Single-molecule spectra split by the interaction with two-level systems (TLSs)

At low temperatures, the fluorescence excitation spectra of single impurity molecules in amorphous or polycrystalline matrices are often split into two or more components by the coupling of the molecule to nearby tunneling systems (TLSs) of the solid. In the case of an asymmetric TLS, the two components of the molecular line will have different intensities. An expression for the ratio of the intensities is derived. Recent experimental data of terrylene in the Shpolʹskiı̆ matrix n-hexadecane and TBT (a substituted terrylene) in the amorphous polymer polyisobutylene are re-evaluated with this model.