Merging an ${\mathrm{HD}}^{+}$ beam with velocity matched electrons in a heavy ion storage ring we observed rapid cooling of the rotational excitations of the ${\mathrm{HD}}^{+}$ ions by superelastic collisions (SEC) with the electrons. The cooling process is well described using theoretical SEC rate coefficients obtained by combining the molecular $R$-matrix approach with the adiabatic nuclei rotation approximation. We verify the $\Delta J=-2$ SEC rate coefficients, which are predicted to be dominant as opposed to the $\Delta J=-1$ rates and to amount to $(1–2)\times {10}^{-6}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$ for initial angular momentum states with $J\le 7$, to within 30%.

• Received 27 February 2009

DOI:https://doi.org/10.1103/PhysRevLett.102.223202