Knowledge Bank

A tunable dye laser (0.05, or 0.005 nm fwhm) has been used to excite $N02$ fluorescence in the 578--612 nm absorption region. The decay of total fluorescence at 585, 593, and 612 nm is highly non-exponential at pressures as low as $10-5$ Torr. The radiative lifetimes of the states contributing to such decays range from about 40--200 $\mu$ sec as determined from a biexponential fit of the decays, with the short lived state(s) being quenched at a rate of about $5\times 10-10$ $cm3$ $sec-1$ and the long, lived state(s) at a much slower rate of $0.75\times 10-10$ $cm3$ $sec-1$. In the weakly absorbing regions at 578, 594, and 603 nm, the decays are nearly exponential with a radiative lifetime and quenching rate constant corresponding to that of the long lived state(s) at 585, 593, and 612 nm. In addition, some experiments have been carried out which show that there is a direct correlation between the degree of non-exponentiality of the fluorescence decay and the strong features in the fluorescence excitation spectrum. The results are discussed in terms of both one and two electronic states perturbed by high vibrational levels of the ground state. The fluorescence lifetime anomaly will also be discussed.