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 2003 Phys. Rev. A 68 012703) and a recent storage-ring measurement at CRYRING (McCall B J et al 2003 Nature 422 500).