Fluorescence lifetime studies of NO2. I. Excitation of the perturbed 2B2 state near 600 nm

Abstract

A pulsed, tunable dye laser (0.05 or 0.005 nm FWHM) was used to measure NO2 fluorescence lifetimes in the 578–612 nm absorption region at pressures of 0.01 to 25 mtorr. When relatively strong absorption features at 585, 593, and 612 nm are excited, the decay of total fluorescence is highly nonexponential, even at pressures as low as 0.01 mtorr, with radiative lifetimes ranging about 20 to 260 μsec, as determined from biexponential fits. In more weakly absorbing regions at 578, 594, and 603 nm, the decays are nearly exponential, with lifetimes of about 200 μsec. Experiments with 0.005 nm bandwidth in the 593 nm region show that there is a one to one correspondence between strong features in the high resolution fluorescence excitation spectrum and the most highly nonexponential decays. These results are explained in terms of a single excited electronic state (2B2) variably vibronically coupled to upper vibrational levels of the ground state. States with a larger fraction of 2B2 parentage have shorter lifetimes and absorb more strongly. Their rotational structure overlaps that of the ubiquitous, more weakly absorbing, longer-lived states with less 2B2 character, so that when the laser frequency corresponds to a transition to one of the shorter-lived states, fluorescence is stronger and decays more nonexponentially. Based on published spectroscopic data, MC–SCF calculations, and density of states estimates, vibronic levels of the perturbed 2B2 states are expected to have lifetimes ranging from about 15 to roughly 100 μsec in the 600 nm region, in qualitative agreement with the experimental findings. These results are discussed in the context of earlier findings from this and other laboratories.