[en] Using laser resonance-enhanced ionisation spectroscopy, we have studied electron (6-350 eV) simulated dissociation of NO₂ coadsorbed with up to 0.75 monolayer of atomic O on Pt(111). Several dramatic effects on NO₂ dissociation occur due to the presence of O. There is a large (x26) enhancement in the specific dissociation yield, a narrowing of the NO translational energy distributions, and a distinct propensity (>4:1 at low J) for populating the upper Ω=3/2 NO spin-orbit level over the Ω=1/2 level. The spin-orbit state distribution of the O(³P_J) dissociation fragment is (5.0):(2.5):(1.0) for K=2, 1 and 0, which is within experimental error of the statistical (T→∞) 2J+1 limit. The enhanced yield probably results from an increased excited state lifetime due to a reduction in substrate charge-transfer screening. We have also detected O(³P_J=2,1,0) and NO X²Π_3/2,1/2(v=5) above an electron (6-350 eV) beam irradiated Pt(111) surface containing coadsorbed O₂ and NO at 90 K. We conclude that both O(³P_J) and NO(v=5) are laser-induced photodissociation fragments of NO₂ desorbates. The NO₂ is probably the reaction product of a collision between an O atom (created by electron-stimulated dissociation of adsorbed O₂) and an NO(a). We correlate the 10 eV NO₂ production threshold with the dissociative ionization of the 3σ_g molecular bonding orbital of O₂(a). (orig.)