Thus, it does not describe the final state of the emitted electron in cases when such interactions cannot be ignored. Replacing the spherical Bessel functions 𝑗 ℓ 𝑘𝑟 in eqn ( ) with Coulomb radial functions from eqn (9), as described in Sec. 4.2, adopts an alternative basis for the final state of the electron, but if only a single expansion center is used, it does not introduce a dipole moment into the system. We now turn to two alternative approaches to model the exit-channel interactions between the emitted electron and the polar neutral residue.