Dynamics of the two-photon photodissociation of NO2: A molecular beam multiphoton ionization study of NO photofragment internal energy distributions

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Abstract
Two-photon photodissociation of NO2 is induced by the output of a pulsed dye laser tuned over the region from 455 to 425 nm. We characterize the dynamics of this process by recording the multiphoton ionization spectrum of the product NO: intensities of spectral features associated with Ã(2Σ+)←X̃(2π3/2,1/2) two-photon resonance enhanced, four-photon ionization of nascent NO reveal its distribution over accessible rovibronic states. A single laser pulse serves both as photodissociation source and probe. Over the wavelengths studied the dominant reaction pathway yields NO(X̃ 2π) and O(1D). Its dynamics in all regions of the photodissociation spectrum, save one, are comparable to those observed for the loss of O(3P) in one photon photolysis at comparable available excess energy. In the region of 427 nm, however, the photodissociation dynamics are dramatically different. Here we find that the photoproduct NO rotational distribution is anomalously cold, apparently limited by the rotational temperature of our supersonic molecular beam, and that a product spin-orbit state 2π1/2 is missing. We argue that this result suggests a linear or near-linear dissociation geometry which imparts very little torque to the departing NO photofragment and places strict symmetry requirements on its spin-orbit state. We offer an interpretation that traces the cause of this anomalous behavior to the participation at the two-photon level of a theoretically predicted linear state of NO2.