Purely rotational coherence effect and time-resolved sub-Doppler spectroscopy of large molecules. II. Experimental

Abstract

In this paper we describe the results of picosecond fluorescence polarization (sub-Doppler) experiments designed to determine the role of purely rotational coherence in two jet-cooled molecules: trans-stilbene and anthracene. Observations of the manifestations of purely rotational coherence in t-stilbene are reported. The relationship of purely rotational coherence to molecular parameters (excited state rotational constants and transition dipole directions) is confirmed by comparison of our measurements with the results of the theory described in paper I [P. M. Felker and A. H. Zewail, J. Chem. Phys. 86, 2460 (1987)]. The sum of rotational constants B′ and C′ of the t-stilbene S1 electronic state is determined with a precision of better than 1 part in 700 (B′+C′=0.5132±0.0007 GHz). The influence of molecular beam expansion conditions and fluorescence detection conditions on our measurements is also investigated and compared with the theroretical findings of paper I. Also measurements of time-resolved and polarization-analyzed fluorescence as a function of excess vibrational energy in the S1 electronic states of both t-stilbene and anthracene are reported. We are able to distinguish the contribution of purely rotational coherence from the contributions of purely vibrational (or rovibrational) coherence to the evolution of fluorescence from the vibrationally excited molecule. The results are first analyzed on the basis of a model in which strict separability of vibrational and rotational motion is assumed. This provides a test of the extent of coupling of these motions and its influence on intramolecular vibrational energy redistribution (IVR).