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Physical Review Letters

Electric dipole moments of impact-excited He atoms

A. S. Aynacioglu, S. Heumann, Gebhard von Oppen

Published Apr 16, 1990• 21 citations
Publisher

Abstract

We excited helium atoms using 10--20-keV ${\mathrm{H}}_{2}^{+}$ (${\mathrm{D}}_{2}^{+}$) ion impact and investigated the intensity of the \ensuremath{\lambda}(${4}^{3}$D-${2}^{3}$P)=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.

DOI
10.1103/physrevlett.64.1879

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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.
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 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.
In a measurement of emission of Balmer-$\ensuremath{\alpha}$ flux from atomic hydrogen or deuterium excited by electron impact in a crossed-beam experiment, a longitudinal electric field which enhances the Balmer-$\ensuremath{\alpha}$ flux by Stark-mixing components of like MJ{M}_{J}MJ​ within the n=3n=3n=3 shell, does so asymmetrically depending on whether the field is applied parallel or antiparallel to the electron beam. The effect is attributed to coherent excitation of states of opposite parity.
The prediction of the energy level shifts and intensity transfer in atomic hydrogen and helium spectra caused by an external electric field was one of the first applications of wave mechanics. Early measurements relied on electrical discharge sources for excited atoms; since discharge processes will not produce excited states with n > 7 in observable densities, very large (10s to 100s kV cm) electric fields were required to resolve spectral modifications. The development of high peak power lasers enabled excitation to high n (Rydberg) states, increasing field sensitivity down to millivolts/cm and allowing comparison of experiment and theory in well-defined conditions. Field modified selection rules can be used to measure the field direction as well as magnitude. Analysis of line shape simulations of Stark modified Rydberg spectra reveals pitfalls in commonly used field measurement techniques. An ambiguity in defining series termination results in a factor of 2 discrepancy in field measurements based on Inglis Teller line merging. Field-induced line broadening (unresolved manifold splitting) in triplet and singlet helium are shown to differ significantly, with triplet broadening nonlinear with field.
Dipole-dipole interactions between helium atoms in Rydberg-Stark states with principal quantum number n=53n=53n=53 and approximately linear Stark energy shifts, resulting from induced electric dipole moments of approximately 7900 D, have been investigated experimentally. The experiments were performed in pulsed supersonic metastable helium beams, with particle number densities of up to $\ensuremath{\sim}{10}^{9}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. In the presence of amplitude-modulated, radio-frequency electric fields, changes in the spectral intensity distributions associated with the transitions to these states that are attributed to dipole-dipole interactions within the ensembles of excited atoms have been observed. The experimental results are in excellent agreement with calculations of the Rydberg energy level structure carried out using Floquet methods, and excitations shared by up to four atoms. The use of these Rydberg-Stark states as sensors for nonresonant broadband radio-frequency electrical noise is also discussed.
A joint experimental and theoretical study of three-photon ionization of the 1s2p3Po1s2p{\phantom{\rule{0.16em}{0ex}}}^{3}{P}^{o}1s2p3Po(${M}_{L}=0,\ifmmode\pm\else\textpm\fi{}1$) states of helium is presented. The ion yield is recorded in the 690--730 nm wavelength range for different laser pulse energies, using an excited helium beam produced by photodetachment of helium negative ions. Two series of asymmetric peaks due to two-photon resonances with 1snp1snp1snp and 1snf1snf1snf Rydberg states are observed. In one series, the peaks have tails towards higher frequencies, while in the other series the tails change direction for higher Rydberg states. An effective Hamiltonian is built in the dressed state picture, and a numerical model simulating the traversal of the helium atom across the laser pulse is developed. The simulated and observed ion yields are in good qualitative agreement. The observed behavior is shown to result from the contributions of two different resonantly enhanced multiphoton ionization processes, depending on the magnetic quantum number ML{M}_{L}ML​ of the initial state. The asymmetry reversal is explained by the strong 1s2p1s2p1s2p--1s3s1s3s1s3s dynamic Stark mixing for ML=0{M}_{L}=0ML​=0.