We demonstrate the mixing of rotational states in the ground electronic state using microwave radiation to enhance optical cycling in the molecule yttrium (II) monoxide (YO). This mixing technique is used in conjunction with a frequency modulated and
chirped continuous wave laser to slow longitudinally a cryogenic buffer gas beam of YO. We generate a measurable flux of YO below 10~m/s, directly loadable into a three-dimensional magneto-optical trap. This technique opens the door for laser cooling of molecules with more complex structure.
