  We demonstrate the coherent optical bichromatic force on a molecule, the
polar free radical strontium monohydroxide (SrOH). A dual-frequency
retro-reflected laser beam addressing the
$\tilde{X}^2\Sigma^+\leftrightarrow\tilde{A}^2\Pi_{1/2}$ electronic transition
coherently imparts momentum onto a cryogenic beam of SrOH. This directional
photon exchange creates a bichromatic force that transversely deflects the
molecules. By adjusting the relative phase between the forward and counter
propagating laser beams we reverse the direction of the applied force. A
momentum transfer of $70\hbar k$ is achieved with minimal loss of molecules to
dark states. Modeling of the bichromatic force is performed via direct
numerical solution of the time-dependent density matrix and is compared with
experimental observations. Our results open the door to further coherent
manipulation of molecular motion, including the efficient optical deceleration
of diatomic and polyatomic molecules with complex level structures.
