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

Abstract

Loosely bound states of jet cooled NO2 just below the first dissociation threshold D-0, with binding energies E-b between 0.8 and 59.3 cm(-1), have been 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 corresponding to a large amplitude motion of an "oxygen atom" and a "NO fragment", Such states, typical for loosely bound nonrigid molecules or van der Waals complexes, are unusual for chemically bound molecules, in this paper we are describing first experiments in which we obtained direct evidence for their existence in NO2. Most of the absorptions from the loosely bound states terminate on a dissociative potential energy surface (PES), so that the corresponding spectrum is a broad unstructured feature, with a blue shift (compared to free NO) increasing with binding energy. Very weak bound-bound transitions have also been observed. The analogy to spectra of NO/Ar van der Waals complexes is discussed.