High resolution spectrum of NO2 loosely bound states: densities of states and long range forces

More than 1700 rovibronic states of low angular momentum (J = 1/2–7/2) have been observed by laser induced fluorescence (LIF) at high precision and resolution, ≃0.004 cm−1, in a range covering 133 cm−1 below the NO2 lowest photodissociation threshold D0. This allowed us to significantly improve and complete previous experimental results devoted to the states lying in the vicinity of D0 (A. Delon, S. Heilliette and R. Jost, Chem. Phys., 1998, 238, 465). The density of loosely bound vibronic levels exceeds the expected density over the entire observed energy range and exhibits an abrupt increase in the last 30 cm−1 below D0. The number of observed vibronic levels between D0 − 133 cm−1 and D0 is about twice the expected number of levels. The rovibronic eigenstates cannot be assigned any other quantum numbers than J and the overall symmetry. This indicates strong intramolecular couplings, and this is consistent with the fact that the densities of observed rovibronic states J = 1/2, 3/2, 5/2 and 7/2 have reached their maximum value, as compared with the density of vibronic levels. Moreover, R0/P2 intensity ratios, involving the same upper levels, scatter around their average value by four orders of magnitude. The above mentioned anomalous properties of the loosely bound states of NO2 (density and intensity ratios) are assumed to be the consequences of the long range electrostatic forces between the fragments NO and O. To quantify this hypothesis, we propose a simple analytic relation between the isotropic contribution of the long range part of the potential energy surface and the density of observed loosely bound states.