We have Heteronuclear diatomic molecules with atomic masses m1 and m2 and an internuclear distance R have rotational energy eigenvalues EJ = BJ(J + 1) , J = 0, 1, 2, . . . when they are considered as a rigid rotor. The rotational constant is given by B = h^2/2µR^2 with the reduced mass µ = m1m2/(m1 + m2) . I need to Describe the frequency spectrum observed when photons are emitted in a pure rotational transition J + 1 -> J . also describe a method to estimate the internuclear distance R from such a rotational spectrum ?. How to proceed in this regard. Regards. QP.