Phonon scattering and exciton linewidths in naphthalene and phenanthrene molecular crystals

Abstract

A short theoretical description of absorption bandwidths of the lowest singlet exciton state of aromatic crystals is given in terms of exciton–phonon coupling. Exciton–phonon scattering, which is mainly due to librational phonons, generates a width which can be described in terms of exciton damping. The scattering may be inter- or intra-component in form. Lattice distortion on excitation contributes a coupling term which results in a phonon progression. The theory is applied to new experimental results for naphthalene and phenanthrene crystals and the phonons involved in the scattering process identified. An order of magnitude estimation of the coupling strength is shown to be consistent with experimental values, the major source of the coupling being via the dispersion (gas to crystal shift) term. Theoretical results for naphthalene are presented showing the effect of amalgamating a weak isolated phonon peak with the absorption band and the high temperature results are interpreted in terms of a strong overlap with an intense vibronic sideband. In addition an analysis of the results of phenanthrene shows the presence of states lying ∼50 cm^–1 below the optical level of the lowest Davydov component.