Now consider an atom which is moving in the formula_26-direction. The Zeeman effect shifts the energy of the formula_27 state lower in energy, decreasing the energy gap between it and the formula_28 state; that is, the frequency associated with the transition decreases. Red-detuned formula_29photons, which only drive formula_30 transitions, propagating in the formula_31-direction thus become closer to resonance as the atom travels further from the center of the trap, increasing the scattering rate and scattering force. When an atom absorbs a formula_29photon, it is excited to the formula_27 state and gets a "kick" of one photon recoil momentum, formula_34, in the direction opposite to its motion, where formula_35. The atom, now in an excited state, will then spontaneously emit a photon in a random direction and after many absorption-spontaneous emission events, the atom will have, on average, been "pushed" back towards the field-zero of the trap. This trapping process will also occur for an atom moving in the formula_31-direction if formula_37photons are traveling in the formula_26-direction, the only difference being that the excitation will be from formula_28 to formula_40 since the magnetic field is negative for formula_41. Since the magnetic field gradient near the trap center is uniform, the same phenomenon of trapping and cooling occurs along the formula_12 and formula_13-directions as well.