Coulomb and centrifugal barrier bound dianion resonances of NO2

New short-lived resonance states of NO22- have been studied by bombarding N O2- anions by low-energy, mono-energetic electrons at the ASTRID heavy-ion storage ring. Storage for several seconds before the measurement ensures fu ll vibrational relaxation of NO2- target anions. The dianion resonances wer e identified by the detection of resonances in the cross section for format ion of neutral NO2. Two resonances were observed: The one of lowest energy is assigned to be the ground state of NO22- based on an ab initio calculati on. This state may be held by a Coulomb barrier alone. A second state of si gnificantly higher energy is argued to be held by a combined Coulomb and ce ntrifugal barrier. Finally, a new scheme in which electron recombination ma y create stable dianions is proposed. (C) 2001 American Institute of Physic s.