Observation of the Electric-Dipole-Forbidden States in Polydiacetylene by Means of Resonance Second-Harmonic Generation Spectroscopy

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Abstract
We have observed resonance second-harmonic generation (SHG) spectra in polydiacetylene (poly [5,7-dodecadiyne-1,12-diyl-bis-ethylurethane], 4U2 for short) films. The resonant peak appears on the high-energy side of the linear absorption peak. The peak observed in SHG spectra is assigned to an A g exciton state, and the linear absorption peak to the lowest B u exciton state. The observation of the dipole-forbidden A g state originates from the resonance enhancement of SHG due to an electric quadrupole transition at the SH frequency region. Thus, the resonance SHG technique serves as a powerful tool for investigating dipole-forbidden states in centrosymmetric media such as π-conjugated polymers. The advantages of this method compared with other methods such as two-photon absorption and electroreflectance will be discussed.