Radiation Pressure Force from Optical Cycling on a Polyatomic Molecule

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.