Laser spectroscopy of NO2 under very high resolution: Fluorescence spectra from selectively excited hfs levels

Absorption spectra of NO₂ with reduced Doppler width and resolved hyperfine structure have been obtained by crossing the beam from a single mode argon laser, tunable around 4880 and 5145 Å, with a well-collimated NO₂ beam at low pressures. With the laser frequency stabilized onto selected NO₂ absorption lines the fluorescence spectra of NO₂ molecules in selectively populated hfs levels of an excited electronic state were examined through a grating monochromator which resolved the different rotational lines. From the fluorescence spectra the quantum numbers N′ and K′ of the emitting levels could be determined. The analysis proved that some emitting levels have K′-values differing from that of the primarily excited level. This implies radiationless transitions between different electronic states in the free molecule, which can change the value of K′ but preserve that of N′.