Interference in two-photon rotational line strengths of diatomic molecules

A general formalism describing two-photon absorption of an isotropic ensemb le of diatomic molecules is presented, and practical simplifications are di scussed. Explicit rotational line strength factors are given for transition s between electronic doublet states, which are intermediate between Hund's angular momentum coupling cases (a) and (b) and including these limiting ca ses. The theory is validated by comparison with an experimental room temper ature 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 resolv ed rotational branches is shown to arise from interfering virtual absorptio n paths, which causes interference between components of the two-photon abs orption tensor. Previous listings of two-photon rotational line strength fa ctors are not correct for transitions between Pi states, since they cannot describe such interference effects.