The energy denominator effect in lambda-doubling

Lambda-doubling is the lifting of the degeneracy of a pair of rotational levels of opposite e/f-symmetry, which is caused by their interaction with rotational levels of an energetically remote electronic state. Historically, this phenomenon has been associated with the symmetry dependence of the matrix elements that appear in the numerator of a second-order perturbation expression. We show that this effect may be present even when there is no rotational interaction or when the off-diagonal matrix element of the rotational Hamiltonian is independent of e/f-symmetry.