Conformational Change in Azobenzene in Photoisomerization Process Studied with Chirp-Controlled sub-10-fs Pulses

Negatively and positively chirped pulses were used to selectively excite vibrationally coherent wave packets in the ground and the excited states, respectively, in an azobenzene derivative molecule in solution. Strong dynamic mode coupling between the N N and C−N stretching modes is observed in real time at ∼1400 and ∼1150 cm -1 in the excited state of the molecule, but it is relatively week in the ground state. Instantaneous frequencies of N N and C−N stretching modes around 1400 and 1150 wavenumbers (corresponding to ∼24 and ∼29 fs vibrational periods, respectively, in both of the states) were found to be modulated on the basis of time-frequency analysis of transmittance-change traces with high time-resolution of a few femtoseconds. In the excited state, the modulation periods of N N and C−N stretching modes are ∼510 and ∼390 fs, respectively, directly indicating the existence of at least one more mode other than the torsion mode coupled to either or two of these stretching modes during the photoisomerization.