FLUORESCENCE SPECTROSCOPY IN THE TIME DOMAIN: THE PROPERTIES OF OPTICALLY EXCITED NITROGEN DIOXIDE

The temporal modulation of fluorescence induced by a static external field after selective optical excitation of the first excited electronic state (('2)B(,2)) of nitrogen dioxide has been studied. The rotationally-resolved radiative lifetime as well as the hyperfine-resolved fluorescence polarization anisotropy and excited state Lande g-factor are extracted from magnetic-field-induced modulation (Zeeman quantum beat) experiments performed on a number of individual levels excited near 600nm. The collisional dynamics of the alignment and population of the optically excited nitrogen dioxide is studied by Zeeman quantum beat experiments carried out in the presence of low pressures of foreign gas. The second-order Stark shifts of single hyperfine levels are studied by time-resolved methods utilizing parallel electric and magnetic fields (Stark-Zeeman quantum beats).; In the absence of collisions, the alignment of the excited state decays at the same rate as its population, indicating no intrinsic dephasing processes exist in the isolated modecule.; The radiative lifetime varies significantly with emitting level even within the same vibronic band. The observed g-factors also deviate from the expected Hund's case (b) value, and the deviation is strongly rotational level dependent but always to smaller magnitude. This deviation correlates roughly with an increase in the observed radiative lifetime of the level. This indicates a rotational-level-dependent perturbation of the electronic character of the first excited electronic state.; Kinetic analysis of the effect of foreign gas pressure on the time-resolved emission of the optically prepared sample indicates that thermally averaged collisions at room temperature are very efficient at removing the population from the level initially excited. The elastic disalignment rate is found to be much slower than this process. About one seventh of the population removed from the initially prepared level is found to emit detectable radiation from different upper states.; The second-order Stark effect is analysed to yield an electric dipole moment for the first excited electronic state of the molecule. Although the derived dipole moment varies with rotational level, its magnitude is approximately the same as that predicted from ab initio calculations.