Investigation of loosely bound states of NO2 just below the first dissociation threshold

Loosely bound states of jet cooled NO2 just below the first dissociation th reshold D-0, with binding energies E-b between 0.8 and 59.3 cm(-1), have be en investigated using pulsed VIS/UV optical double resonance spectroscopy. The measured UV spectra of these states in a spectral region where free NO absorbs have been found to depend strongly on the binding energy E-b = D-0 - E. This suggests that the states just below the dissociation threshold D- 0 may be regarded (at least in part) to belong to a family of states corres ponding to a large amplitude motion of an "oxygen atom" and a "NO fragment" , Such states, typical for loosely bound nonrigid molecules or van der Waal s complexes, are unusual for chemically bound molecules, in this paper we a re describing first experiments in which we obtained direct evidence for th eir existence in NO2. Most of the absorptions from the loosely bound states terminate on a dissociative potential energy surface (PES), so that the co rresponding spectrum is a broad unstructured feature, with a blue shift (co mpared to free NO) increasing with binding energy. Very weak bound-bound tr ansitions have also been observed. The analogy to spectra of NO/Ar van der Waals complexes is discussed.