Coherence in He I and II Excited States Produced by an Asymmetric Collision

In an external magnetic field, quantum beats at the Larmor frequency $\omega$ are observed in circularly polarized light as a result of coherent orientation of fast He i and ii states by a thin foil with exit-surface normal not coincident with the beam direction. Effects resulting from variation of detection geometry, quantum state, incident ion energy, foil tilt angle, and foil material are discussed. An interpretation in terms of an impulsive asymmetric electric interaction is considered.