The problem of determining the contribution of the exciton–photon coupling to the absorption line shape or width of an exciton is addressed. In this regard, both the spatially homogeneous (polariton) and inhomogeneous wave models are considered. For the latter model, the theory is complex although a line shape expression is derived for the case of high exciton velocity and low oscillator strength. The related problem of determining the conditions under which the strong exciton–photon coupling limit is attained is also discussed. It is in this limit that light attenuation is independent of the oscillator strength. The theory plus new experimental data on the a exciton of naphthalene argue for the inadequacy of the polariton model. On the other hand, reasonable agreement between the data and the spatially inhomogeneous wave theory developed here is obtained. Thus, it is possible to estimate that the exciton–photon coupling contribution to the a-exciton linewidth is 0.26 cm−1. This is the major portion of the 2 K linewidth of 0.3 cm−1.

High resolution (-0.1 em-I) one-photon absorption profiles of the a band of naphthalene at 31475 cm-J 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-J have been observed. The a band profile exhibits an asymmetry which increases with the temperature. The high E side is close to Lorentzian 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 (T 2 ) of the a exciton. The data and theory are consistent with the exciton-phonon scattering contribution to T2 being due to low frequency (wefT-IS cm-I ) acoustical phonons. However, existing theories on asymmetric but homogenous exciton line shapes fail to account for the observed asymmetries.4790

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