Gases, collision-induced light scattering

Several types of collision-induced light scattering spectra are known. We have already mentioned the depolarized translational spectra of rare gas pairs and bigger complexes which arise from the anisotropy of the diatom polarizability. Contrary to the infrared inactivity of like pairs, e.g., Ar-Ar like pairs are Raman active. Furthermore, polarized translational spectra... 

Collision-induced light scattering is the name for the Raman spectroscopy that arises from the interaction-induced variations of the polarizability of a sample. For a description of the scope of CILS, we consider briefly two somewhat extreme cases, a tenuous gas and a liquid. 

M. Berard and P. Lailemand. Collision induced light scattering in low density neon gas. Molec. Phys., 34 251-260 (1977). 

S. Bratos, B. Guillot, and G. Birnbaum. Theory of Collision Induced Light Scattering and Absorption in Dense Rare Gas Fluids. In G. Birnbaum (ed.). Phenomena Induced by Intermolecular Interactions, Plenum Press, New York, 1985, pp. 363-381. 

The collision-induced light scattering in the helium gas has been subject of many experimental studies (35)-(42). Most of these measurements were done at high densities (35)-(37), so the reported Raman intensities were affected by three-body contributions, and pure pair spectra had to be separated out (37) by applying simplified models. Only the polarized... 

Collision-induced Raman Light Scattering in Atomic Gas... 

In the collision-induced Raman experiments the laser light of wavenumber co0 is scattered inelastically by the interacting atoms in the gas. The intensities of the depolarized, D(v), and polarized, P(v), scattered light are given by323 324,... 

The Raman spectra of solids have a more or less prominent collision-induced component. Rare-gas solids held together by van der Waals interactions have well-studied CILS spectra [656, 657]. The face-centered, cubic lattice can be grown as single crystals. Werthamer and associates [661-663] have computed the light scattering properties of rare-gas crystals on the basis of the DID model. Helium as a quantum solid has received special attention [654-658] but other rare-gas solids have also been investigated [640]. Molecular dynamics computations have been reported for rare-gas solids [625, 630, 634]. 

Consequently, the light scattered from spherical molecules is not expected to be de polarized.5 Nevertheless as we see in Chapter 14, even inert gas atoms depolarize th< light (i.e., Ivh 0) this arises from the anisotropy induced by collisions, an effec which is discussed in Section 10.1 and Chapter 14. 

In this contribution we present two laser spectroscopic methods that use coherent resonance Raman scattering to detect rf-or laser -induced Hertzian coherence phenomena in the gas phase these novel coherent double resonance techniques for optical heterodyne detection of sublevel coherence clearly extend the above mentioned previous methods using incoherent light sources. In the case of Doppler broadened optical transitions new signal features appear as a result of velocity-selective optical excitation caused by the narrow-bandwidth laser. We especially analyze the potential and the limitations of the new detection schemes for the study of collision effects in double resonance spectroscopy. In particular, the effect of collisional velocity changes on the Hertzian resonances will be investigated. 

---