Resonance Raman spectroscopy RRS

In most work on electrochemical systems, use is made of two effects that greatly enhance the Raman signals. One is resonance Raman spectroscopy (RRS), wherein the excitation wavelength corresponds to an electronic transition in an adsorbed molecule on an electrode surface. The other effect is surface-enhanced Raman spectroscopy (SERS), which occurs on certain surfaces, such as electrochemically roughened silver and gold. This effect, discovered by Fleischmann et al. (1974), yields enhancements of 10 to 10 . The vast majority of publications on Raman studies of electrochemical systems use SERS. The limitations of SERS are that it occurs on only a few metals and the mechanism of the enhancement is not understood. There is speculation that only a small part of the surface is involved in the effect. There is a very good review of SERS (Pemberton, 1991). 

Extensive studies of enzyme-substrate complexes by resonance Raman spectroscopy (RR) have prompted the synthesis of new peptide bond modifications such as thionoesters and dithioesters (Scheme l7)t82-83l within simple model substrates. The resulting acyl-enzyme complexes are especially amenable to RR analysis with cysteine proteases such as papain due to formation of the transient dithioester intermediates. 

Wamser and co-workers have investigated the nature of the binding of 28 onto the TiC>2 electrodes by using X-ray photoelectron spectroscopy (XPS) and Resonance Raman Spectroscopy (RRS). The XPS spectra of TiC>2 revealed the changes of the binding energy of both O (1 s) and Ti (2p3/2) upon adsorption of 28. 

The classical Raman effect produces only very weak signals. There are two techniques which very successfully enhance this effect. The resonance Raman spectroscopy RRS is making use of the excitation of molecules in a spectral range of electronic absorption. The surface-enhanced Raman spectroscopy SERS employs the influence of small metal particles on the elementary process of Raman scattering. These two techniques may even be combined surface-enhanced resonance Raman effect SERRS. Such spectra are recorded with the same spectrometers as classical Raman spectra, although different conditions of the excitation and special sample techniques are used (Sec. 6.1). 

Resonance Raman spectroscopy (RR), Sec. 6.1, is a powerful technique in the field of enzymology. An extremely accurate structure-reactivity correlation was obtained by observing the wavenumber of the carbonyl group during catalytic attack by acyl serin proteases (Tonge and Carey, 1990). Evidence of bonds formed with substrates at more... 

In Eq. (2), is Ihe pth component of the transition dipole moment for the electronic transition, r) <— i, between initial state / and excited state r is the frequency of this transition and iT is a damping factor related to the lifetime of the excited state 8). Equation (2) shows that the Raman intensity can be increased dramatically when the wavelength of the exciting laser is in resonance with an electronic absorption of the sample the process is referred to as resonance Raman spectroscopy (RRS). 

Resonance Raman spectroscopy (RRS) if the wavelength of the incident radiation is chosen so that it coincides with an absorption band of the scattering molecules, the resonant Raman scattering cross-sections may be up to 10 times the cross-sections for normal Raman scattering. In such cases it is possible to detect monolayers (e.g., of dye molecules) at surfaces. This has indeed been demonstrated [68,69]. Recently RRS has found many new applications, mainly in biological studies. 

This search led to the introduction of resonance Raman spectroscopy (RRS) in environmental applications in which the excitation wavelength is in the band envelope (or near the band envelope in the case of preresonance) of an electronic transition. This results in a 10 -10 increase over NRS signals with a corresponding decrease in detection levels [10]. However, because visible and ultraviolet excitation must be used in achieving resonance, fluorescence background tends to be a greater problem with RRS than NRS, particularly when using near-infrared (NIR) excitation with NRS. 

Resonance Raman spectroscopy (RRS) leads to increased selectivity in Raman spectral measurements. The Raman spectrum of individual components in a complex mixture can be selectively enhanced by a judicious choice of laser wavelength. Only the Raman bands of the chromophore which is in resonance at the wavelength of excitation are significantly enhanced. Raman bands of non-absorbing species are not enhanced and do not interfere with those of the chromophore. Clearly, resonance Raman is a very sensitive analytical tool capable of providing detailed molecular vibrational information. 

Since its discovery in 1928, Raman spectroscopy has evolved in terms of the fundamental understanding of the process, instrumentation and applications. More advanced techniques such as Resonant Raman Spectroscopy (RRS) " ... 

In the above, we have assumed that the exciting light is not oscillating at an absorption frequency of the molecule so that the process does not involve an excited electronic state of the molecule. However, if the exciting light is chosen to coincide with an electronic absorption, the Raman spectra are greatly intensified. This phenomenon is very useful in obtaining spectra when the normal (nonresonance) Raman (NR) spectrum is too weak to be detected and is termed resonance Raman spectroscopy (RRS). 

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