First observation of dynamic intensity borrowing induced by coherent molecular vibrations in J-aggregates revealed by sub-5-fs spectroscopy

Sub-5-fs spectroscopy of porphyrin J-aggregates reveals for the first time coherent molecular vibration coupled to the Frenkel exciton. The oscillations with the frequency of 244cm^-1 are described by a plus-cosine function for bleaching and a minus-cosine function for induced absorption. The coherent oscillation is explained by a modulated transition dipole moment, which is due to the transfer of an oscillator strength from the intense B-band to the weak Q-band through the ruffling mode with 244cm^-1-frequency.