Vibrational chirp in the dynamic Stokes-shift process due to ultrafast geometrical relaxation in a polydiacetylene

Abstract

Relaxation kinetics of photoexcitation in a blue-phase polydiacetylene has been investigated by difference absorption spectroscopy with extremely high time- and energy-resolution using broadband sub-$5\text{\ensuremath{-}}\mathrm{fs}$ optical pulses. One-fs-step tracking of an induced emission peak has enabled us to separate the vibrational dynamics during geometrical relaxation and internal conversion into modulations of transition-peak energy and transition probability. The peak energy shows redshift of about $60\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ and the instantaneous frequency of the ``$\mathrm{C}\mathrm{C}$'' stretching mode is positively ``chirped'' starting from $1600\ifmmode\pm\else\textpm\fi{}100\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ $(21\ifmmode\pm\else\textpm\fi{}2\phantom{\rule{0.3em}{0ex}}\mathrm{fs})$ and ending in $2100\ifmmode\pm\else\textpm\fi{}100\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ $(16\ifmmode\pm\else\textpm\fi{}1\phantom{\rule{0.3em}{0ex}}\mathrm{fs})$. This positive chirp indicates a huge dynamic structural change from ${(\mathrm{C}R\mathit{C}\mathit{C}\mathrm{C}{R}^{\ensuremath{'}})}_{n}$--like to ${(\mathrm{C}R\mathrm{C}\mathrm{C}\mathrm{C}{R}^{\ensuremath{'}})}_{n}$--like structured bonds in the polymer backbone during the internal conversion process within $100\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$. This corresponds to the reduction of the bond length from $1.32\ifmmode\pm\else\textpm\fi{}0.04\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ to $1.19\ifmmode\pm\else\textpm\fi{}0.04\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ by about $0.13\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, which was estimated from the bond lengths in literatures determined by the x-ray analysis for several polydiacetylenes.