Raman resonance effects

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 be set to a selected wavelength over the UV/VIS range of 200-800 nm. This permits maximum flexibility in the choice of excitation wavelength. 

In this section we briefly discuss some special techniques of linear and nonlinear Raman spectroscopy that have special advantages for different applications. These are the resonance Raman effect, surface-enhanced Raman signals, Raman microscopy, and time-resolved Raman spectroscopy. 

The Raman scattering cross section can be increased by several orders of magnitude if the excitation wavelength matches an electronic transition of the molecule, that is, when it coincides with or comes close to a line in an electronic absorption band. In this case, the denominator in (8.12) becomes very small for the neighboring lines in this band, and several terms in the sum (8.12) give large contributions to the signal. 

In stimulated Raman scattering the strongest Raman transition will have the largest gain and reaches threshold before the other transitions can develop. Just above threshold we therefore expect only a single Raman line in the stimulated Raman spectum, while at higher pump powers more lines will appear. 

Resonance Raman scattering is particularly advantageous for samples with small densities, for example, for gases at low pressures, where the absorption of the incident radiation is not severe and where nonresonant Raman spectroscopy might not be sufficiently sensitive. 

If the excited state lies above the dissociation limit of the upper electronic state, the scattered Raman light shows a continuous spectrum. The intensity profile of this spectrum yields information on the repulsive part of the potential in the upper state. 

The Raman lines appear predominantly at those downward transitions that have the largest Franck-Condon factors, that is, the largest overlap of the vibrational wavefunctions in the upper and lower state (Fig. 3.24). Unlike the nonresonant case, in spontaneous resonance Raman scattering a larger number of Raman lines may 

In the subsequent derivations all electronic and vibrational states are considered as non-degenerate. 

The tfaeoiy of intensities of re onantly enhanced Raman lines is based on the Kramers-Heisenberg-Dirac dispersion equation [266,267], The problem is analyzed in teims of vibronic interactions in die molecule. The JK matrix component of the molecular polarizability for the gi gf transition in die Raman spectrum can be expressed as follows [256,257] 

Mj and are the respective dipole moment operators and Eq is die excitation light energy. In resonance conditions the first term in Eq. (8.66) becomes dominant since the denominator (E y - gi - Eo) rapidly decreases. In this case, a phenomenological damping constant F is introduced in the denominator expression. Thus, retaining the resonant term in Eq. (8.66) only, we obtain 

This equation can be further developed if the electronic wave function is expanded in a Taylor series along the normal coordinates of the molecule. This is known as die Herzbeig-Teller expansion [268] and its application to Eq. (8.67) yields [255,256] 

In these equations (Mp)g = [ olMpieo] (p = J K 6, 4 = g. e, s, t) are the electric transition dipole moments at the equilibrium molecular geometiy and -[4ol(3H/aQa)oleo]- (3H/3Qa)o is the vibronic coupling operator for the norma) mode a. H is the electronic Hamiltonian of the molecule. 

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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. 

Experimentally, several precautions must be taken if reliable Raman data are to be obtained from solution studies. Firstly, the instrumental slit-width should be appreciably smaller than the half-width of the band to be studied. This means that slits wider than 2 cm-1 are to be avoided. Secondly, photolytic decomposition of the sample and local boiling of the solvent have also to be avoided. Careful choice of laser frequencies, use of a low incident power and, if necessary, sample spinning are indicated. The need for a relatively high solute concentration usually means that there is little choice of solvent. Particularly for coloured samples the presence of a vestigal resonance Raman effect must be tested by measurements with a variety of... 

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)... 

When the frequency of a laser falls fully into an absorption band, multiple phonon processes start to appear. Leite et al 2° ) observed /7 h order ( = 1, 2. 9) Raman scattering in CdS under conditions of resonance between the laser frequency and the band gap or the associated exciton states. The scattered light spectrum shows a mixture of fluorescent emission and Raman scattering. Klein and Porto 207) associated the multiphonon resonance Raman effect with the fluorescent emission spectrum, and suggested a possible theoretical approach to this effect. 

Resonance Raman effects in halogen gases have been observed by Holzer etal. 207a). with an appropriate choice of exciting lines from an argon laser either resonance Raman effect or resonance fluorescence could be observed. The difference between the two spectra is discussed. In the case of a strong resonance Raman effect, overtone sequences up to the 14 harmonic could be observed. 

Figure 1. Diagrams of potential energy, V, versus Internuclear separation, q, for a molecule undergoing vibrational excitation by (a) the Raman effect or (b) a resonance Raman effect (hVfj-hvg) or a pre-resonance effect h >

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 spectra of the radical cations derived from l,r-dialkyl diquaternary salts of 4,4 -bipyridine have received attention, the UV -997,1029,1036.1056 ig spectfa having been well studied. -1057-1059 jg eyjfjejjt from some of these studies that the radical cations are in equilibrium with dimeric species. The Raman spectra of paraquat and its radical cation adsorbed at a silver electrode have also been investigated, whereas a resonance Raman effect with radical cations of viologens has been noted. Other Raman studies at metal... 

High electron density in the intemuclear region of multiple metal bonds leads to a large polarizability and thus the vibrational features are amenable to Raman studies. Resonance Raman effects are common owing to the presence of... 

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 Raman scattering signal can also be enhanced if one chooses an excitation wavelength corresponding to an electronic transition of the molecule of interest. This resonance Raman effect can enhance the signal by two to six orders of magnitude [55]. Hence, exploiting both the surface enhancement and the molecular resonance leads to extremely low detection limits (e.g., picomolar and below). 

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