Vibrational Properties of p-Terphenyl

Vibrational properties associated with the intra- and intermolecular terms of pristine p-terphenyl at low temperatures are analyzed in detail by Raman spectroscopy. Nearly all of the vibrational modes exhibit anomalous behaviors at the transition temperature of about 193 K on the frequencies, widths, and intensities. Meanwhile, the drastic drops of the spectrum weight result in the splits of many peaks like the lattice vibrational peaks and the peaks located at around 1220, 1280, and 1600 cm^-1. All the anomalies result from the drastic decrease of the vibrational anharmonic coupling effects in the crystalline p-terphenyl after entering into the ordered state. The rapidly declining anharmonicity also makes contributions to the anomalous behaviors of the intensity ratios of the 1220, 1280, and 1600 cm^-1 modes, as well as the energy separations between the combination bands and the fundamental bands. Our work is of great significance to understand the internal vibrational properties of p-terphenyl.