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Physical Review Letters

Measurement of S−P Coherence in the Beam-Foil—Excited n=2 State of Atomic Hydrogen

Andreas Gaupp, H. J. Andrä, J. H. Macek

Published Feb 11, 1974• 69 citations
Publisher

Abstract

The intensity of beam-foil-excited $\mathrm{Ly}\ensuremath{-}\ensuremath{\alpha}$ 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 $S\ensuremath{-}P$ co-herence term ${\ensuremath{\sigma}}_{\mathrm{SP}}$ has been determined. The magnitude of ${\ensuremath{\sigma}}_{\mathrm{SP}}$ 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.

DOI
10.1103/physrevlett.32.268

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The intensity of beam-foil-excited Ly\ensuremath−\ensuremathα\mathrm{Ly}\ensuremath{-}\ensuremath{\alpha}Ly\ensuremath−\ensuremathα 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\ensuremath−PS\ensuremath{-}PS\ensuremath−P co-herence term \ensuremathσSP{\ensuremath{\sigma}}_{\mathrm{SP}}\ensuremathσSP​ has been determined. The magnitude of \ensuremathσSP{\ensuremath{\sigma}}_{\mathrm{SP}}\ensuremathσSP​ 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.
An expression is given for the Stark-induced beat signal in the Lyman-$\ensuremath{\alpha}$ radiation from hydrogen atoms in a beam-foil experiment. This expression includes possible contributions to the beat signal from coherent excitation of SSS and PPP 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.
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We have developed an experimental method for determining the initial density matrix of the foil-excited atoms. The excited hydrogen (n=2)(n=2)(n=2) atoms pass through a region of a variable electric field ($\ensuremath{-}200\mathrm{k}\mathrm{V}/\mathrm{m}$ to +200kV/m+200\mathrm{k}\mathrm{V}/\mathrm{m}+200kV/m), and the intensity of the Lyman-\ensuremath{\alpha} line (121.6 nm) emitted in a free space downstream is observed. By using a vacuum ultraviolet polarizer, the dependence of the intensities of the polarized components on the electric field, including the field-dependent quantum beats, was measured and it was numerically fitted to a calculation. Thus all the initial density matrix elements were determined in a single measurement. It was found that, in the energy range 100--180 keV, just after the excitation, the electron cloud has higher density in the front side of the ion and the electron probability current is backwards.
We excited helium atoms using 10--20-keV H2+{\mathrm{H}}_{2}^{+}H2+​ (D2+{\mathrm{D}}_{2}^{+}D2+​) ion impact and investigated the intensity of the \ensuremath{\lambda}(43{4}^{3}43D-23{2}^{3}23P)=447 nm line as a function of an electric field -2 kV/cm\ensuremath{\lesssim}F\ensuremath{\le}+2 kV/cm applied antiparallel and parallel to the ion beam. The recorded signals were strongly asymmetric with respect to the sign of the electric field. This asymmetry is explained by assuming a coherent excitation of He levels with different angular momenta (4d,4f and 5f,5g). These measurements demonstrate that angular momentum coherences of impact-excited states can also be observed in nonhydrogenic systems.
This paper presents measurements of the Lyman-$\ensuremath{\alpha}$ photons emitted when target gases He, Ar, O2{\mathrm{O}}_{2}O2​, and N2{\mathrm{N}}_{2}N2​ are bombarded by a beam of ground-state hydrogen atoms. The collisions take place in the presence of an axial electric field and the observed Lyman-$\ensuremath{\alpha}$ brightness depends on field direction. This observation is interpreted as evidence for excitation of hydrogen atoms to states which are linear superpositions of 2s2s2s and 2p2p2p. Measurements are presented of a "mixed" cross section ${\ensuremath{\sigma}}_{\mathrm{sp}}$ which characterizes the degree of coherent excitation. The real part of ${\ensuremath{\sigma}}_{\mathrm{sp}}$ measures the electric dipole moment created by the collision, while the imaginary part measures the rate at which the moment is changing. It is found that these quantities vary substantially over the energy range 3-30 keV, with maximal values being about as large as permitted theoretically. Coherent excitation should be present to some degree in all inelastic collisions and, when measurable, provides new tests of scattering theory. Some implications are also discussed for precision frequency measurements and searches for weak-interaction effects in atomic physics.
Four experiments involving individual and combined effects of electric fields and multiple exciter foils on beam-foil-excited hydrogen atoms are described.The use of multiple exciter foils shows that excitation equilibrium does not occur in the thin carbon foils used here. The change of the rate of decay of Ly-α flux emitted by beam-foil excited hydrogen atoms in an electric field has been used to determine the relative populations of the 2s and 2p levels at the foil. The 2s level is overpopulated with respect to the 2p level when one foil is used and two exciter foils enhance the overpopulations. Photoelectric measurements of Ly-α flux and photographic measurements of the decay of Balmer-δ flux emitted by atoms in an electric field show that quasiperiodic intensity variations exist on the decay curve. The variations are attributed to the effect of the electric field but discrepancies arise between the experimental frequencies and those calculated from Stark-effect theory.