Theoretical analysis of the resonance CARS spectra of diacetylene single crystals

A theoretical model is presented which describes the excitation and line profiles of resonance CARS spectra obtained from diacetylene single crystals containing few short polymer chains. The model is based on a crude Born‐Oppenheimer approximation. The electronic wavefunctions are calculated by means of an LCAO MO (linear combination of atomic molecular orbital) method in the Hückel approach. The Franck—Condon integrals are calculated under the assumption of harmonic potential functions. The model includes a distribution of polymer chain lengths and inhomogeneous broadening due to distortions of the monomer host lattice. The simulations of the CARS excitation and line profiles taken from FBS and TS/FBS diacetylene crystals are based on results of former calculations of absorption and linear resonance Raman spectra of these materials. Fairly good agreement between model and experimental CARS spectra is obtained. Strong resonance behaviour can be seen by varying the frequency of the pump laser. Interference effects occur for neighbouring lines, which can also be simulated by the theoretical model. Possibilities for applications of diacetylene polymers as non‐linear optical devices are discussed.