COHERENT EXCITATION OF A HE-ASTERISK BEAM OBSERVED IN ATOMIC MOMENTUMDISTRIBUTIONS

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