Storage Ring Experiments with Cold Molecular Ions: the H₃⁺ Puzzle

Experiments on the dissociative recombination (DR) of low-energy electrons with triatomic hydrogen ions (H3+) using merged beams in storage rings are discussed, focusing on the ro-vibrational relaxation and state diagnostics of the stored ion beam, on studies of the three-body breakup following DR, and on new observations showing a variation of the DR rates during the continuous interaction of the stored ions with a cold electron beam. Linear fragmentation patterns in the three-body DR are discussed in the light of the H3 electronic potential surfaces. Measured energy dependencies of the H3+ DR rate coefficient are compared to recent theoretical predictions [Kokoouline, V. and Greene, C. H., Phys. Rev. A 68, 012703 (2003)] and a recent storage-ring measurement at CRYRING [McCall, B. J. et al., Nature 422, 500 (2003)]