Molecular vibration of several modes in blue-phase polydiacetylene-3-butoxycarbonylmethylurethane (PDA-3BCMU) was real-time observed by 5-fs pump-probe measurement. The contribution of the vibrational wave packets in the ground state and in the excited state in the signal were separated by multichannel measurement. The $\mathrm{C}\mathrm{C}$ stretching mode in the ground state starts to oscillate $\pi$-out-of-phase with the $\mathrm{C}\mathrm{C}$ 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 $\mathrm{C}\mathrm{C}$ and $\mathrm{C}\mathrm{C}$ stretching modes there were determined by singular value decomposition method to be $1472\pm 6{\mathrm{cm}}^{-1}$ and $2092\pm 6{\mathrm{cm}}^{-1}$, respectively. The double and triple bond stretching frequencies in the ground state are $1463\pm 6{\mathrm{cm}}^{-1}$ and $2083\pm 6{\mathrm{cm}}^{-1}$, respectively.

• Received 11 May 2004

DOI:https://doi.org/10.1103/PhysRevB.70.214301