Picosecond excitation and selective intramolecular rates in supersonic molecular beams. II. Intramolecular quantum beats and IVR

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Abstract
An account of observations pertinent to quantum beat-modulated fluorescence decays of jet-cooled anthracene excited to S1+1380 cm−1 is given. Based on both spectral and temporal characteristics of the beats, combined with theoretical expectations of such characteristics, it is shown that the interference phenomenon arises from the coupling of zero-order vibrational levels within the S1 manifold. (Consistent with this interpretation is the observed absence of a magnetic field effect on the beats.) As such, it is shown to be a manifestation of restricted IVR. The results of measurements of rotational band contours and the effect of carrier gas on beat parameters are used to assess the role of rotations in the coupling between vibrational levels. Specific vibration–vibration coupling schemes are briefly discussed within the context of the observations.