Q1(m). Hydrogen rotational spectra

Answer in brief

H₂ has no permanent electric dipole but has anisotropic polarisability.

Key points

• Electric-dipole microwave rotational absorption requires a permanent molecular dipole. Homonuclear H₂ has none, so ordinary electric-dipole pure rotational transitions are forbidden.
• Raman scattering couples through the induced dipole, μ_(ind)=α E. The polarisability of hydrogen differs parallel and perpendicular to the bond, enabling rotational modulation.
• Rotational Raman transitions obey ΔJ=±2 (with ΔJ=0 for Rayleigh scattering). Nuclear-spin statistics affect line intensities, but do not remove the Raman activity.