Raman effect spectra

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 nature of the spectrum and the terminology of the Raman effect are summarized in Fig. 1. 

Chapter 3 is devoted to pressure transformation of the unresolved isotropic Raman scattering spectrum which consists of a single Q-branch much narrower than other branches (shaded in Fig. 0.2(a)). Therefore rotational collapse of the Q-branch is accomplished much earlier than that of the IR spectrum as a whole (e.g. in the gas phase). Attention is concentrated on the isotropic Q-branch of N2, which is significantly narrowed before the broadening produced by weak vibrational dephasing becomes dominant. It is remarkable that isotropic Q-branch collapse is indifferent to orientational relaxation. It is affected solely by rotational energy relaxation. This is an exceptional case of pure frequency modulation similar to the Dicke effect in atomic spectroscopy [13]. The only difference is that the frequency in the Q-branch is quadratic in J whereas in the Doppler contour it is linear in translational velocity v. Consequently the rotational frequency modulation is not Gaussian but is still Markovian and therefore subject to the impact theory. The Keilson-... 

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. 

Beattie, I.R., Ozin, G.A., and Perry, R.O., Gas phase Raman spectra of P4, P2, As4 and As2. Resonance fluorescence spectrum of 80Se2. Resonance fluores-cence-Raman effects in the gas-phase spectra of sulfur and iodine. Effect of pressure on the depolarization ratios for iodine,. Chem. Soc., Perkin I, 2071, 1970. 

Figure 1.14 The spectral manifestation of the Raman effect, (a) The spectrum of the incident light, (b) The spectrum due to scattered (Rayleigh and Raman) light, (c) The Raman spectrum. The relative intensities of the incident, Rayleigh, and Raman hnes are quite different in real...

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. 

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)... 

This phenomenon is known as the inverse Raman effect and was first observed by Stoicheff The inverse Raman spectrum is the analogue of the stimulated Raman emission spectrum and therefore theories of the stimulated Raman effect apply to both emission and absorption. There are, however, significant differences in the corresponding spectra ... 

The Raman effect is analogous to fluorescence except that it is not wavelength dependent and does not require the molecule to have a chromophore. The energy shift in cm" due to inelastic scattering of laser radiation is measured rather than wavelength. The shifts measured correspond to the wavenumbers of the bands present in the middle-IR spectrum of the molecule. 

When an incident beam of radiation of frequency y falls on a molecule, some radiation is scattered and in this scattered radiation we get, as well as v, frequencies y where vp is a fundamental frequency. This is called the Raman effect and when a fundamental frequency appears in the Raman spectrum it is said to be Raman active. 

I. Is it possible to observe a shift in coherent Raman scattering in the three-level system with A-type coupling We have done an experiment to obtain a femtosecond Raman gain spectrum in polydiacetylenes. The Raman spectrum is shifted to the red under increased pump (to i) intensity. By changing o>2> the amplification peak signal is to be shifted to lower frequency. If the optical Stark effect is observed, then, in principle, it should be possible to observe the effect of a high field on the coherent Raman spectrum (see Fig. 1). 

There are advantages in the Raman field over its absorption counterpart. Thus, many of the functional groups which, due to their very high (dfijdq) values, tend to obscure the infrared spectrum (e.g. vC-O-, vC-F, vO-H and <50-H) rarely give any trouble in this respect in the Raman effect. Those familiar with the infrared spectrum of poly-(tetra-fluoro ethylene) or of polyoxymethylene will confirm this when they compare their data with Fig. 10 and 11, respectively. In addition, all the spectra given in this review were recorded on readily available samples — with no sample preparation. 

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... 

Fig. 8.12 The Raman effect. Monochromatic light of frequency vQ is scattered by a sample, either without losing energy (Rayleigh band) or inelastically, in which a vibration is excited (Stokes band), or a vibra-tionally excited mode in the sample is de-excited (anti-Stokes band). The spectrum is that of the light scattered by the sample. The energy level diagrams illustrate that the scattering process occurs via highly unstable states of high energy.

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