  We demonstrate and characterize Zeeman-Sisyphus (ZS) deceleration of a beam
of ytterbium monohydroxide (YbOH). Our method uses a combination of large
magnetic fields ($\sim$ 2.5 T) and optical spin-flip transitions to decelerate
molecules while scattering only $\sim$ 10 photons per molecule. We study the
challenges associated with the presence of internal molecular perturbations
among the excited electronic states and discuss the methods used to overcome
these challenges, including a modified ZS decelerator using microwave and
optical transitions.
