To promote an HCl molecule from the "v" = 0 to the "v" = 1 state, we would expect to see an infrared absorption about "ν"o = "ν"e + 2"x"e"ν"e = 2880 cm−1. However, this absorption corresponding to the Q-branch is not observed due to it being forbidden by symmetry. Instead, two sets of signals (P- and R-branches) are seen owing to a simultaneous change in the rotational state of the molecules. Because of quantum mechanical selection rules, only certain rotational transitions are permitted. The states are characterized by the rotational quantum number "J" = 0, 1, 2, 3, ... selection rules state that Δ"J" is only able to take values of ±1.