Depolarized Rayleigh spectroscopy of small alkanes with picosecond relaxation times

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Abstract
Depolarized Rayleigh scattering is studied as a function of temperature in a group of small alkanes under conditions where the observed relaxation time is in the picosecond time range. The molecules studied include the n-alkanes from pentane to nonane, 2-methyl butane, 2,3-dimethyl butane, 2,4-dimethyl pentane, 2,2,4,4-tetramethyl pentane, and 3,3-diethyl pentane. In the n-alkanes, 2-methyl butane, and 2,4-dimethyl pentane, the spectrum is dominated by the molecular optical anisotropy and the second rank reorientational relaxation time is determined. In the other systems, the depolarized Rayleigh spectrum is determined by the relaxation of intermolecular optical anisotropy due to dipole-induced dipole interactions between pairs of density fluctuations. The observed linewidth can be accurately predicted using the theory of Madden [Mol. Phys. 36, 365 (1978)]. The present results are intended to serve as an accurate data base for the rapidly improving computer calculations of the dynamics of small alkanes.