Knowledge Bank

A tunable dye laser has been used to excite the fluorescence excitation spectrum of a supersonic molecular beam of 2% $NO2$ in argon. The fluorescence excitation spectrum has been measured over region 5700 to 6700 {\AA} at a resolution of 1 GHz. Because of the rotational cooling which takes place during the supersonic expansion, only a few rotational levels have any significant population. Therefore, each vibronic band is well resolved and a rotational and fine structure assignment is easily possible. All observed vibronic bands are consistent with parallel selection rules and this suggests that most of the oscillator strength in this wavelength region is associated with the $X~2A1$ to $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$B$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ electronic transition. The effective rotational and fine structure constants of the upper electronic state show substantial variation from band to band indicating that the upper electronic state is strongly perturbed. Observation of ``hot” bands arising from molecules originally in the V$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ = 1 vibrational state of the ground electronic state indicates that there is little or no vibrational cooling during the supersonic expansion.