Polyacetylene third harmonic generation

The existence of the 2 Ag state below the PBu state in polyacetylene thin films was suggested by a number of experiments. Third harmonic generation (THG) and two-photon absorption by Halvorson and Heeger (1993) indicates that a Ag state lies below 1.1 eV in energy, while hnear absorption, which locates the state, typically rises at 1.8 eV and peaks at 2.0 eV (Vardeny 1993), as illustrated in Fig. 10.1. Peaks at 0.6 and 0.89 eV were observed in the THG spectrum by Farm et al. (1989). From the discussion of THG in Chapter 8, we know that this data implies some combination of Ag states at twice the photon energy (namely 1.2 and 1.8 eV), and states at three times the photon energy (namely 1.8 and 2.7 eV). Fann et al. interpreted the experiments as indicating the Ag and states virtually coincident at 1.8 eV. 

Due to their structural simplicity, polyenes and polyacetylene have been the focus of many experimental and theoretical studies dealing with the nonlinear optical response of conjugated systems [126-135]. Experimentally, third-harmonic generation (THG) measurements in polyacetylene indicate the appearance of two distinct peaks attributed to two- and three-photon resonances [127, 128]. Calculations carried out on short polyenes by Garito and co-workers in the framework of a Complete Neglect of Differential Overlap/Configuration Interaction (CNDO/CI)... 

Drury, M. R., Observation of third harmonic generation in oriented Durham polyacetylene. Solid State Comrmm., 68, 417, 1988. 

KRAUSZ F., WINTER E. and LEISING G., (1989). "Optical Third Harmonic Generation in Polyacetylene", Phys. Rev. B39. 3701-3710. 

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