Interference in two-photon rotational line strengths of diatomic molecules

A general formalism describing two-photon absorption of an isotropic ensemble of diatomic molecules is presented, and practical simplifications are discussed. Explicit rotational line strength factors are given for transitions between electronic doublet states, which are intermediate between Hund's angular momentum coupling cases (a) and (b) and including these limiting cases. The theory is validated by comparison with an experimental room temperature spectrum of the C(2)Pi (upsilon' = 0) <-- X(2)Pi (upsilon " = 0) two-photon band of NO near 382 nm. The absence of certain Lambda-doublet resolved rotational branches is shown to arise from interfering virtual absorption paths, which causes interference between components of the two-photon absorption tensor. Previous listings of two-photon rotational line strength factors are not correct for transitions between Pi states, since they cannot describe such interference effects.