Time domain investigation of excited-state vibrational motion in organic molecules by stimulated emission pumping

A novel technique is demonstrated for the study of molecular vibrational dynamics in the structurally relaxed electronically excited state. The molecule is first optically excited and allowed to vibrationally relax. During the excited-state lifetime, emission is stimulated impulsively with a resonant <10-fs pulse, inducing vibrational coherence in both coupled electronic states. These are detected by an equally short probing pulse. In this first report, the methodology is applied to a substituted oligo(phenylene vinylene) in solution, and a complex vibrational response consisting of contributions from a number of active normal modes is observed. Our analysis shows that modulations observed in the experiment result mainly from vibrational coherences in the excited electronic state. The most prominent modulation is observed at 1589 cm^-1 and is assigned to the excited-state potential. The strong similarity of this frequency with that detected in resonant Raman scattering (1591 cm^-1) supports electronic structure calculations for this class of molecules. The spectroscopic scheme demonstrated here should prove to be useful in many other systems for obtaining excited-state vibrational dynamics.