Infrared multiphoton excitation and inverse electronic relaxation in SO2

Visible luminescence (λ=270–470 nm) has been observed from S16O2 and S18O2 at pressures of 0.2 to 20 Torr following irradiation by an intense infrared laser (λ=9.3 μm). Our experiments show that the luminescence is not due to dielectric breakdown or recombination of dissociation fragments, but rather is fluorescence from the first excited singlet states of SO2 following inverse electronic relaxation from highly excited vibrational levels of the ground electronic state. Crossover from the ground to excited electronic states may also be collisionally assisted. Spectroscopic and kinetic measurements are consistent with previous studies on 1B1 emission from SO2. The pressure dependence of the fluorescence yield exhibits two distinct pressure regimes, while the dependence of visible emission on laser pump wavelength follows the small signal infrared absorption spectrum. The threshold for detection of fluorescence is 17–20 J/cm2 with 9.3 μm radiation as the excitation source. These observations are discussed in terms of recently proposed theories which describe the photophysics of vibrationally excited states coupled to a radiative continuum through higher electronic states.