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Physical Review A

Electric dipole moments of H(n=2) induced in H+ -He and H-He collisions

R. Hippler, O. Plotzke, Wolfgang Harbich, H. Madeheim, Hans Kleinpoppen, Hans O. Lutz

Published Mar 1, 1991• 17 citations
Publisher

Abstract

Electric dipole moments of H(n=2) induced in ${\mathrm{H}}^{+\mathrm{\ensuremath{-}}}$He and H-He collisions are investigated at incident energies of 5--25 keV. For these collision systems, an energy-dependent forward-backward asymmetry (dipole moment) of the emitted Lyman-\ensuremath{\alpha} radiation as a function of an external electric field is observed. In ${\mathrm{H}}^{+\mathrm{\ensuremath{-}}}$He collisions, the measured dipole moment is positive, corresponding to an electron trailing behind the proton. The same analysis applied to the H-He system shows the electron riding in front of the proton. The observed differences between H and ${\mathrm{H}}^{+}$ impact are partly explained with the help of a simple model which assumes different cross sections for excitation into the different Stark states.

DOI
10.1103/physreva.43.2587

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Physical Review A•Jul 1, 1983•18 citations
Electric dipole moments of H(n=2) induced in H+\ensuremath−{\mathrm{H}}^{+\mathrm{\ensuremath{-}}}H+\ensuremath−He and H-He collisions are investigated at incident energies of 5--25 keV. For these collision systems, an energy-dependent forward-backward asymmetry (dipole moment) of the emitted Lyman-\ensuremath{\alpha} radiation as a function of an external electric field is observed. In H+\ensuremath−{\mathrm{H}}^{+\mathrm{\ensuremath{-}}}H+\ensuremath−He collisions, the measured dipole moment is positive, corresponding to an electron trailing behind the proton. The same analysis applied to the H-He system shows the electron riding in front of the proton. The observed differences between H and H+{\mathrm{H}}^{+}H+ impact are partly explained with the help of a simple model which assumes different cross sections for excitation into the different Stark states.
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.
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.
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.
Four signals characterizing the Balmer-alpha radiation resulting from 40-80-keV collisions of H+{\mathrm{H}}^{+}H+ on He were measured as a function of an applied transverse electric field. Detailed analysis allowed the axially symmetric density matrix of the H(n=3)\mathrm{H} (n=3)H(n=3) atom formed in the collision to be obtained. The imaginary parts of the off-diagonal density-matrix elements are presented as the distribution of the electronic current density in the atom. One of the first-order moments ${〈\stackrel{\ensuremath{\rightarrow}}{\mathrm{A}}\ifmmode\times\else\texttimes\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{L}}〉}_{z}^{s}$ of the current distribution is given.
The theory of collisions of Rydberg atoms with rare-gas atoms is generalized to include the effects of an applied static electric field. Two effects occur. First, the field causes the states to split, and the increased energy separations lead to a decrease in the inelastic cross sections. Second, there is a change in the form of the wave functions, because the various Stark states in the field have electron charge distributions that may be localized in very different regions of space. The present work focuses on this second effect. With the description of the initial and final Stark states of the Rydberg atom by the proper electronic wave functions obtained in parabolic coordinates, cross sections are obtained for a variety of specific transitions of Rydberg atoms (n=10,15,and20n=10, 15, \mathrm{and} 20n=10,15,and20) caused by collisions. The calculations are strictly valid for hydrogen, but exhibit general features that are expected to apply to other Rydberg atoms as well. The results show a dramatic effect on the cross section for particular transitions depending on the degree of overlap of the initial and final charge distributions. In addition, it is observed that transitions are favored that involve small changes in the azimuthal quantum number mmm. The origin of this tendency is discussed.