  We demonstrate slowing and longitudinal cooling of a supersonic beam of CaF
molecules using counter-propagating laser light resonant with a closed
rotational and almost closed vibrational transition. A group of molecules are
decelerated by about 20 m/s by applying light of a fixed frequency for 1.8 ms.
Their velocity spread is reduced, corresponding to a final temperature of about
300 mK. The velocity is further reduced by chirping the frequency of the light
to keep it in resonance as the molecules slow down.
