Surface polaritons on molecular crystals: An experimental study of anthracene

We report studies of surface polaritons on the (ab) face of crystalline anthracene by the method of attenuated total reflection. We find that: (a) At low temperature (less than 100 °K) a surface polariton propagating down the b axis on the (ab) face is a well defined excitation. The estimated dephasing time at 16 °K is 0.1 psec, and the surface polariton can move coherently along the surface for up to 2 μm. (b) At higher temperatures the surface polariton is not a well defined excitation by virtue of scattering by lattice phonons. (c) The single particle 391 cm−1 vibronic dipolar surface polariton is also a well defined mode at low temperature. (d) The two particle scattering resonance arising from the 391 cm−1 vibration is seen in both the region of nonradiative modes and the region of radiative modes. (e) Three types of nonradiative polariton mode are observed below the exciton origin in thin crystals. Each of these is characterized by a different form of dispersion relation.