Vibrational dephasing in organic solids: Temperature dependence of a Raman active localized internal mode of naphthalene

Abstract

For a localized vibrational transition which is subject to both inhomogeneous and homogeneous broadenings, study of temperature dependence of its linewidth can still be used to identify the mechanism of vibrational dephasing. In the Raman spectra of naphthalene, temperature dependence of the line shape, the linewidth, and the vibrational frequency is investigated for the 764 cm−1 internal mode which is localized even in the neat crystal. At 2 °K the line is found to be a Gaussian and, thus, inhomogeneously broadened. At higher temperatures the line shapes are between a Lorentzian and a Gaussian. A computer simulation method is used to separate the homogeneous and the inhomogeneous components of the linewidth. The temperature dependence of the homogeneous linewidth is explained by a mechanism of dephasing which involves a T2′ process due to an off-diagonal mixed mode quartic anharmonic interaction with 140 cm−1 phonons. This process produces a modulation broadening but no T1 relaxation. The observed temperature dependence of the vibrational frequency is explained by a larger contribution from thermal expansion and a smaller contribution of opposite sign from a diagonal quartic anharmonic interaction with the 140 cm−1 phonon.