Vibrational and rotational cooling of ${\mathrm{H}}_3^{+}$

The vibrational relaxation of ${\mathrm{H}}_3^{+}$ molecules from a conventional plasma ion source is studied performing Coulomb explosion imaging on the ions extracted from a storage ring after variable times of storage. Storage for 2 s is found sufficient for radiative relaxation of the breathing excitation and the fragment velocity distribution in the breathing coordinate then agrees well with simulations based on the calculated ground-state wave function. The radiative decay of the two lowest pure breathing levels ${(1,0\mathrm{}}^0)$ and ${(2,0\mathrm{}}^0)$ is seen to be considerably faster than expected from rotationless calculations. Assuming a high rotational excitation of the ${\mathrm{H}}_3^{+}$ ions, as suggested already in earlier experiments, the theoretical transition probabilities of the University College London line list for ${\mathrm{H}}_3^{+}$ [L. Neale, S. Miller, and J. Tennyson, Astrophys. J. 464, 516 (1996)] can explain the increase of the vibrational cooling rates and reproduce the observed decay curve for the lowest breathing-excited level, confirming the absolute transition probabilities of these line tables. The observations give evidence for a quasistable population of high-lying rotational levels in the stored ion beam, relevant for the interpretation of storage ring measurements on the rate coefficients for dissociative recombination of ${\mathrm{H}}_3^{+}$ ions with low-energy electrons.