Vibronic-level dependence of radiative and nonradiative processes in naphthalene vapor

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Abstract
The vibronic‐level dependence of radiative and nonradiative processes in naphthalene vapor has been investigated under collision‐free conditions, with particular attention to the effect of molecular rotation and to the contamination caused by simultaneous excitation of levels other than the one under study. Fluorescence excitation spectra, lifetimes, and sensitized biacetyl–phosphorescence excitation spectra were measured by using a pulsed dye laser with a bandwidth of 0.5 cm−1. From the resulting data, the radiative and intersystem‐crossing rate constants (kf and kisc,respectively), as well as the fluorescence and intersystem‐crossing yields, were determined for 20 single vibronic levels of S1 up to an excess vibrational energy of about 2000 cm−1. The value of kf varies greatly with vibronic level. The levels involving the 8(b1g) vibrational mode have particularly large kf values, and those involving 8(ag), 6(ag), and 4(ag) modes moderately large ones. The observed variation of kf is explained in terms of the transition moment induced by vibronic interaction of S1 with the other singlet states. The value of kisc also varies with vibronic level. A sudden increase and pronounced fluctuation of the kisc value are found at an excess energy of about 1000 cm−1, suggesting the presence of a new crossing channel related presumably to T3. The fluorescence quantum yield measured along the rotational contour of the 8(b1g)01 absorption band shows a rather small variation; moreover, the lifetimes are nearly constant over the contour of the 8(b1g)10 band and of the 0–0 band. The observed variation of the fluorescence yield along the rotational contour is interpreted as due largely to excitation of sequence bands.

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