i l e iδ l [j l (kr) cos δ l -y l (kr) sin δ l ]P l (cos θ), (10) where δ l describe the phase shifts obtained due to scattering of the l-wave, j l (kr) and y l (kr) are the spherical Bessel functions 97 and P l (cos(θ)) is the Legendre polynomial 97 . Of course, we can also expand the unperturbed plain wave in the same manner by putting δ l = 0 for all orbital quantum numbers l ∈ N ⩾0 , thus