Two-photon absorption by rotating diatomic molecules

Explicit forms for the two-photon absorption cross section in linearly and circularly polarized light for rotating diatomic molecules have been developed. The ratio (σλλ/σ _cc ) of the cross sections for individual rotational transitions is precisely two-thirds for all branches of ΔΩ = 0, ±1, ±2 transitions except for the unique case—the Q branch of a ΔΩ=0 transition. In the latter case the ratio becomes J-dependent and may range from infinity to 4/1 (for high J) for different relative contributions to the absorption cross section of intermediate states having either the same or different electronic symmetry. These contributions are thus experimentally accessible in this favourable situation.