The photodissociation of NO2 by visible and ultraviolet light

Abstract

We present velocity map images of the NO, O(³P_J) and O(¹S₀) photofragments from NO₂ excited in the range 7.6 to 9.0 eV. The molecule was initially pumped with a visible photon between 2.82–2.95 eV (440–420 nm), below the first dissociation threshold. A second ultraviolet laser with photon energies between 4.77 and 6.05 eV (260–205 nm) was used to pump high-lying excited states of neutral NO₂ and/or probe neutral photoproducts. Analysis of the kinetic energy release spectra revealed that the NO photofragments were predominantly formed in their ground electronic state with little kinetic energy. The O(³P_J) and O(¹S₀) kinetic energy distributions were also dominated by kinetically ‘cold’ fragments. We discuss the possible excitation schemes and conclude that the unstable photoexcited states probed in the experiment were Rydberg states coupled to dissociative valence states. We compare our results with recent time-resolved studies using similar excitation and probe photon energies.