An optical study of the temperature dependent scattering of the a exciton (polariton) of naphthalene

High resolution (∼0.1 cm−1) one-photon absorption profiles of the a band of naphthalene at 31 475 cm−1 from pure strain-free mounted (PSF) crystals have been measured between ∼2 and 40 K. Low temperature linewidths (FWHM) as narrow as 0.4 cm−1 have been observed. The a band profile exhibits an asymmetry which increases with the temperature. The high E side is close to Lorenztian at all temperatures (particularly for T≳8 K) and tails more steeply than the low E side. Arguments are presented for the asymmetric line shape being homogeneous so that the T-dependent FWHM are a direct measure of the transverse optical dephasing (T2) of the a exciton. The data and theory are consistent with the exciton–phonon scattering contribution to T2 being due to low frequency (ωeff∼15 cm−1) acoustical phonons. However, existing theories on asymmetric but homogenous exciton line shapes fail to account for the observed asymmetries.