  We demonstrate strong coupling between molecular excited states and surface
plasmon modes of a slit array in a thin metal film. The coupling manifests
itself as an anti-crossing behavior of the two newly formed polaritons. As the
coupling strength grows, a new mode emerges, which is attributed to long range
molecular interactions mediated by the plasmonic field. The new, molecular-like
mode repels the polariton states, and leads to an opening of energy gaps both
below and above the asymptotic free molecule energy.
