Rate – equation treatment of radiation force on a three – level cascade atom

A rate equation is derived for the momentum acquired by a three – level atom in a ladder configuration interacting with two thermal radiation fields. The equation is solved in conjunction with the standard rate equations for the populations of the atomic levels to obtain an expression for the radiation force exerted on the atom. The force is evaluated using data that correspond to these used elsewhere for the 2 3S1 – 2 3P2 – 3 3D3 cascade in metastable helium. It is found that the radiation force on a two – level atom is well enhanced for all times if a third level is exploited in theinteraction, in agreement with the findings of a Bloch – equation treatment of the same problem available in literature.