Intramolecular vibrational redistribution in alkylbenzenes. II. Spectroscopy and dynamics

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Abstract
Dispersed fluorescence spectra from 00, 6b1, and 121 excitation of ethylbenzene, pentylbenzene, and hexylbenzene are computed within the normal mode approximation and compared with experiment. The harmonic model is shown to be capable of describing the appearance of the ‘‘new shifted spectrum,’’ originally identified as a ‘‘relaxed spectrum,’’ which becomes increasingly dominant with increasing chain length. Thus, this feature cannot be a consequence of intramolecular vibrational redistribution (IVR). By contrast, the appearance of substantial spectral broadening can only be accounted for through the introduction of anharmonic coupling. An assessment of standard assumptions in the analysis of IVR in ‘‘ring+tail’’ molecules is also provided. Results show that some crucial assumptions, e.g., that electronic transitions are not accompanied by changes in chain excitation, are unjustified.