FEMTOSECOND HOLE-BURNING SPECTROSCOPY WITH STIMULATED-EMISSION PUMPING AND SUPERCONTINUUM PROBING

A novel broadband femtosecond version of the stimulated emission pumpi ng (SEP) technique is demonstrated. A nonstationary ground state of a molecular sample in the condensed phase is prepared by two optical pul ses. The first picosecond PUMP pulse resonantly excites the sample. Th e second femtosecond DUMP pulse, which is tuned to the molecular fluor escence band, is applied after relaxation in the excited state and cre ates a ''particle'' in the ground state and a ''hole'' in the excited state. The relaxation of this system is probed by a femtosecond superc ontinuum. An advantage of the proposed scheme is that the hole contrib ution is constant for certain conditions, and hence, the transient abs orption spectrum of the particle may be isolated. As an application of the technique, the ground-state evolution of coumarin 102 in acetonit rile is studied. Intramolecular vibrational redistribution (IVR), with a characteristic time tau(IVR) similar to 10 fs, is observed in the f requency domain. Subsequently, the absorption band shifts to the blue and shows isosbestic points in the course of relaxation. (C) 1998 Amer ican Institute of Physics.