Reflection and diffraction of atomic de Broglie waves by an evanescent laser wave

The diffraction theory of a reflection grating for atoms based on a standing light wave is developed in a particular dressed-state approximation. The basis states take into account internal and external atomic degrees of freedom simultaneously and correspond to diffraction orders in the asymptotic region. In this basis the atom is described as an effective multilevel system. In an adiabatic approximation for the motion perpendicular to the grating, these levels serve as position-dependent potentials. An incoming de Broglie wave is coupled to higher diffraction orders by nonadiabatic transitions near avoided crossings of the potentials. Numerical results confirm the qualitative predictions following from the physical picture of the diffraction process and lead to an improved description of experiments.