Fig. 2: Energy level scheme, calculated state-specific DR rate coefficients, and predicted cooling curves.

a Energy level scheme and radiative transitions for all D₂H⁺ states with J < 3. The states are sorted into the respective ortho and para manifolds, which are strictly separated. The labels in color denote the rotational quantum numbers \({J}_{{K}_{{{{\rm{a}}}}}{K}_{{{{\rm{c}}}}}}\). The lifetimes of the excited states are given in gray below each level. The individual energies are given to the left of each level (in meV). The dark gray arrows indicate major radiative transitions, while the thinner arrows in light gray indicate minor transitions (where applicable). b Individual theoretical merged-beams DR rate coefficients α^mb for the lowest rotational states. The curves were derived from the theoretical DR cross sections (see Supplementary Information), which were convolved with the experimental velocity distribution for comparison to the experimental results. c Simulated relative populations for the rotational states of D₂H⁺ as a function of storage time within the CSR. The initial temperature of the ensemble was set to 3000 K. While the initial distributions are spread over hundreds of rovibrational states, here we plot only the lowest rotational states with J < 3, which remain populated after 1000 s of storage. Ortho and para states are plotted separately. The population of all states with J > 2 is summed up and depicted by the gray solid lines.