An experimental and theoretical study of the negatively charged helium dimer, He<SUB>2</SUB><SUP>-</SUP>

Translational energy spectrometry measurements have been made on He<SUB>2</SUB><SUP>-</SUP> ions resulting from 3 keV double-electron capture collisions between He<SUB>2</SUB><SUP>+</SUP> and N<SUB>2</SUB>. The endoergic energy defect for this reaction is measured to be 43 eV. Potential energy curves have been computed for He<SUB>2</SUB><SUP>+</SUP> (X <SUP>2</SUP>Σ <SUP>+</SUP>), He<SUB>2</SUB><SUP>-</SUP> (<SUP>4</SUP>Σ<SUP>+</SUP>) and He<SUB>2</SUB>(<SUP>3</SUP>Σ<SUB>+</SUB>) in order to elucidate the negative ion formation mechanism. The vertical electron affinity for He<SUB>2</SUB><SUP>-</SUP> is determined to be +0.15 eV, and a lower limit on its lifetime is measured to be 1.6 μ s