Fluorescence excitation spectra have been recorded for the first 6000 cmminus;1of the ultraviolet spectrum ofh8hyphen; andd8hyphen;naphthalene cooled in a supersonic free jet. Measured profiles of vibronic bands in these spectra display a monotonically increasing width as a function of vibrational energy (Ev) in the excited electronic state. The high fluorescence quantum yield and relatively constant fluorescence lifetimes in these spectral regions require the measured line broadening to be assigned to intramolecular vibrational relaxation (IVR) within the excited electronic state. The rate of this IVR process as measured by the width of the broadened profiles increases smoothly from 9times;1010secminus;1atEv=3068 cmminus;1to 7times;1011secminus;1atEv=5200 cmminus;1forh8hyphen;naphthalene. Line profiles ofd8hyphen;naphthalene in this same spectral region are found to be sim;50percnt; wider.
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