Role of negative ion resonances in electron scattering from atoms and molecules

Transient negative ions (resonances) formed during the collision of an elec tron with an atom or molecule have been extensively studied for over thirty years. The continued interest in these states, both experimentally and the oretically, stems from the profound erects that they can have on electron s cattering cross sections and the role that electron{electron correlations p lay in their formation and quasi-stability. A selective discussion of examp les of such resonances, involving one, two and three excited electrons is g iven for a wide range of atomic and molecular systems.