Before we can trap the molecules in a magneto-optical trap, they have to be slowed down. We make a beam of CaF molecules moving at 150m/s using cryogenic buffer gas cooling. The radiation pressure of counter-propagating laser light slows down the molecular beam. The frequency of the light is chirped to follow the changing Doppler shift as the molecules decelerate.

The graph shows the speed distributions that we measure. The black curves are obtained when no laser slowing is applied and the coloured ones when the slowing is applied with various frequency chirps. The radiation pressure slows down the molecules and also bunches up their velocity distribution. The dashed lines are the results of our numerical modelling. Using this method, we are able to slow molecules down to the capture velocity of a magneto-optical trap. The next step is to trap them.