Excitonic and Vibronic Spectra of a Two-Dimensional Graphene-like Molecular Lattice

Recently, graphene has shown interesting physical properties with promising applications. In this paper, we study the excitonic and vibronic spectra of Frenkel excitons (FEs) in a two-dimensional hexagonal graphene-like lattice with two identical and nonequivalently positioned molecules per unit cell. The excitonic Hamiltonian and the excitonic levels with transition dipole moments perpendicular to the layer or parallel to it represent the electronic excitations of this hexagonal lattice. The vibronic spectra consist of both types of dipole active FEs plus one or several quanta of intramolecular vibrations coupled with FEs via linear and quadratic coupling. The expressions and numerical simulations of the absorption spectra manifest excitonic, one-particle (bound), and many-particle unbound exciton-phonon states depending on the strength of the linear exciton-phonon coupling as well as Davydov splitting of the vibronic spectra.