INTRAMOLECULAR VIBRATIONAL ENERGY REDISTRIBUTION IN ISOLATED NAPHTHALENE

The rate and extent of intramolecular vibrational redistribution (IVR) following optical excitation in an isolated naphthalene molecule has been measured using the technique of supersonic free-jet spectroscopy. Naphthalene, exicted to its first (('1)B(,3u)) or second (('1)B(,2u)) excited singlet state is found to undergo a rapid vibrational redistribution which is quite global in nature. The rate of the IVR process is measured by observation of the fluorescence excitation linewidths as a function of vibrational energy (E(,v)). A smooth, monotonic increase in the vibronic linewidths as vibrational energy is increased is observed, corresponding to redistribution rates ranging from 9 x 10('10) sec('-1) at E(,v) = 3068 cm('-1) to 7 x 10('11) sec('-1) at E(,v) = 5200 cm('-1). The extent of the redistribution is monitored via single vibronic level fluorescence spectra of naphthalene taken in a free-jet. At low vibrational excitation the fluorescence spectra display sharp, well resolved features, indicating that little redistribution has occurred on the time-scale of the fluorescence (nsec). However, at energies above E(,v) = 2122 cm('-1) the spectra become diffuse. Computor modeling of these spectra shows that many zero-order levels must be involved in the redistribution process, at least all of the quasi-resonant levels of the same symmetry as the originally pumped level. Therefore, even at moderate vibrational energies, an isolated molecule the size of naphthalene undergoes a rapid flow of vibrational energy among much of the energetically available vibrational phase space, following optical excitation