Evidence of Superelastic Electron Collisions from H2+ Studied by Dissociative Recombination Using an Ultracold Electron Beam from a Cooler Ring

The dissociative recombination rate of H⁺₂ was measured as a function of the storage time with an ultracold electron beam in a storage ring. The H⁺₂ ions vibrationally relax with time, and almost reach the vibrational ground state at a time of 25 s after injection. The magnitude of the ground-state population after 25 s far exceeds that expected based on the initial ground-state population and its decay. This indicates the existence of a superelastic collision process, where electrons are scattered from excited molecular ions and gain energy by vibrational deexcitation.