George A. Hagedorn, Vidian Rousse, Steven W. Jilcott The AC Stark Effect, Time-Dependent Born-Oppenheimer Approximation, and Franck-Condon Factors (68K, latex plus 3 files for a figure) ABSTRACT. We study the quantum mechanics of a simple molecular system that is subject to a laser pulse. We model the laser pulse by a classical oscillatory electric field, and we employ the Born--Oppenheimer approximation for the molecule. We compute transition amplitudes to leading order in the laser strength. These amplitudes contain Franck--Condon factors that we compute explicitly to leading order in the Born--Oppenheimer parameter. We also correct an erroneous calculation in the mathematical literature on the AC Stark effect for molecular systems.