Collision-Induced Dissociation of Highly Excited NO₂ in the Gas Phase and on MgO (100) Surfaces

Collision-induced dissociation (CID) of NO₂ in highly excited mixed ²A₁/²B₂ states is studied in crossed molecular beams at collision energies of ∼2000 cm^-1 and on crystalline MgO(100) at collision energies of ∼2000 and 4400 cm^-1. The yield spectra obtained by scanning the excitation laser wavelength while monitoring NO fragments show features identical to those in the fluorescence excitation spectrum of NO₂, but the yield of CID decreases exponentially with the increase of the amount of energy required to reach the threshold for the monitored NO state. The results are discussed in terms of a mechanism in which highly excited NO₂ undergoes further activation by collisions, followed by unimolecular decomposition. The NO product spin-orbit excitations are sensitive to the chemical identity of the collider and bear the imprints of exit-channel interactions, which are more significant on the MgO(100) surface than in the gas-phase.