Spectroscopy Raman effects

Raman Spectroscopy (Raman Effect, and Normal Raman Scattering) When light is scattered from a molecule most photons are elastically scattered. The scattered photons have the same energy (frequency) and, therefore, wavelength, as the incident photons. However, a small fraction of light (approximately 1 in 107 photons) is scattered at optical frequencies different from, and usually lower than, the frequency of the incident photons. The process leading to this inelastic scatter is the termed the Raman effect. Raman scattering... 

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 incident radiation should be highly monochromatic for the Raman effect to be observed clearly and, because Raman scattering is so weak, it should be very intense. This is particularly important when, as in rotational Raman spectroscopy, the sample is in the gas phase. 

Before discussing specific examples of the application of Raman spectroscopy to studying adsorbate-adsorbent interactions, it will be necessary, at this juncture, to explain the nature of the Raman effect. 

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. 

Inelastic scattering of light due to the excitation of vibrations had already been predicted in 1923 [37] and was confirmed experimentally a few years later by Raman [38], Because at that time the Raman effect was much easier to measure than infrared absorption, Raman spectroscopy dominated the field of molecular structure determination until commercial infrared spectrometers became available in the 1940s [10]. 

The Raman effect by Neil Everett, Bert King and Ian Clegg in Chemistry in Britain, July 2000, p. 40, is a good general introduction, written for scientists with no prior experience of Raman spectroscopy. Each of the books cited above under general reading discuss Raman spectroscopy, but in greater depth. 

Inelastic photon scattering processes are also possible. In 1928, the Indian scientist C. V. Raman (who won the Nobel Prize in 1930) demonstrated a type of inelastic scattering that had already been predicted by A. Smekal in 1923. This type of scattering gave rise to a new type of spectroscopy, Raman spectroscopy, in which the light is inelastically scattered by a substance. This effect is in some ways similar to the Compton effect, which occurs as a result of the inelastic scattering of electromagnetic radiation by free electrons. 

Orendorff, C.J., Ducey, M.W., Jr., Pemberton, J.E., and Sander, L.C., Structure-function relationships in high density octadecylsilane stationary phases by Raman spectroscopy 3. Effects of self-associating solvents, AnoZ. Chem., 3360, 2003. 

Doyle, C.A., Vickers, T.J., Mann, C.K., and Dorsey, J.G, Characterization of C-18-bonded liquid chromatographic stationary phases by Raman spectroscopy the effect of temperature, J. Chromatogr. A, 877, 41, 2000. 

We will first discuss spontaneous Raman spectroscopy with lasers (linear Raman effect) and then briefly some investigations of the nonlinear Raman effect. 

With the available high-power lasers the nonlinear response of matter to incident radiation can be studied. We will briefly discuss as examples the stimulated Raman effect, which can be used to investigate induced vibrational and rotational Raman spectra in solids, liquids or gases, and the inverse Raman effect which allows rapid analysis of a total Raman spectrum. A review of the applications of these and other nonlinear effects to Raman spectroscopy has been given by Schrotter2i4)... 

The distinctive feature of the Raman effect compared with fluorescence or infrared spectroscopies is that the photon is not actually absorbed in the... 

The most serious problem associated with conventional Raman spectroscopy is the minuteness of the effect. Even in the most favorable cases, only on the order of 10 of the incident intensity is converted into signal. With the development of lasers and the improvements in detection sensitivity, Raman spectroscopy has made considerable progress in recent years. Still, the lack of sensitivity has restricted Raman use. Nevertheless, the magnitude of the Raman effect can be greatly increased using nonlinear Raman processes. If... 

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