Optical Stark Effect in the Two-Photon Spectrum of NO

A large optical Stark effect has been observed in the two-photon spectrum $X{}_{}{}^2{}_{}{}^{}\Pi \to A{}_{}{}^2{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}$ in nitric oxide. It is explained as a near-resonant process in which the upper state of the two-photon transition is perturbed by interactions with higher-lying electronic states coupled by the laser field. A theoretical analysis is presented along with Stark parameters determined from ab initio wave functions. The synthetic spectrum reproduces the major experimental features.