The Resonance Raman Effect

Lasers that have wavelengths in the UV and visible regions of the spectrum are used for resonance Raman spectroscopy. Tunable dye lasers are often used these lasers can 

First excited electronic state with 4 vibrational levels shown 

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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. 

In the following sections, we first show the phonon dispersion relation of CNTs, and then the calculated results for the Raman intensity of a CNT are shown as a function of the polarisation direction. We also show the Raman calculation for a finite length of CNT, which is relevant to the intermediate frequency region. The enhancement of the Raman intensity is observed as a function of laser frequency when the laser excitation frequency is close to a frequency of high optical absorption, and this effect is called the resonant Raman effect. The observed Raman spectra of SWCNTs show resonant-Raman effects [5, 8], which will be given in the last section. 

The resonance Raman effect — review of the theory and of applications in inorganic chemistry. R. J. H. Clark and B. Stewart, Struct. Bonding (Berlin), 1979, 36, 1-80 (110). 

Clarke RJH, Stewart B (1979) The Resonance Raman Effect. Review of the Theory and of AppUcations in Inorganic Chemistty 36 1-80... 

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 first laser Raman spectra were inherently time-resolved (although no dynamical processes were actually studied) by virtue of the pulsed excitation source (ruby laser) and the simultaneous detection of all Raman frequencies by photographic spectroscopy. The advent of the scanning double monochromator, while a great advance for c.w. spectroscopy, spelled the temporary end of time resolution in Raman spectroscopy. The time-resolved techniques began to be revitalized in 1968 when Bridoux and Delhaye (16) adapted television detectors (analogous to, but faster, more convenient, and more sensitive than, photographic film) to Raman spectroscopy. The advent of the resonance Raman effect provided the sensitivity required to detect the Raman spectra of intrinsically dilute, short-lived chemical species. The development of time-resolved resonance Raman (TR ) techniques (17) in our laboratories and by others (18) has led to the routine TR observation of nanosecond-lived transients (19) and isolated observations of picosecond-timescale events by TR (20-22). A specific example of a TR study will be discussed in a later section. 

In addition to experiments which were possible with conventional lamps but can be much more easily performed with lasers, there are some investigations which have to be done within certain exposure times or signal-to-noise ratios and these have only been possible since lasers have been developed. This group includes the electronic Raman effect 195-197) observation of Raman scattering in metals where the scattering quasi particles are phonons, Raman studies of vibrational spectra in semiconductor crystals or the resonance Raman effect 200-202)... 

The exact features of molecular and electronic structure which give rise to the resonance Raman effect are not well understood. 

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 normal Raman spectrum obtained with 647.1 nm excitation serves as a comparison for the Raman spectra obtained with excitation frequencies of 488.0 and 514.5 nm, which lie within the 5- 5 absorption band. The tremendous enhancement of the i>,(Mo-Mo) alg mode, the high overtone progression in v, the increase in overtone bandwidth with increasing vibrational quantum number, and the increased intensity of the overtones relative to the fundamental as the excitation frequency approaches the electronic absorption maximum are all attributable to the resonance Raman effect. Polarization... 

The photons excite the molecules to a virtual electronic state, (Fig. 12.6a) from which emission occurs emission to the ground vibrational state is Rayleigh scattering. If the photons have a very high energy then the virtual state is within the vibration levels of the excited electronic state. In this case there is a much greater interaction between the radiation and the molecules and an increase in intensity of the Raman effect by a factor of 104-106—the resonance Raman effect (Fig. 12.6b). 

The resonance Raman effect has been applied to electrochemical cells, generally with laser excitation21,22. As it is possible to construct cells that are transparent to IR, it is not necessary to use transparent electrodes. The Raman results are useful for mechanistic diagnosis and for investigating the vibrational and electronic properties of the species under study. 

Fig. 12.6. Raman spectroscopy—radiation emission, (a) The normal Raman effect. R represents Rayleigh scattering (b) The resonance Raman effect.

The resonance Raman effect was corroborated by confocal Raman mapping of mixtures of M0O3 and M0O2 and of orthorhombic Mo4On. Raman signals were recorded after dilution of the compounds 1 100 in BN, and statistical data analysis was performed (Dieterle et al., 2001, 2002). When the Raman spectrum was excited at 532 nm, (i.e., at a frequency close to the minimum absorption in the UV-vis spectrum) and the integration time was set to 200 s, only the characteristic bands of M0O3 could be detected. Excitation at 632.8 nm produced, albeit at an... 

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