A comparative study of the fluorescence lifetimes of 9-cyanoanthracene in a bulb and supersonic free jet

Abstract

The fluorescence decays of 9-cyanoanthracene (9CNA) in a supersonic free jet have been studied for a large number of vibrational bands including the electronic band origin. The fluorescence lifetime τf varied from vibrational level to level but decreased from 28.0±0.2 at the 0–0 band origin to 4.2±0.1 ns at the excess energy of 2045 cm−1. In a bulb, however, τf of 9CNA was no more than 3.2 ns and independent of both temperature (433–553 K) and excess energy (∼4800 cm−1). We attempted to interpret the very short and temperature invariant τf of 9CNA in the bulb in terms of the vibrational level specific τf found in the supersonic jet. No simple way to rationalize the findings in the bulb was found but the results suggested an important role of optically nonactive vibrational modes in determining the radiationless transition. The nonexponential decay with the lifetimes of 3.9±0.4 and 27.3±0.2 ns found for the partially cooled state of 9CNA, which was created by seeding 9CNA in helium at the pressure of 200 Torr, can be taken as the evidence supporting this view.