Again, [ A y , H ] = 0 , and in addition: A y L z = − − L z A y [as L z = − − i ℏ ℏ ( x ∂ ∂ ∂ ∂ y − − y ∂ ∂ ∂ ∂ x ) ]. It follows immediately that if Λ Λ ≠ ≠ 0 the action of the operator A y on an eigenstate corresponding to the eigenvalue Λ Λ ℏ ℏ of L z converts this state into another one corresponding to the eigenvalue − − Λ Λ ℏ ℏ , and that both eigenstates have the same energy. The electronic terms such that Λ Λ ≠ ≠ 0 (that is, the terms Π Π , Δ Δ , Φ Φ , . . . . . . . . . . . . . . . . ) are thus doubly degenerate, each value of the energy corresponding to two states which differ by the direction of the projection of the orbital angular momentum along the molecular axis. This twofold degeneracy is actually only approximate and it is possible to show that the interaction between the electronic and rotational motions leads to a splitting of the terms with Λ Λ ≠ ≠ 0 into two nearby levels, which is called Λ Λ -doubling.