Atomic quasi-Bragg-diffraction in a magnetic field

We report on a technique to split an at. beam coherently with an easily adjustable splitting angle. In our expt. metastable helium atoms in the |{1s2s}3S1 M=1 state diffract from a polarization gradient light field formed by counterpropagating .sigma.+ and .sigma.- polarized laser beams in the presence of a homogeneous magnetic field. In the near-adiabatic regime, energy conservation allows the resonant exchange between magnetic energy and kinetic energy. As a consequence, sym. diffraction of |M=0 or |M=-1 atoms in a single order is achieved, where the order can be chosen freely by tuning the magnetic field. We present exptl. results up to sixth-order diffraction (24[planck]k momentum splitting, i.e., 2.21 m/s in transverse velocity) and present a simple theor. model that stresses the similarity with conventional Bragg scattering. The resulting device constitutes a flexible, adjustable, large-angle, three-way coherent at. beam splitter with many potential applications in atom optics and atom interferometry.