Coherent Excitation of a He* Beam Observed in Atomic Momentum Distributions

We report on an experimental investigation of coherent optical excitation processes for an atomic two-level system in metastable helium. A simple absolute measure of the excitation probability is obtained from the deflection of an atomic beam, as excited atoms receive an additional momentum corresponding to a single photon recoil. With this method, we investigate the excitation process as a function of laser power, wavefront curvature and interaction time. Rabi oscillations are seen in the center-of-mass motion of an atomic ensemble for small wavefront curvatures, which change to adiabatic transfer produced by a Doppler detuning chirp for large wavefront curvatures. The adiabatic transfer provides a simple method for complete excitation of an atomic two-level system.