Key points are not available for this paper at this time.
Excited atoms incident upon the entrance port of a maser cavity will be reflected half of the time, if the atoms are slow enough, even when the maser field is in its vacuum state. This happens because the interaction strength, between the atom and the maser field, changes strongly when passing from the exterior to the interior of the cavity.
Englert et al. (Tue,) studied this question.