  It is shown that the translational degrees of freedom of a large variety of
molecules, from light diatomic to heavy organic ones, can be cooled
sympathetically and brought to rest (crystallized) in a linear Paul trap. The
method relies on endowing the molecules with an appropriate positive charge,
storage in a linear radiofrequency trap, and sympathetic cooling. Two
well--known atomic coolant species, ${}^9{\hbox{Be}}^+$ and
${}^{137}{\hbox{Ba}}^+$, are sufficient for cooling the molecular mass range
from 2 to 20,000 amu. The large molecular charge required for simultaneous
trapping of heavy molecules and of the coolant ions can easily be produced
using electrospray ionization. Crystallized molecular ions offer vast
opportunities for novel studies.
