Kinetics of Nitrogen Dioxide Fluorescence

The fluorescence lifetime and intensity of gas‐phase nitrogen dioxide ${(}^2{B}_1)$ have been measured as a function of excitation wavelength, fluorescence wavelength, and pressure (0.5–50 mtorr). The phase‐shift method was used; this technique allows lifetime measurements to be obtained with signal intensities of 100 counts per sec and lower. The excitation source, tunable throughout the visible region, had a half‐width bandpass as low as 15 Å. Fluorescence wavelength separation was accomplished with 15 interference filters between 400 and 8000 Å. The radiative lifetimes range from 55 to 90 μsec for excitation from 3980 to 6000 Å and tend to increase with excitation wavelength; however, the lifetimes exhibit considerable variation within a narrow excitation region. The fluorescence sample was contained in a 33‐cm‐diam spherical bulb; apparent fluorescence lifetimes in smaller cells were reduced because of migration of excited molecules (under collision‐free conditions) and wall quenching. In order that the measured lifetimes exhibit no more than 5% error, observation must be extended beyond the excitation region to a distance five times the product of the lifetime by the most probable velocity. The Stern–Volmer analysis of fluorescence kinetics has been generalized to a multilevel system under conditions of modulated excitation and phase‐sensitive detection. Analysis of fluorescence data in terms of this mechanism yields the product of energy‐transfer rate constants times efficiencies (amount of energy lost by the excited polyatomic molecule per effective collision). This analysis implies the loss of at least one vibrational quantum (average value 1230 cm⁻¹) per gas‐kinetic collision over the entire range from 12 500–25 000 cm⁻¹ energy above the ground state. This result indicates that molecules in the ${}^2{B}_1$ excited electronic state are rapidly interconverted to the high vibrational levels of the ground electronic state.