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Physical Review Letters

Coherent Excitation of S and P States of the n=2 Term of Atomic Hydrogen

T. G. Eck

Published Jul 30, 1973• 76 citations
Publisher

Abstract

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 $S$ and $P$ 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.

DOI
10.1103/physrevlett.31.270

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Physical Review Letters•Mar 28, 1983•17 citations
An expression is given for the Stark-induced beat signal in the Lyman-\ensuremathα\ensuremath{\alpha}\ensuremathα 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.
A beam–foil experiment is proposed for observation of a phase shift π in a beat intensity of light emitted by hydrogen and antihydrogen atoms in an external electric field. This effect is based on the existence of the terms linear in the electric field in the expression for the radiation probability of an atom. The linear terms have an opposite sign for hydrogen and antihydrogen atoms and lead to such a difference in a beat signal. Experimental conditions for observation of this effect are discussed.
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.
Intensity beats in the emission of $\mathrm{L}{\mathrm{y}}_{\ensuremath{\alpha}}$ from beam-foil excited hydrogen in various static electromagnetic field configurations have been studied. The observed beat frequencies agree with the calculated values, which are independent of the field directions. The beat amplitudes, however, show significant dependence on the field directions relative to the beam. The theory for emission of light from the decay of coherently excited states is applied in the sudden perturbation limit. Agreement with experiment can only be obtained under the assumption that the foil excitation generates initial coherent wave functions ${\ensuremath{\Psi}}_{\mathrm{I},\mathrm{I}\mathrm{I}}=\ensuremath{\alpha}|p,\frac{1}{2},\ifmmode\pm\else\textpm\fi{}\frac{1}{2}〉+\ensuremath{\beta}|s,\frac{1}{2},\ensuremath{\mp}\frac{1}{2}〉$, where ∣l,j,mj〉|l,j,{m}_{j}〉∣l,j,mj​〉 notation and the beam as quantization axis are used. The ratio $\frac{\ensuremath{\alpha}}{\ensuremath{\beta}}$ is then determined, which gives the ratio of the excitation cross section of the ppp to that of the sss states as $\frac{{\ensuremath{\sigma}}_{p}}{{\ensuremath{\sigma}}_{s}}>~4\ifmmode\pm\else\textpm\fi{}1$ at a beam energy of 200 (keV) per atom.
We have measured Lyman-\ensuremath{\alpha} quantum beats from coherently excited hydrogen atoms produced in 85--235-keV proton collisions with a thin helium jet target. Intensity measurements of the radiation emitted at 90\ifmmode^\circ\else\textdegree\fi{} to the beam direction as a function of distance downstream from the target in electric fields both parallel and antiparallel to the beam axis allow extraction of the coherence between the s and p states by Eck's method [Phys. Rev. Lett. 31, 270 (1973)]. Because the jet target allows the collisions to occur in the applied field, we have eliminated the problems associated with field inhomogeneities when gas target cells are used. A strong-intensity peak emitted by excited helium in the target allows a determination of the starting point of the evolution in the field as well as a determination of the effects of experimental averaging. We present the s-p coherence parameters as a function of the projectile energy and compare them with recent theoretical calculations.
Zero-field quantum beats were observed in $\mathrm{L}\mathrm{y}\ensuremath{-}\ensuremath{\beta}$ emission (1026 \AA{}) from hydrogen atoms formed by beam-foil excitation, and the energy dependence of the magnetic-substate cross sections for n=3n=3n=3, l=1l=1l=1 states was determined for beam energies in the range from 230 to 500 keV/atom.
Hydrogen atoms are excited into high Rydberg states following fast proton impact on a thin carbon foil. The effects of various electromagnetic fields on these atoms are observed by measuring the integral ionization yield in an additional constant electric field. These experiments reveal oscillations in that yield, regularly spaced as a function of electric field. A model is outlined that quantitatively describes the observed field dependence. This new phenomenon indicates selective excitation of the Stark substates.