Relative fine-structure intensities in two-photon excitation

Relative intensities for different fine-structure components of the two-photon transitions $2p {}_{}{}^3{}_{}{}^{}P\to 3p {}_{}{}^3{}_{}{}^{}P$ in oxygen and $2p^3{}_{}{}^4{}_{}{}^{}S_{}^o{}_{}{}^{}\to 2p^{2}3p^4{D}^o$ in nitrogen had been measured and found to agree well with calculations involving a single virtual intermediate level. They are in direct conflict, however, with a two-photn selection rule $\Delta J\ne 1$ derived in an earlier but erroneous purely theoretical treatment of these transitions. Five other experiments are also briefly examined, with the conclusion that relative fine-structure intensities in two-photon transitions are well understood as straightforward extensions of angular momentum coupling in single-photon cases, in accordance with allowed $\Delta J=0\mathrm{}$, ±1, and ±2 transitions.