Editing Rotational transition (section)

=== Rotational spectrum ===
{{main|Rotational spectroscopy}}
When a rotational transition occurs, there is a change in the value of rotational quantum number {{math|''J''}}. Selection rules for rotational transition are,
when {{math|1=Λ = 0}}, {{math|1=Δ''J'' = ±1}} and
when {{math|Λ ≠ 0}}, {{math|1=Δ''J'' = 0, ±1}} 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 {{math|''J''}}.

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
<math display="block"> \hbar \omega = E_r(J+1)-E_r(J) = 2B(J+1) </math>
Thus values of {{math|''B''}}, {{math|''I''<sub>0</sub>}} and {{math|''R''<sub>0</sub>}} of a substance can be determined from observed rotational spectrum.