  Atomic physics was revolutionized by the development of forced evaporative
cooling: it led directly to the observation of Bose-Einstein condensation,
quantum-degenerate Fermi gases, and ultracold optical lattice simulations of
condensed matter phenomena. More recently, great progress has been made in the
production of cold molecular gases, whose permanent electric dipole moment is
expected to generate rich, novel, and controllable phases, dynamics, and
chemistry in these ultracold systems. However, while many strides have been
made in both direct cooling and cold-association techniques, evaporative
cooling has not yet been achieved due to unfavorable elastic-to-inelastic
ratios and impractically slow thermalization rates in the available trapped
species. We now report the observation of microwave-forced evaporative cooling
of hydroxyl (OH) molecules loaded from a Stark-decelerated beam into an
extremely high-gradient magnetic quadrupole trap. We demonstrate cooling by at
least an order of magnitude in temperature and three orders in phase-space
density, limited only by the low-temperature sensitivity of our spectroscopic
thermometry technique. With evaporative cooling and sufficiently large initial
populations, much colder temperatures are possible, and even a
quantum-degenerate gas of this dipolar radical -- or anything else it can
sympathetically cool -- may now be in reach.
