Dissociation of highly excited NO₂ induced by collisions with Ar, CO, and O₂

The gas-phase collision-induced dissociation (CID) of highly excited, mixed ²A₁/²B₂ states of NO₂ with Ar, CO, and O₂ is studied in crossed beams experiments with state-resolved detection of products. Both the internal and translational energy of the reactants are varied independently. The state-specific relative yield of the NO product as a function of the initial NO₂ excitation is reported for excitation energies hv from dissociation threshold D₀ to D₀ - hv = 1000 cm^-1. The relative collision energies are 750-2400 cm^-1. The structure of the CID yield spectrum is similar to that observed in the fluorescence excitation spectrum of NO₂, and the scaling of the collisional energy transfer efficiency can be described by an exponential decay law. NO is detected using laser ionization, and rotational and spin-orbit distributions are determined. CID is described fairly well by assuming a unimolecular decomposition (UMD) of NO₂, collisionally excited to a range of excess energies above D₀ with excitation probability determined by the exponential gap law. The average energy transferred per activating collision is in the range 110-310 cm^-1, dependent on both the relative collision energy and the nature of the collider. Higher product spin-orbit excitations are observed with CO and O₂ than with Ar which suggests a stronger exit-channel interaction with these colliders.