Sub-5-fs real-time spectroscopy of several molecular systems

We have performed two-pump-probe experiments, azobenzene derivative and trans-polyacetylene (t-PA), using ultrashort visible pulse with photon energy extending from 1.6 to 2.3 eV. The photo-isomerization dynamics of azobenzene derivative was studied by pump-probe spectroscopy using sub-5-fs laser pulses. Time–frequency analysis of molecular vibration modes revealed that the mechanism of photo-isomerization must be described by a multi-dimensional model. t-PA with a degenerate ground state has a nonlinear excitation of soliton after photoexcitation, due to the electron–phonon coupling. The excess energy of an excited electron–hole pair over a soliton pair creation induces a breather oscillation characterized by collective stretching vibration of the carbon–carbon bonds. A time–frequency analysis of pump-probe signal shows that instantaneous frequencies of stretching modes are modulated for ∼50 fs after excitation and the modulation period is 44±3 fs consistent with the theoretical expectation, clearly verifying the breather.