsorption on TS-1 that demonstrate that the number of NH3 molecules adsorbed per Ti atom under saturation conditions was close to two, suggesting that virtually all Ti atoms are involved in the adsorption and have completed a 6-fold coordination Ti(NH3)204. The reduction of the tetrahedral symmetry of Ti4+ ions in the silicalite framework upon adsorption of NH3 or H20 is also documented by a blue shift of the Ti-sensitive stretching band at 960 cm-1 (43,45,134), by a decrease of the intensity of the XANES pre-edge peak at 4967 eV (41,43,134), and by the extinction of the resonance Raman enhancement of the 1125 cm-1 band in UV-Raman spectra (39,41). As an example, spectra in Figs. 15 and 16 show the effect of adsorbed water on the UV-visible (Fig. 15), XANES (Fig. 16a), and UV-Raman (Fig. 16b) spectra of TS-1. 

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). 

The authors stress the advantages of pre-resonance intensity enhancement effect accompanying the FT-Raman measurements allowing to detect carotenoids even in trace amounts with simultaneous elimination of background fluorescence of the plant sample. These special advantages of FT-Raman spectroscopy applied to 2D Raman mappings enabled to illustrate the distribution of individual plant carotenoids independently from each other in the same sample [4]. Furthermore, the high potential of FT-Raman microspectroscopy to obtain detailed information about microstructure and chemical composition of fennel fruits, chamomile inflorescence, and curcuma roots was demonstrated [11]. 

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