Fluorescence decay of aromatic vapours. II. Single vibronic level lifetimes of the isolated naphthalene molecule

The lifetimes of single vibronic levels of naphthalene-h8 and ?d8 have been studied under collision-free conditions in the vapour. The lifetime of the vibration less 1B3u state of naphthalene-d8 was found to be 485±5 ns. At sufficiently high resolution of the exciting light the fluorescence decay is exponential. The mean fluorescence lifetime of the prepared state decreases with increasing vibrational energy content but not as a smooth function of energy. The ratio of lifetimes for the 0° and 8(b18)1 states is explicable entirely in terms of the differing radiative pathways for electronic relaxation. The quantum yields for these states of naphthalene-h8 are estimated to be of the order of 0.45 and 0.63 for the 0° and 8(b18)1 state respectively. The lifetimes of comparable states show a normal deuterium isotope effect; for higher energies where the errors are larger the evidence is less certain.