Theoretical and experimental approaches to enhanced two-photon absorption

A selection rule for two-photon transition is considered based on the alternancy symmetry and the result is adopted to explain the experimental and calculation results of two-photon absorption (TPA) properties of two diacetylene derivatives with neutral and charged peripheral groups. The discussion successfully explains the difference in strength of the TPA bands and the existence of the higher-lying TPA states that is strongly allowed and plays a main role to the resonance enhancement of TPA.