Rotational Band Contour Studies of Single Vibronic Levels in Jet-Cooled trans-Stilbene

The rotational band contours of approximately 40 vibrational levels in the first excited singlet state (S_1) of supersonic jet-cooled stilbene are obtained and analyzed by use of computer simulations. No substantial differences are found in contour shapes of different levels, which is attributed to low symmetry and/or extensive torsional motion. Approximately 35 levels are studied by frequency resolving their emission, but no dependence of contour shape on detection conditions (specific mode detected, relaxed vs. unrelaxed emission) is observed. The use of selective detection to resolve overlapping levels in excited states is discussed qualitatively and quantitatively, with excellent agreement between experiment and theory. Finally, high-resolution (0.05 cm^(-1)) maps of the two known regions in S_1 exhibiting evidence of dissipative intramolecular vibrational redistribution (IVR) are presented to clarify the level structure involved