Line shape of a transition between two levels in a three-level Λ configuration

We report on our study of the line shape of a transition between two levels in a three-level Λ configuration. By using Poisson statistics under the assumption that the atom stays in a two-level steady state before it is optically pumped to the reservoir state, we arrive at a simple analytic expression for the line shape of a three-level atom. This expression reveals a new type of saturation in the time domain, which is conceptually different from that of power-broadening in a two-level atom. It can also be used as a basis for more complicated situations of Doppler-broadened gaseous samples or pump-and-probe spectroscopy. We tested the theory experimentally in an ideal situation of slow pulsed 85Rb atoms and found excellent agreement. Application to measurements of a branching ratio or a Franck-Condon factor of a diatomic molecule is discussed.