Measurement of S-P coherence in the beam-foil-excited n = 2 state of atomic hydrogen

The intensity of beam-foil-excited Ly-α radiation has been measured in electric fields alternately parallel and antiparallel to the beam. Measurements were made at three energies---110, 165, and 210 keV---and three field strengths. From these data the S-P co-herence term ${\sigma }_{SP}$ has been determined. The magnitude of ${\sigma }_{SP}$ is found to be 0.07 at 110 keV, 0.19 at 160 keV, and 0.22 at 210 keV. These results establish that beam-foil-excited hydrogen atoms have a nonzero electric dipole moment.