  We demonstrate a scheme for magneto-optically trapping strontium monofluoride
(SrF) molecules at temperatures one order of magnitude lower and phase space
densities three orders of magnitude higher than obtained previously with
laser-cooled molecules. In our trap, optical dark states are destabilized by
rapidly and synchronously reversing the trapping laser polarizations and the
applied magnetic field gradient. The number of molecules and trap lifetime are
also significantly improved from previous work by loading the trap with high
laser power and then reducing the power for long-term trapping. With this
procedure, temperatures as low as 400 $\mu$K are achieved.
