  Calculations have been carried out for the vibrational quenching of excited
H$_2$ molecules which collide with Li$^+$ ions at ultralow energies. The
dynamics has been treated exactly using the well known quantum coupled-channel
expansions over different initial vibrational levels. The overall interaction
potential has been obtained from the calculations carried out earlier in our
group using highly correlated ab initio methods. The results indicate that
specific features of the scattering observables, e.g. the appearance of
Ramsauer-Townsend minima in elastic channel cross sections and the marked
increase of the cooling rates from specific initial states, can be linked to
potential properties at vanishing energies (sign and size of scattering
lengths) and to the presence of either virtual states or bound states. The
suggestion is made that by selecting the initial state preparation of the
molecular partners, the ionic interactions would be amenable to controlling
quenching efficiency at ultralow energies.
