  We demonstrate multiple photon cycling and radiative force deflection on the
triatomic free radical strontium monohydroxide (SrOH). Optical cycling is
achieved on SrOH in a cryogenic buffer-gas beam by employing the rotationally
closed $P\left(N''=1\right)$ branch of the vibronic transition
$\tilde{X}^{2}\Sigma^{+}\left(000\right)\leftrightarrow\tilde{A}^{2}\Pi_{1/2}\left(000\right)$.
A single repumping laser excites the Sr-O stretching vibrational mode, and
photon cycling of the molecule deflects the SrOH beam by an angle of
$0.2^{\circ}$ via scattering of $\sim100$ photons per molecule. This approach
can be used for direct laser cooling of SrOH and more complex, isoelectronic
species.
