Determination of Excited -State Rotational Constants and Structures by Doppler-Free Picosecond Spectroscopy

Picosecond time-resolved fluorescence measurement of purely rotational coherence is developed as a Doppler-free technique for the determination of rotational constants of large molecules in their excited states. We present detailed analyses of purely rotational coherence measurements, supplying new information about the rotational constants and structures of the first excited electronic states of t-stilbene, four t-stilbene van der Waals complexes, and anthracene, including values for all three anthracene S₁ rotational constants. Evidence is considered in the case of stilbene for a transition dipole with a significant component perpendicular to the a inertial axis, and the consequences of such a dipole are explored by way of numerical simulations. Excited-state structures are proposed for stilbene and stilbene–rare-gas complexes and comparisons made with model calculations of the van der Waals potential. Application of the new spectroscopic technique to molecules of large asymmetry is demonstrated by the analysis of fluorene and fluorine–argon measurements, and the results are compared with data from previously published high-resolution frequency domain studies.