Inelastic electron collisions of the isotopically symmetric helium dimer ion He2+4 in a storage ring

Rate coefficients for dissociative recombination (DR), dissociative excitation (DE), and vibrational excitation between the helium dimer ion He2+4 and electrons from a few meV up to 40 eV were measured using fast (3.8 and 8.3 MeV) ion beams stored for up to 85 s. Vibrational relaxation to greater than 95% in the v=0 level was achieved by collisions with cold electrons over 50 s. Low-energy, strongly v -dependent DR rate coefficients are given for v=0, 1, and ≥2. The rate coefficients at higher energies for v=0, with DR and DE given on an absolute scale, are compared to results from recent wave-packet calculations on the fast dissociation dynamics of the doubly excited helium dimer, where the three processes occur as competing reaction channels. Also given are rate coefficients for vibrationally superelastic electron collisions at near 10 meV average energy and the approximate vibrational excitation cross section for fast collisions with the residual gas.