Are asymmetric stretch Raman spectra by centrosymmetric molecules depolarized?: the 2ν3 overtone of CO2.

Molecular vibrations that are not totally symmetrical give rise to depolarized lines [P. Atkins and J. de Paula, Atkins' Physical Chemistry (Oxford University Press, UK, 2006), p. 464]. But in the case of stretching vibrations in centrosymmetric molecules, the statement has so far not been conclusively verified. It is the purpose of this article to report a rigorous experimental and theoretical analysis of the 2ν(3) band of CO(2)--the first overtone of the asymmetrical stretch vibration. The anisotropic spectrum was extracted and its spectral moment calculated from light-scattering measurements, taken at room temperature and for a wide range of CO(2)-gas densities. Evidence for a near-entirely depolarized Raman band is provided, with integrated depolarization ratio η(int)=6/7.16, closely approaching the upper bound η(max)=6/7. Agreement with theoretical predictions is found, on the basis of quality ab initio data for polarizability properties, provided that electro-optical and mechanical anharmonicity and intermode coupling effects between symmetric ν(1) and antisymmetric ν(3) stretching vibrations are incorporated.