Excitation profile, resonance Raman effect

Inagaki F, Tasumi M and MiyazawaT (1974) Excitation profile ofthe resonance Raman effect of )3-carotene. J Mol Spectrosc 50 286-303... 

In the case of polyenes, the term I scattering is dominant in the Raman spectra resonant with the 1 flu excited states22,26 the contribution of the term m scattering is important in the Raman spectra resonant with the 2 Ag excited states27. In addition, an interference effect between the 1 flu and 2 kg states in a Raman-intensity vs excitation-photon-energy plot (which is called an excitation profile) of the in-phase C=C stretch has been suggested28. 

The final two examples of the determination of excited state distortions are large bimetallic compounds whose electronic absorption spectra are broad and featureless. We must turn entirely to resonance Raman spectroscopy to measure the distortions because all of the information in the electronic spectrum is buried under the envelope. Fortunately, the resonance Raman profiles contain a great deal of information. These molecules were chosen as illustrative examples precisely because the resonance Raman spectra are so rich. The spectrum contains long overtone progressions and combination bands. Excitation profiles of not only the fundamentals but also of overtones and combination bands will be used to determine the distortions. The power of time-dependent theory from Section III.F and experimental examples of the effects of A on fundamentals, overtones, and combination bands are shown. The calculated distortions provide new insight about the orbitals involved in the electronic transition. 

It is inappropriate to go into the theory underlying this effect here, but it should be noted that not all Raman-active modes of a sample give enhanced band intensities in RR scattering. The mode selectivity depends on the symmetry of the sample molecule and the nature of its electronic transitions, which are actively involved in the Raman process. Thus, by changing the Raman excitation wavelength from coincidence with one electronic absorption band to another of a different symmetry type, the pattern of vibrational band resonance enhancements may change drastically. This dependence of Raman band intensity on the wavelength of the excitation source is known as the "excitation profile and may also be used to provide information on the nature of the electronic transitions. 

Infrared and Raman Spectroscopy. Resonance Raman spectra of aW-trans- and 15-CW-/3-carotene have been compared.The ps resonance Raman spectrum of /8-carotene has been described,and solvent effects on the excitation profile of the line of jS-carotene have been studied. Model calculations have been used to interpret observed jS-carotene Raman spectra and excitation profiles. Raman scattering spectra of j8-carotene-l2 complexes have been determined. Resonance Raman spectra of carotenoids have been used as an intrinsic probe for membrane potential, e.g. neurosporene [7,8-dihydro-(/r,(/r-carotene (183)] in chromatophores of Rhodopseudomonas sphaeroides. ° Resonance Raman spectroscopy and i.r. spectroscopy have been used in studies of the chromophore of visual pigments and visual cycle intermediates and of bacteriorhodopsin and its photocycle intermediates. ... 

Figure 9 (Henneker et al., 1978a) shows an example of Raman scattering by a non totally symmetric mode involved in (linear) Herzberg-Teller coupling of two excited states, d> and <1> . It is an elaboration of Fig. 7 in that the result of resonance with both excited states is shown. For convenience, we have assumed that both transitions -> and <5, -> are allowed and have the same transition moment, but different polarization. This assumption makes the Rayleigh profile symmetric relative to the two states, but has no effect on the structure of the fundamental REP since its matrix elements are proportional to the product of the two transition moments. The difference in intensity of the overtones in the and <1> band region reflects the frequency differences cOg — co and cOg — co . Figure 9 is based...

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