Collision-induced rotational Raman

A. D. Buckingham and G. C. Tabisz. Collision induced rotational Raman scattering. Opt. 

A. R. Penner, N. Meinander, and G. C. Tabisz. The spectral intensity of the collision induced rotational Raman scattering by gaseous CH and CH4-inert gas mixtures. Molec. Phys., 54 479 92 (1985). 

Much of the early work in CILS was done with the rare gases. The reasons are conceptual simplicity as well as fairly well established data, such as the permanent polarizabilities and interaction potentials. The absence of allowed Raman spectra further simplifies the analysis only the translational CILS spectra are observed in this case. However, since the earliest days of CILS studies, the optically isotropic gases such as CH4 and SFg have also been of considerable interest. Besides translational CILS, these show collision-induced rotational and rotovibrational bands [240, 241, 258, 324, 338, 342, 347],... 

We mention that molecules like CH4 are optically isotropic only if they are not rotating. If rotationally excited, centrifugal forces appear that distort the spherical symmetry and rotation-induced Raman lines arise whose intensities are proportional to the density at any fixed temperature. These are usually superimposed with the collision-induced rotational components whose intensities vary with a higher than the first power of density. 

D. P. Shelton and G. C. Tabisz. Binary collision induced light scattering by isotropic molecular gases II. Molecular spectra and induced rotational Raman scattering. Molec. Phys., 40 299-308 (1980). 

Thirdly, the collision-induced Raman resonances can expand the application of Raman spectroscopy to situations where the conventional Raman susceptibility vanishes. Andrews et al., for example, have demonstrated that sharp vibrational transitions in an initially unpopulated excited electronic state of a molecule can be observed as extra four-wave mixing resonances, even though they cannot be observed in absorption because of rapid dephasing of the electronic transition. In these experiments pentacene molecules were doped in a benzoic acid crystal, and phonon scattering rather than collisions provided the dephasing mechanism. CARS of equally-populated ground state rotational levels in molecules would also become possible by observing collision-induced resonances. 

A second way to induce molecular vibrations is to irradiate a sample with an intense source of monochromatic radiation, usually in the visible or near-infrared region, this leads to the Raman effect, which can be regarded as an inelastic collision between the incident photon and the molecule. As a result of the collision, the vibrational or rotational energy of the molecule is changed by an amount AE, . For energy to be conserved, the energy of the scattered photon h v, must differ from that of the incident photon h Vj by an amount equal to AE , ... 

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