Optically induced spin precession and echo in an atomic beam

We report on spin precession experiments using a ⁷Li atomic beam, which offers a perturbation-free environment to investigate spin dynamics under the influence of external fields. The ground-state magnetic moments perform a Larmor precession in a ``fictitious magnetic field'' induced by an off-resonant, circularly polarized laser beam. For a fictitious magnetic field parallel to a real one, we observe additional rotation of the spins, whereas an echo is observed if both fields are perpendicular with respect to each other. The systematics of these phenomena are studied.