Scattering by Molecules

An additional type of scattering is obtained due to the fact that molecules themselves are vibrating at frequencies corresponding to various normal modes of motion. These characteristic vibrational frequencies can mix with the exciting light to form sum and difference frequencies in the scattered radiation. Although weak, these frequencies may be detected as shifts from the Rayleigh frequency and are called normal Raman spectra. The measure of these shifts reflects the characteristic vibrations of the molecule and may be utilized as a complement to infrared spectroscopy. 

In the above, we have assumed that the exciting light is not oscillating at an absorption frequency of the molecule so that the process does not involve an excited electronic state of the molecule. However, if the exciting light is chosen to coincide with an electronic absorption, the Raman spectra are greatly intensified. This phenomenon is very useful in obtaining spectra when the normal (nonresonance) Raman (NR) spectrum is too weak to be detected and is termed resonance Raman spectroscopy (RRS). 

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Raman effect When light of frequency Vo is scattered by molecules of a substance, which have a vibrational frequency of j, the scattered light when analysed spectroscopically has lines of frequency v, where... 

Gersten, J. and Nitzan, A. (1980) Electromagnetic theory of enhanced Raman scattering by molecules adsorbed on rough surfaces. J. Chem. Phys., 73, 3023-3037. 

H. Chew and D.-S. Wang, Double resonance in fluorescent and Raman scattering by molecules in small particles, Phys. Rev. Lett. 49, 490 192 (1982). 

Long, D. A., The Raman Effect A Unified Treatment of the Theory of Raman Scattering by Molecules, J. Wiley Sons, New York, 2002. 

An excellent concise treatment of scattering—by molecules and particles, single and multiple—at an intermediate level is Chapter 14 of Stone (1963). Among the books devoted entirely to scattering by particles, that by Shifrin (1951) most closely resembles ours in that it discusses optical properties of bulk matter as well. Biit the two books that have influenced us most are those of van de Hulst (1957) and Kerker (1969) we are indebted to both authors. Another book on scattering, which emphasizes polydispersions, is by Deirmendjian (1969). 

The fluctuation theory of scattering by molecules is treated in books by Bhagavantam (1942), Fabelinskii (1968), and Chu (1974, Chap. 3). 

Early theoretical treatments [3,4] of optically active scattering by molecules did little to arouse renewed interest in a measurement of ROA. The decisive cross-terms between the electric dipole-electric dipole polarizability and the optical activity tensor were missed, and effects predicted for the optical activity tensor alone were too small to be practically useful. It was only after the proper cross-terms were identified [5] that new interest in the measurement of ROA accrued, and that the existence of the phenomenon was finally proved [6,7], It was not long before the first measurement of whole ROA spectra was demonstrated [8],... 

Smith IM, Thompson DG, Blum K (1998) Chiral effects in electron scattering by molecules using a continuum multiple scattering method. J Phys B 31 4029... 

Blum K, Thompson DG (1997) Chiral effects in electron scattering by molecules. In Bederson BWalther H (ed) Advances in atomic, molecular, and optical physics. Academic Press, San Diego, pp 39-86... 

A third way (Creighton, 1988) in which the surface may modify the intensity of Raman scattering by molecules adsorbed at the surface is by perturbing the molecules as a result of the bonding interaction with the surface. However, it turned out that this chemical effect on the Raman spectra is relatively small, and it is not normally considered to be a part of SERS, though it is inseparable from it. Several other SERS mechanisms have also been proposed (see for instance the discussion by Moskovits, 1985) which by now are widely thought to make at most only minor contributions to the enhancements observed in SERS. 

Moskovits M (1978) Surface-roughness and enhanced intensity of Raman-scattering by molecules adsorbed on metals. J Chem Phys 69(9) 4159-4161... 

Wang DS, Kerker M (1981) Enhanced Raman-scattering by molecules adsorbed at the surface of colloidal spheroids. Phys Rev B 24(4) 1777-1790... 

Under the assumption that half of the energy scattered by molecules in the atmosphere reaches the ground, according to [5.42], we obtain... 

J. A. Janik A. Kowalska (1965). In Thermal Neutron Scattering, (Ed.) P.A. Egelstaff, Academic Press, London and New York, Chapter 9. The theory of neutron scattering by molecules. 

Electron diffraction is the other of the two important sources of gas-phase structural data. As discussed by Hedberg (this volume), the intensity of electrons scattered by molecules is modulated by the interatomic distances, both bonded and nonbonded. Since interatomic distances enter explicitly into electron diffraction determinations, the method is in some ways more direct than spectroscopy. Moments of inertia are functions of Cartesian coordinates of individual atoms rather than distances between atoms. On the other hand, electron diffraction is much more susceptible to experimental error than spectroscopic techniques.15 Problems with structural determinations by spectroscopic methods often stem almost entirely from model error, whereas in electron diffraction both experimental and model error are important. Experimental and model error in electron diffraction are discussed elsewhere in this volume by Hedberg, and we shall confine ourselves here to definitions of the various structural parameters that arise in electron diffraction studies and the relationships among them and spectroscopic quantities. 

McCartney, E.J., Optics of the Atmosphere Scattering by Molecules and Particles, Wiley and Sons, 1976. 

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