Application of Raman scattering spectroscopy in analysis of inorganic gases (Review)

Autor: A. A. Solov'ev, A. A. Petrov, V. M. Nemets
Rok vydání: 1987
Předmět:
Zdroj: Journal of Applied Spectroscopy. 47:975-985
ISSN: 1573-8647
0021-9037
DOI: 10.1007/bf00667685
Popis: Raman scattering of light is an inelastic interaction of the photons of the probing radiation with the molecules of the scattering material. In this process absorption of the incident photon and emission of % new photon, as happens in the case of fluorescence, do not occur, but rather the electronic shell of the molecule is perturbed (polarized) as a result of its interaction with the electromagnetic field of the incident photon. If the energy of this photon h~o is not high enough to excite the molecule from the ground state into a higher energy state, then for some time, determined by the period of time it interacts with the incident photon, the molecules is in an intermediate, so-called virtual, state, and then it returns to one of the stationary vibrational-rotational energy levels of the ground electronic stated scattering the photon with a charged energy h~ i. Thus in the interaction there occurs only a partial exchange of energy between the incident photon and the molecule in the scattering material. Figure la shows a simplified diagram illustrating this phenomenon. The molecule in the ground vibrational state (v = 0), on interacting with the incident photon with energy hv 0 scatters a photon with lower energy h(v 0 - vl), forming the so-called Stokes component of the RS spectrum. At the same time, part of the energy of the incident photon is absorbed by the molecule, increasing its internal energy. The molecule in the excited vibrational state, on interacting with the incident photon, gives up to its part of the energy and scatters the photon with a higher energy h(~o + v~), forming the anti-Stokes component of the RS spectrum. Since the molecules in the gas are in chaotic thermal motion, the scattered radiation is isotropic and incoherent, and for this reason this phenomenon is called spontaneous Raman scattering (SRS) of light. From the viewpoint of analytical applications, resonance Raman (RRS) and hyper-Raman (HRS) scattering are also interesting. The former arises when the frequency of the probing radiation approaches the characteristic absorption frequency of the molecule, i.e., when the virtual levels (Fig. la) approach the electron-vibrational energy levels of the molecules. The latter is a multiphoton which, as a rule, occurs with the participation of the electronic energy states and for this reason in practice it is very difficult to observe this type of RS in gases. A more detailed exposition of the theory of different types of RSL can be found in [1-4]. In the RS method analytical information is extracted from measurements of parameters of the spectral lines, such as the intensity I and the wave number (A~, cm-:), characterizing the shift of the RS lines relative to the exciting radiation ~o. The first parameter carries information about the number of scattering particles, whereas the second one enables identification of the measured signal with a definite type of particle. The intensity of the lines in the SRS spectrum is directly proportional to the intensity of the incident radiation, the number density of scattering particles, and the scattering cross section. From the viewpoint of analytical application of the phenomenon of spontaneous RS the most important problem is to increase the intensity of the scattered radiation since the intensity of RS equals 10 -7i0-" and in the best case 10-5-10 -6 times the intensity of the probing radiation. For this reason intense laser radiation is now employed as the probing radiation.
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