State-resolved dynamics in highly excited states of NO2: collisional relaxation and unimolecular dissociation

Abstract

Recent state-resolved investigations of unimolecular dissociation and collisional relaxation of NO₂ at chemically significant internal energies are outlined. Two powerful double-resonance techniques are described which permit the investigation of these processes on a quantum-state-resolved level of detail. A sequential optical double-resonance technique with sensitive laser-induced fluorescence detection has been employed for assignments of the molecular eigenstates of NO₂ in the energy range at 17 700 cm^–1. Subsequently, we were able to measure state-to-state rotational and vibrational energy transfer in NO₂–NO₂ self-collisions using a time-resolved double-resonance technique. From these data, direct information about propensity rules and intermolecular interactions for rotational and vibrational energy transfer in NO₂ self-collisions at high vibrational excitation could be obtained. In addition, we have used a folded high-resolution V-type double-resonance technique in a free jet to access and to assign rovibronic states of NO₂ above and below the dissociation threshold, E₀. From the double-resonance spectra, linewidths at around 25 130 cm^–1 as a function of internal energy, E, and total angular momentum, J, could be extracted. Specific rate constants, k(E, J), calculated from the homogeneous linewidths, have been compared with results from SACM calculations, predictions from a statistical random matrix model, and ps time-domain measurements.