The dissociative recombination rate of ${\mathrm{H}}_2^{+}$ was measured as a function of the storage time with an ultracold electron beam in a storage ring. The ${\mathrm{H}}_2^{+}$ 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.

• Received 23 February 1999

DOI:https://doi.org/10.1103/PhysRevLett.83.2163