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1 An expression is given for the Stark-induced beat signal in the Lyman-o; radiation from hydrogen atoms in a beam-foil experiment. This expression includes possible contributions to the beat signal from coherent excitation of S and P states in the beam-foil process. For the case where the electric field is applied parallel to the beam, the coherentexcitation contributions can be separated experimentally from those resulting purely from coherence produced by the electric field, by simply reversing the direction of the electric field.The recently developed beam-foil spectroscopic technique 1 has been used to investigate excited states of many atoms and ions. Among the most interesting results of these experiments have been the observations of periodic spatial fluctuations, or "beats/' in the intensity of the radiation emitted by light atoms and ions (H, He, Li + ) subsequent to their exit from the exciting foil. Since all the particles in the beam have nearly the same velocity, these spatial beats are equivalent to beats in time measured from the instant of excitation at the foil.Macek 2,3 has explained these temporal beats as the result of coherent excitation of neighboring fine-structure levels. With the beam direction as the axis of quantization, the symmetry of the experiment about this direction restricts the possibility of coherence to states of the same m ; . Further, the validity of Russell-Saunders coupling for these light atoms requires that only states of the same m L and m s can be coherently excited. Thus, beats between L Jttnj states with the same L and nij but different J (such as P 3/2 ,i/2 and Pi/g, i/ 2 ) ma Y more properly be thought of as the result of coherence induced by the fine-structure interaction (proportional to L • §) rather than the excitation process itself. 4 This is implicit in Macek's analysis 3 and has been explicitly stressed by Sellin. 5,6 Macek 3 has pointed out the possibility of beats between states with the same m ; but different L (such as S 1/2tl/2 beating with D 3/2tl/2 or 2> 5/2il/2 ). Such beats, which cannot be ruled out by any symmetry argument, require true coherence in the excitation process in the sense that the initial relative phases of these states averaged over successive atoms in the beam must not be totally random. There has been only one published report of the observation of such "true" coherence beats, that by Burns and Hancock. 7 In their beam-foil investigation of the Balmer-jS line of hydrogen, S-D beat signals appeared to be present along with fine-structure-induced beats between the P 3/2 and P 1/2 states and between the D 5/2 and D 3/2 states. Extraction of the amplitudes of the component signals required computer fitting of a rather complicated theoretical expression to the observed beat pattern. The purpose of the present Letter is to discuss coherent excitation contributions to the Stark-induced beat signal in the Lyman -a line of hydrogen. We shall show how these contributions can be separated experimentally from those resulting purely from cohe...

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