Measurements of the hydrogen 2s-2p coherence using the quantum-beat technique

We have measured Lyman- quantum beats from coherently excited hydrogen atoms produced in 85235-keV proton collisions with a thin helium jet target. Intensity measurements of the radiation emitted at 90°to the beam direction as a function of distance downstream from the target in electric fields both parallel and antiparallel to the beam axis allow extraction of the coherence between the s and p states by Ecks method [Phys. Rev. Lett. 31, 270 (1973)]. Because the jet target allows the collisions to occur in the applied field, we have eliminated the problems associated with field inhomogeneities when gas target cells are used. A strong-intensity peak emitted by excited helium in the target allows a determination of _the starting point of the evolution in the field as well as a determination of the effects of experimental averaging. We present the s-p coherence parameters as a function of the projectile energy and compare them with recent theoretical calculations.