'Anomalous' density of states and rotational selection rules of loosely bound states of NO2

We report experimental results concerning the laser-induced fluorescence (LIF) spectrum and the laser-induced dispersed fluorescence (LIDF) spectrum of NO2 recorded by exciting very loosely bound levels located just below the dissociation threshold D-0 at 25128.57 cm(-1) (NO2 --> NO((2)Pi(1/2))+ O(P-3(2))). We had already mentioned (R. Jest, J. Nygard, A. Pasinski, A. Delon, J. Chem. Phys. 105 (1996) 1287) the 'anomalously' high density of rovibronic eigenstates observed by LIF close to the dissociation threshold D-0. Here we present more detailed results about the sudden increase of the density within the last 20 cm(-1) below D-0, which reaches more than 5 times its 'normal' value (smooth extrapolation) at D-0. In addition to this, the LIDF spectra show evidence of strongly 'anomalous' rotational selection rules for the transitions implying these loosely bound states. Clearly these 'anomalies' are observable because the eigenstates are mixture of 'normal' bright states and some additional states which exist only very close to D-0. Two reasonable origins can be invoked for these additional states: either the long-range electrostatic interactions between the fragments O and NO (involving dipole, quadrupoles and dispersion), or additional electronic states which may be repulsive at short range but slightly attractive at long range. These two possibilities are discussed. (C) 1998 Elsevier Science B.V. All rights reserved.