Resonance Raman effects transition

Raman spectroscopy is primarily useful as a diagnostic, inasmuch as the vibrational Raman spectrum is directly related to molecular structure and bonding. The major development since 1965 in spontaneous, c.w. Raman spectroscopy has been the observation and exploitation by chemists of the resonance Raman effect. This advance, pioneered in chemical applications by Long and Loehr (15a) and by Spiro and Strekas (15b), overcomes the inherently feeble nature of normal (nonresonant) Raman scattering and allows observation of Raman spectra of dilute chemical systems. Because the observation of the resonance effect requires selection of a laser wavelength at or near an electronic transition of the sample, developments in resonance Raman spectroscopy have closely paralleled the increasing availability of widely tunable and line-selectable lasers. 

The Raman scattering signal can also be enhanced if one chooses an excitation wavelength corresponding to an electronic transition of the molecule of interest. This resonance Raman effect can enhance the signal by two to six orders of magnitude [55]. Hence, exploiting both the surface enhancement and the molecular resonance leads to extremely low detection limits (e.g., picomolar and below). 

The resonance Raman effect was explained by a resonant excitation of the IVCT transition associated with the fivefold-coordinated Mo5+ centers. The Raman scattering intensity results then from the relative efficiencies of the resonant absorption of the incident photons and the resonant reabsorption of the Raman-scattered photons. The resonance enhancement was further demonstrated by investigation of a series of M0O3 v samples with various degrees of reduction (Dieterle and Mestl, 2002 Dieterle et al., 2002). 

Resonance Raman effects, which arise from resonant coupling of the exciting laser light to electronic transitions, can be exploited to overcome difficulties with detection limits and are of particular interest for the characterization of working catalysts that contain reduced transition metal ions. 

As already pointed out, this description of the Raman effect is based on the polarizability theory (Placzek, 1934) which is valid in a good approximation if the exciting frequency is much higher than the frequency of the vibrational transition // , but lower than the frequency of the transition to the electronic excited state If, on the other hand, is approaching then resonances occur which considerably enhance the intensities of the Raman lines, i.e., the resonance Raman effect. This effect and its applications are described in Sec. 6.1 and also in Secs. 4.2 and 4.8. 

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-... 

In the adiabatic Bom-Oppenheimer approximation, the eigenstates of the unperturbed molecule, i. e. i >, / >, and r >, may be expressed as products of electronic, vibrational, and rotational states. In the following we restrict our discussion to the vibrational resonance Raman effect and hence, assuming that the molecule is initially in the vibrational state u, > and that the Raman transition starts and terminates in the ground electronic state g >, we may write... 

In the ideal limit of the normal (non-resonance) Raman effect the energy of the exciting radiation is far removed from any transition energy of the system. That is... 

Iridium(iv).—Reduction of IrCli to IrClg by various transition-metal ions does not change the inner co-ordination sphere. A resonance Raman effect has been observed for IrClg". Anomalous polarization was found on all Raman bands and this was ascribed to Jahn-Teller distortions in the excited electronic state. I.r. and... 

Figure 17.2.7 Schematic views of Raman scattering. Excitation (E) to a nonstationary virtual state is followed by Rayleigh scattering (R ) with no change in energy, or Raman scattering (R and R2) with energy changes equal to vibrational quanta, (a) The normal Raman effect involves excitation in a nonabsorbing region, (b) The resonance Raman effect involves excitation very near an allowed absorption transition.

Figure 8.3 Schematic representation of two eletronic states (ground and excited) and their respective vibrational levels (the eletronic and vibrational levels are not represented on the same scale). The arrows Indicate the types of transitions that can occur among the different levels. It Is Important to say that in the case of Raman scattering, if the laser line (XJ used has energy similar to one electronic transition of the molecule, the signal can be intensified by a resonance process, know as the resonance Raman effect. In the figure, and laser line and scattering frequencies, respectively (just the Stokes, Vs < Vg, component is shown in the diagram)...

Resonance Raman spectroscopy is a particularly powerful probe for studying the structures and dynamics of macrocyclic supramolecular compounds, e.g., metallo-porphyrins. These compounds have strongly allowed electronic transitions in the visible or UV regions that can enhance the intensity of some Raman-active vibrations by a factor of 10 -10", i.e., resonance enhancement or resonance Raman effect. 

According to (3.12), the Raman scattering cross section increases considerably if the laser frequency co matches a transition frequency coij of the molecule (resonance Raman effect) [310, 311]. With tunable dye lasers and optical frequency doubling this resonance condition can often be realized. The enhanced sensitivity of resonant Raman scattering can be utilized for measurements of micro-samples or of very small concentrations of molecules in solutions, where the absorption of the pump wave is small in spite of resonance with a molecular transition. 

The Raman scattering can be enhanced by a factor of the order of 10 if the exciting line coincides with an allowed electronic transition (resonance Raman effect). During recent years several types of coherent Raman spectroscopy [6.127] have been introduced, e.g. coherent anti-Stokes Raman spectroscopy (CARS) (Sects.8.6 and 10.1.4) and Raman gain spectroscopy. 

The main exceptions to the Av = 1 selection rule occur in electronic spectra (Section 9.5), and in the resonance Raman Effect (Section 8.3.3), where progressions due to transitions with Av = 0, 1, 2, 3. . . can also be observed. 

In normal Raman spectroscopy a sample is placed in a (monochromatic) laser beam and the very weak scattered light of lower frequency is studied. In such a study the colour of the laser light is usually chosen to be away from any absorption band of the sample because such a choice reduces the risk that the focused laser beam will destroy the sample by heating it. In the resonance Raman effect the laser beam colour is deliberately chosen to coincide with an absorption band—an electronic transition—of the sample. Whilst this may lead to the destruction of the sample, for favourable cases it leads to Raman scattering which is much stronger than normal. This, in turn, means that the laser power can be reduced, improving the chances of sample survival. The spectra obtained from compounds showing such a resonance Raman effect are both simpler and more complicated than normal Raman spectra. They are simpler because, often, only totally symmetric vibrational modes are seen. The reason for this is that if the electronic... 

Using the resonance Raman effect, Hester and Nour have assigned the Fe(II) -> Co(III) intervalence transitions in the complexes [(NC)5Fe(II)CNCo(III)(CN)5] and [(NC)5Fe(II)CNCo(III)(edta)f . Detailed analysis of the CN stretching modes also confirms the ground state valency assignments as shown. In the linear chain complexes [Pt(LL)2][Pt(LL)2X2]X4 (LL = diamine, X = Br or l ) the Pt(II)-Pt-Pt(IV) intervalence band is coupled to the symmetrical stretch, X-Pt-X of the Pt(IV) units, as in other complexes of this type. 

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