In order to have a cooling effect, there must be some dissipation of energy. Selection rules for dipole transitions dictate that for this example, Δ Δ J = − − 1 , + 1 and Δ Δ M J = 0 , − − 1 , + 1 with relative intensities given by the square of the Clebsch-Gordan coefficients. Suppose we start with a single atom in the ground state, J = 1 2 , in the M J = 1 2 state at z = 0 with velocity in the +z direction.