Role of the middle and long range parts of the NO2 potential energy surfaces: anomalous density of states and recombination rate constant

Abstract

Using an accurate ab initio potential energy surface of NO₂, the density of vibrational states of NO₂ has been calculated up to the dissociation threshold. It is shown that the semi-classical approach, based on phase space volume derivation, cannot be used in the vicinity of the dissociation threshold, due to long-range interactions. Consequently, a new adiabatic method is introduced that benefits from the quantum nature of the NO intramolecular mode, while properly taking into account the C_2v symmetry. Finally, the spin–orbit interactions are shown to play an important role for the observed anomalous density of states just below the dissociation threshold. On the other hand the effect of the middle and long range parts of the potential energy surface on the high pressure limit of the recombination rate constant is investigated within adiabatic capture theory. It is shown that the middle range forces are crucial to reproduce the law of variation of the experimental rate coefficient as a function of temperature and that the spin-orbit interactions play a role only at very low energy.