Spectroscopy resonance Raman excitation

Resonance Raman Spectroscopy. If the excitation wavelength is chosen to correspond to an absorption maximum of the species being studied, a 10 —10 enhancement of the Raman scatter of the chromophore is observed. This effect is called resonance enhancement or resonance Raman (RR) spectroscopy. There are several mechanisms to explain this phenomenon, the most common of which is Franck-Condon enhancement. In this case, a band intensity is enhanced if some component of the vibrational motion is along one of the directions in which the molecule expands in the electronic excited state. The intensity is roughly proportional to the distortion of the molecule along this axis. RR spectroscopy has been an important biochemical tool, and it may have industrial uses in some areas of pigment chemistry. Two biological appHcations include the deterrnination of helix transitions of deoxyribonucleic acid (DNA) (18), and the elucidation of several peptide stmctures (19). A review of topics in this area has been pubHshed (20). 

We have reported the first direct observation of the vibrational spectrum of an electronically excited state of a metal complex in solution (40). The excited state observed was the emissive and photochemically active metal-to-ligand charge transfer (MLCT) state of Ru(bpy)g+, the vibrational spectrum of which was acquired by time-resolved resonance Raman (TR ) spectroscopy. This study and others (19,41,42) demonstrates the enormous, virtually unique utility of TR in structural elucidation of electronically excited states in solution. 2+... 

Raman (R) and resonance Raman (RR) spectroscopy detects vibrational modes involving a change in polarizability. For RR, enhancement of modes is coupled with electronic transition excited by a laser light source. This technique is complementary to IR and is used for detection of v(O-O) and v(M-0), especially in metalloproteins. In porphyrins, one may identify oxidation and spin states. 

Increasing the solvent polarity results in a red shift in the -t -amine exciplex fluorescence and a decrease in its lifetime and intensity (113), no fluorescence being detected in solvents more polar than tetrahydrofuran (e = 7.6). The decrease in fluorescence intensity is accompanied by ionic dissociation to yield the t-17 and the R3N" free radical ions (116) and proton transfer leading to product formation (see Section IV-B). The formation and decay of t-17 have been investigated by means of time resolved resonance Raman (TR ) spectroscopy (116). Both the TR spectrum and its excitation spectrum are similar to those obtained under steady state conditions. The initial yield of t-1 is dependent upon the amine structure due to competition between ionic dissociation and other radical ion pair processes (proton transfer, intersystem crossing, and quenching by ground state amine), which are dependent upon amine structure. However, the second order decay of t-1" is independent of amine structure... 

This initial report was followed closely by the UV resonance Raman spectra of uridine (UMP), cytidine (CMP) and guanidine (GMP) monophosphates by Nishimura, et al. [149] and the application of UV resonance Raman spectroscopy to nucleic acids and their components started in earnest. In the years that followed, Peticolas and Spiro provided much of the research effort in this area. For nucleosides and nucleotides, Peticolas studied guanosine [150], UMP [151-154], GMP [152, 155], AMP [144, 152, 156] and CMP [153], Spiro was the only one to measure the UV resonance Raman spectra of TMP, in addition to those of all the other naturally occurring nucleotides [157, 158], For all of these nucleotides, UV resonance Raman excitation profiles have been determined. 

Raman and Infrared Spectroscopy. Two reviews deal with resonance Raman spectroscopy of carotenoid-containing biomolecules and micro-organisms152 and of carotenoids and chlorophylls in photosynthetic bacteria.153 The resonance Raman excitation profile of lycopene in acetone has been determined.154 Calculations previously used for (3-carotene do not explain the lycopene data. Several papers report detailed studies of the time-resolved resonance Raman spectra of... 

In each carotenoid, the 2A (0-0) energy for the absorptive transition determined by resonance-Raman excitation profile in the crystalline state is in complete agreement with that for the emissive transition determined by fluorescence spectroscopy in n-hexane solution, a fact which strongly suggests that neither the Stokes shift nor the dependence on the polarizabiliry of the environment (in -hexane solution vs. in crystal) is present in this particular electronic state. 

Because of the advantages mentioned in the preceding section, resonance Raman (RR) spectroscopy has been applied to vibrational studies of a number of inorganic as well as organic compounds. It is currently possible to cover the whole range of electronic transitions continuously by using excitation lines from a variety of lasers and Raman shifters [26]. In particular, the availability of excitation lines in the U V region has made it possible to carry out UV resonance Raman (UVRR) spectroscopy [104]. 

Here, A and B are fit parameters. The discussed model is valid only for off-resonance Raman excitation. In resonance Raman spectroscopy, there are other factors which can also broaden the linewidth of the LO phonon peak and are not considered in this model. 

Resonance Raman (RR) spectroscopy, as with conventional Raman spectroscopy, is a spectroscopic technique helpful for studying characteristic vibrational, rotational, and other low-frequency modes of matter. However, this method differs from conventional Raman spectroscopy in the excitation wavelength, which is tuned to be near an electronic transition and the intensity of vibration associated with the transition increases tremendously and the remaining Raman signals are overwhelmed. As this approach can be used to study some certain vibrational modes without interruption of other signals, it is especially useful for determining the structure or components of complex samples, e.g. biomolecules. Moreover, the detection limits is about 10 times larger than conventional Raman spectroscopy and, hence, substances of much lower concentrations in complex systems can be studied. 

This spectrum is called a Raman spectrum and corresponds to the vibrational or rotational changes in the molecule. The selection rules for Raman activity are different from those for i.r. activity and the two types of spectroscopy are complementary in the study of molecular structure. Modern Raman spectrometers use lasers for excitation. In the resonance Raman effect excitation at a frequency corresponding to electronic absorption causes great enhancement of the Raman spectrum. 

The pioneering use of wavepackets for describing absorption, photodissociation and resonance Raman spectra is due to Heller [12, 13,14,15 and 16]- The application to pulsed excitation, coherent control and nonlinear spectroscopy was initiated by Taimor and Rice ([17] and references therein). 

Myers A B and Mathies R A 1987 Resonance Raman intensities A probe of excited-state structure and dynamics Biological Applications of Raman Spectroscopy yo 2, ed T G Spiro (New York Wiley-Interscience) pp 1-58... 

Infrared and Raman spectroscopy each probe vibrational motion, but respond to a different manifestation of it. Infrared spectroscopy is sensitive to a change in the dipole moment as a function of the vibrational motion, whereas Raman spectroscopy probes the change in polarizability as the molecule undergoes vibrations. Resonance Raman spectroscopy also couples to excited electronic states, and can yield fiirtlier infomiation regarding the identity of the vibration. Raman and IR spectroscopy are often complementary, both in the type of systems tliat can be studied, as well as the infomiation obtained. 

Wang C, Mohney B K, Williams R, Hupp J T and Walker G C 1998 Solvent control of vibronic coupling upon intervalence charge transfer excitation of (NC)gFeCNRu(NH3)g- as revealed by resonance Raman and near-infrared absorption spectroscopies J. Am. Chem. Soc. 120 5848-9... 

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