Measurement ofS−PCoherence in the Beam-Foil—Excitedn=2State of Atomic Hydrogen

The intensity of beam-foil-excited $\mathrm{Ly}-\alpha$ 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 ${\sigma }_{\mathrm{SP}}$ has been determined. The magnitude of ${\sigma }_{\mathrm{SP}}$ 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.