Thermoreflectance study of polydiacetylene-bis (toluene sulphonate) single crystal (PDA-TS)

Abstract
For the first time studies have been made of the polarised thermoreflectance of an oriented PDA-TS single crystal. The spectra in the photon energy range 1.9-2.5 eV are taken at room and liquid nitrogen temperature. From the different sensitivities to temperature the structures at 2.00 eV and 2.44 eV are assigned to excitonic and interband transitions, respectively. The binding energies of the excitons at room temperature (0.44 eV) and those corresponding to the low-temperature split band gap have been estimated to be nearly identical: 0.43 eV and 0.44 eV. The main thermoreflectance structures are assigned to a Frenkel exciton interacting with phonons.