  We perform magnetically-assisted Sisyphus laser cooling of the triatomic free
radical strontium monohydroxide (SrOH). This is achieved with principal optical
cycling in the rotationally closed $P\left(N"=1\right)$ branch of either the
$\tilde{X}^{2}\Sigma^{+}\left(000\right)\leftrightarrow\tilde{A}^{2}\Pi_{1/2}\left(000\right)$
or the
$\tilde{X}^{2}\Sigma^{+}\left(000\right)\leftrightarrow\tilde{B}^{2}\Sigma^{+}\left(000\right)$
vibronic transitions. Molecules lost into the excited vibrational states during
the cooling process are repumped back through the $\tilde{B}\left(000\right)$
state for both the $\left(100\right)$ level of the Sr-O stretching mode and the
$\left(02^{0}0\right)$ level of the bending mode. The transverse temperature of
a SrOH molecular beam is reduced in one dimension by two orders of magnitude to
$\sim700\ {\rm \mu K}$. This approach opens a path towards creating a variety
of ultracold polyatomic molecules, including much larger ones, by means of
direct laser cooling.
