by above expression. Rotational spectrum When a rotational transition occurs, there is a change in the value of rotational quantum number . Selection rules for rotational transition are, when , and when , as absorbed or emitted photon can make equal and opposite change in total nuclear angular momentum and total electronic angular momentum without changing value of . The pure rotational spectrum of a diatomic molecule consists of lines in the far infrared or microwave region. The frequency of these lines is given by Thus values of , and of a substance can be determined from observed rotational spectrum. See also Vibrational transition Notes References Chemical physics