Electro-modulation spectroscopy on excitons in /spl beta/-carotene and related polyenes

Summary form only given. Excited states in the linear conjugated molecules, including finite length polyenes and longer chain polymers, are of current interest both from basic solid-state exciton spectroscopy as well as from technological interest in the remarkable nonlinear optical properties of these organic materials. It has been suggested that the photo-excited electronic states in the linear conjugated systems are described in terms of quasi-one-dimensional Wannier-type excitons extended along the conjugated backbone chains. These excitons are also responsible for the large third-order nonlinear optical susceptibilities reported on various kinds of conjugated polymers and polyenes. Yet, details of the exciton structures are still left to be explored both from experimental and theoretical view points. In this paper, we report on the results of electro-absorption measurements on the thin films of /spl beta/-carotene and related linear polyenes, such as diphenyl octatetraene and diphenyl-hexatriene. The electro-modulation technique is known to be useful for detection of the forbidden (two-photon alloed) /sup 1/Ag excitons, which play an important role in the nonlinear optical processes together with the allowd /sup 1/Bu excitons. Experimental results are presented and discussed in terms of the exciton model which has been successfully applied for linear conjugated systems such as polydiacetylenes and polysilanes.