Ultrafast real-time vibronic coupling dynamics of a breather soliton in trans-polyacetylene with a few-optical-cycle-pulse laser

The dynamics preceding the spatial separation of a charged soliton pair after photoexcitation in trans-polyacetylene was successfully investigated by using ultrafast spectroscopy with a 6.2 fs pulse laser. It was directly verified that after photoexcitation, the electron–hole pair relaxes with a breather mode (i.e. multi-quanta vibronic states), as theory predicts, with an electron–hole pair lifetime of 33–50 fs. By applying spectrogram analysis to the time trace of the absorbance change, the ultrafast amplitude and frequency modulations of CC and CC stretching modes, induced by breathers and lasting no longer than 100 fs, can be observed simultaneously for the first time. The frequency shifts of both modes were in good agreement with a simulation based on the Su–Schrieffer–Heeger model. It was found that the intensities of transition dipoles changed due to breathers, whereas transition energies were dominantly modulated by CC stretching modes as recent theoretical work predicted.