Polarized Raman spectra of n-alkane single crystals with orthorhombic polyethylene-type sublattice

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Abstract
Polarized Raman spectra were measured on single-crystal specimens of orthorhombic n-C35H72 and monoclinic n-C36H74, both consisting of the orthorhombic polyethylene (o-PE) subcells. Relative magnitudes of the squared elements of the derived polarizability tensor were evaluated for every zone-center fundamentals of the o-PE lattice. These values were proved to reproduce well the observed Raman intensities of uniaxially oriented and unoriented samples of PE. The polarizations, the frequencies, and the intensity ratios of the components of the doublets due to the crystal-field splitting were measured for all the internal modes except for the CH2 stretching vibrations, using a single-crystal specimen of n-C35H72 cooled at liquid helium temperature. They agreed with the results of the normal coordinate treatment of the o-PE lattice. The Raman activity of the B3g component of the CH2 rocking mode was found to vanish as predicted by the oriented-gas model approximation.