Relative fine-structure intensities in two-photon excitation

Relative intensities for different fine-structure components of the two-photon transitions 2p ³P→3p ³P in oxygen and 2p³⁴Sᵒ→2p²3p⁴Dᵒ 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 ΔJ≠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 ΔJ=0, ±{}1, and ±{}2 transitions.