If the molecule is not in a Σ Σ state, so that Λ Λ ≠ ≠ 0 , transitions with Δ Δ ℑ ℑ = 0 are allowed. This gives rise to a further branch of the vibrational-rotational spectrum, called the Q branch. The frequencies ω ω Q corresponding to the lines in this branch are given by a quadratic function of ℑ ℑ if B v and B v + 1 are unequal, and reduce to the single frequency: ℏ ℏ ω ω Q = E ( v + 1 , ℑ ℑ ) − − E ( v , ℑ ℑ ) = ℏ ℏ ω ω 0 if B v + 1 = B v .