Real-time spectroscopy to clarify the mechanism of ultrafast nonlinearity

In order to clarify the mechanism of ultrafast optical nonlinearity in polymer especially polydiacetylene real-time measurement of molecular vibration which can provide information of time-resolved structural changes associated with huge electronic spectral change was performed. Molecular vibration of several modes in blue-phase polydiacetylene-3-butoxycarbonylmethylurethane (PDA-3BCMU) was time resolved by 5-fs pump-probe measurement. The contributions of the vibrational wavepackets in the ground state and in the excited state in the signal were separated by multi-channel measurement. The C=C stretching mode in the ground state starts to oscillate π-out-of-phase with the C≡C stretching mode. The structure of PDA-3BCMU in the geometrically relaxed state is not pure butatriene-type but more like acetylene-type. The frequencies of C=C and C≡C stretching modes there were determined by singular value decomposition method to be 1472 ± 6 cm-1 and 2092 ± 6 cm-1, respectively. The double and triple bond stretching frequencies in the ground state which are 1463 ± 6 cm-1 and 2083 ± 6 cm-1, respectively. Frequencies of both modes are increased in the configuration after the geometrical relaxation after the generation in the excited state upon excitation.