Dynamics of NO2 Electronic States Excited by a Tunable Dye Laser

Abstract

A tunable organic dye laser has been used as a narrow band (∼0.8 Å) radiation source to excite NO2 molecules under essentially collision-free conditions in the spectral range 4515–4605 Å. The fluorescence which is observed in a very large cell is decidedly nonexponential, so that the decay function for each experiment is characterized as both nonexponential and exponential, the latter for purposes of comparison with other experiments and for purposes of discussion in standard terms. The lifetimes which are obtained in this way show a marked structural pattern when displayed as a spectrum, with individual values ranging from 62 to 75 μsec for observation of fluorescence at wavelengths longer than 5200 Å. In addition, the dependence of lifetime and nonexponentiality, as well as their spectral variations, on observation wavelength, pressure, and fluorescence observation geometry are investigated over a shorter spectral region. A small amount of high resolution spectral data is reported which seems to indicate the presence of some unresolved, quasicontinuous absorption underlying the obvious bands of the visible spectrum. The effects of restrictive fluorescence measurement geometry on lifetime measurements are explored, with particular attention to other investigations, for which the shorter, nonvarying lifetime observations may be explained. Finally the observed dynamics and spectroscopy of NO2 are discussed in terms of the following hypotheses: (1) Transitions to both the 2B1 and 2B2 electronic states occur together over a large portion of the visible spectrum, (2) both of these excited states are clearly perturbed, and (3) the 2B2 transition is the stronger of the two.