Raman Spectra
Along with infrared spectroscopy and microwave spectroscopy, a further important method of investigating the rotational and vibrational spectra of molecules is Raman spectroscopy. It is based on the inelastic scattering of light from molecules, which is k
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Along with infrared spectroscopy and microwave spectroscopy, a further important method of investigating the rotational and vibrational spectra of molecules is Raman spectroscopy. It is based on the inelastic scattering of light from molecules, which is known as the Raman effect. The rotational and vibrational frequencies of the molecules which scatter the light are present in the scattered spectrum as difference frequencies relative to the elastically-scattered primary light. In this chapter, we shall explain how this scattered-light spectrum arises and what information it contains (Sects. 12.1-12.3). In the final section, 12.4, we then discuss the statistics of nuclear spin states and their influence on the rotational structure of the spectrum. This last section refers not only to the Raman effect, but also to rotational spectra in general, and can thus be regarded as a complement to Chap. 9.
12.1 The Raman Effect As we have already seen, light can be emitted or absorbed by molecules, if the resonance condition .1E = hv is fulfilled. In addition, however, as we know from classical physics, light of any wavelength can be scattered. Elastic or Rayleigh scattering, as described by classical physics, is explained in terms of the force acting on the electronic shells of the molecule due to the E vector of the light. This force induces an electric dipole moment Pind = aE, which oscillates at the frequency of the light and, acting as a Hertzian dipole, emits on its own part a light wave of the same frequency. This scattered radiation is coherent with the radiation field which induces it. In the year 1928, Raman observed frequency-shifted lines in the spectrum of scattered light. The frequency shift relative to the primary light corresponded to the vibrational and rotational frequencies of the scattering molecules. This process, which had been predicted theoretically by Smekal in 1925, is called the Raman effect and forms the basis of Raman spectroscopy. The Raman-scattered light, in contrast to Rayleigh scattering, is not coherent with the primary light. Frequency shifts to smaller energies (Stokes lines) as well as shifts to higher energies (anti-Stokes lines) are observed. The frequency shift is independent of the frequency of the primary light and is a unique property of the scattering molecules. The entire Raman spectrum of a molecule is shown schematically in Fig. 12.1. It is the structure of this spectrum which we want to elucidate. H. Haken et al., Molecular Physics and Elements of Quantum Chemistry © Springer-Verlag Berlin Heidelberg 2004
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12 Raman Spectra Rayleigh line
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Fig. 12.1. The total Raman spectrum of a diatomic molecule, represented schematically. The Rayleigh line at the frequency vp of the primary light is surrounded directly by the rotational Raman lines. Spaced at frequency shifts corresponding to the molecular vibrations, Vvib, are the rotational-vibrational Raman lines, the Q, S, and 0 branches. The corresponding antiStokes lines at vp + Vvib are much weaker and usua
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