Abstract
A novel broadband femtosecond version of the stimulated emission pumping (SEP) technique is demonstrated. A nonstationary ground state of a molecular sample in the condensed phase is prepared by two optical pulses. The first picosecond PUMP pulse resonantly excites the sample. The second femtosecond DUMP pulse, which is tuned to the molecular fluorescence band, is applied after relaxation in the excited state and creates a “particle” in the ground state and a “hole” in the excited state. The relaxation of this system is probed by a femtosecond supercontinuum. An advantage of the proposed scheme is that the hole contribution is constant for certain conditions, and hence, the transient absorption spectrum of the particle may be isolated. As an application of the technique, the ground-state evolution of coumarin 102 in acetonitrile is studied. Intramolecular vibrational redistribution (IVR), with a characteristic time τIVR∼10 fs, is observed in the frequency domain. Subsequently, the absorption band shifts to the blue and shows isosbestic points in the course of relaxation.
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