Abstract

Molecular systems in the condensed phase show ultrafast vibronic dynamics on a timescale well below 100 fs. Recently, coherent molecular polarisations and subsequent incoherent relaxation processes were investigated by optical spectroscopy in the 10 to 20 fs range [1,2], In this paper the vibrational and vibronic dynamics of large dye molecules (IR 125) in solution is studied on a 20 femtosecond time scale. The fundamental and second harmonic pulses [3] of a modelocked Ti:sapphire laser facilitate temporally and spectrally resolved experiments with 30 fs pump pulses resonant to short-wavelength S₀-S_n transitions and 20 fs probe pulses overlapping both the S₀-S₁ absorption and emission bands. In Fig. 1 (a) spectrally and temporally resolved transmission changes (T-T₀)/ T₀ after excitation at 425 nm are plotted as a function of the delay time between pump and probe pulses (T, T₀ are the transmission of the sample with and without excitation). The probe pulses centred at 850 nm were spectrally dispersed after interaction with the sample (detection bandwidth 10 nm). Around delay zero, we observe a pronounced coherent signal with sign and amplitude depending on the spectral position within the probe pulses. This coherent coupling of the two electronic transitions is due to a coherent vibrational motion in the electronic ground state monitored by the probe pulses and induced by the pump pulses via resonant impulsive stimulated Raman scattering [1].

© 1996 Optical Society of America