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Physical Review

Periodic Intensity Fluctuations of Balmer Lines from Single-Foil Excited Fast Hydrogen Atoms

Ivan A. Sellin, Charles D. Moak, P. M. Griffin, John A. Biggerstaff

Published Aug 5, 1969• 34 citations
Publisher

Abstract

Protons with velocities ranging from 3.2 \ifmmode\times\else\texttimes\fi{} ${10}^{8}$ to 6.3 \ifmmode\times\else\texttimes\fi{} ${10}^{8}$ cm/sec were passed through a carbon foil producing excited hydrogen atoms which decayed in flight after emergence. With the foil in magnetic fields from 2 to 25 G, the lines ${\mathrm{H}}_{\ensuremath{\alpha}}$ through ${\mathrm{H}}_{\ensuremath{\epsilon}}$ showed periodic intensity fluctuations due to Stark mixings of fine-structure levels caused by motional electric fields (6-158 V/cm). An extension of the Bethe-Lamb theory of Stark quenching provides good agreement between a small number of theoretically dominant frequencies and the measured fluctuation frequencies. Apparent discrepancies between theory and experiment noted by other experimenters are shown to be removed by our calculations except in one measurement on ${\mathrm{Ly}}_{\ensuremath{\alpha}}$. The necessity of using double foils postulated in the previous work is discussed. Our analysis suggests the preferential alignment of the final-state angular momenta perpendicular to the beam direction. Reasons for this alignment are given.

DOI
10.1103/physrev.184.56

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Protons with velocities ranging from 3.2 \ifmmode\times\else\texttimes\fi{} 108{10}^{8}108 to 6.3 \ifmmode\times\else\texttimes\fi{} 108{10}^{8}108 cm/sec were passed through a carbon foil producing excited hydrogen atoms which decayed in flight after emergence. With the foil in magnetic fields from 2 to 25 G, the lines H\ensuremathα{\mathrm{H}}_{\ensuremath{\alpha}}H\ensuremathα​ through H\ensuremathϵ{\mathrm{H}}_{\ensuremath{\epsilon}}H\ensuremathϵ​ showed periodic intensity fluctuations due to Stark mixings of fine-structure levels caused by motional electric fields (6-158 V/cm). An extension of the Bethe-Lamb theory of Stark quenching provides good agreement between a small number of theoretically dominant frequencies and the measured fluctuation frequencies. Apparent discrepancies between theory and experiment noted by other experimenters are shown to be removed by our calculations except in one measurement on Ly\ensuremathα{\mathrm{Ly}}_{\ensuremath{\alpha}}Ly\ensuremathα​. The necessity of using double foils postulated in the previous work is discussed. Our analysis suggests the preferential alignment of the final-state angular momenta perpendicular to the beam direction. Reasons for this alignment are given.
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.
Passage of a beam of foil-excited He+{\mathrm{He}}^{+}He+ ions through a small electric field induced quasiperiodic fluctuations in the intensity of the light emitted from levels n=7,8,9,and10n=7, 8, 9, \mathrm{and} 10n=7,8,9,and10. Both He3{\mathrm{He}}^{3}He3 and He4{\mathrm{He}}^{4}He4 beams were used. Only one foil was needed, in contrast to the double-foil arrangement reported necessary for hydrogen. The observations are discussed in terms of Stark-induced interference of the fine-structure levels of He ii, but discrepancies between the data and the calculations are large.
A more extensive analysis of the effects of different assumptions about initial state populations is added to a previous paper on the coherent excitation of hydrogen fine structure by foils. It is found that the previously postulated alignment of angular momenta perpendicular to the beam direction is a sufficient but not necessary assumption; an alternative assumption that the broad lll-state distribution of the collision products has a maximum for ddd states also matches the experimental traces adequately. A method of obtaining initial state populations in a manner which suppresses the corrections required for cascade effects is discussed. The method avoids the errors of extrapolating decay-curve fits.
Within the limits set by our experimental apparatus, large-amplitude zero-field oscillations of H fine-structure amplitudes treated in a recent letter of Macek were not observed in polarization---versus---flight-path measurements on ${\mathrm{H}}_{\ensuremath{\beta}}$, ${\mathrm{H}}_{\ensuremath{\gamma}}$, and ${\mathrm{Ly}}_{\ensuremath{\alpha}}$ emissions from suddenly excited H atoms (50- and 150-keV incident protons). Random initial phases for orbital and magnetic substates and approximately equal magnetic substate populations are indicated, in contrast to binary electron capture in gases. A successful test of the theoretical polarization of 2s2s2s Stark quench radiation was also made.
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.
General equations are presented for determining the field-dependent variation of the wavefunctions, matrix elements and intensities of dipole transitions between Stark levels of a hydrogen atom in a field range where the fine-structure effects may be neglected. The Sturm-series resolution for the reduced Coulomb-Green function is used to derive analytical expressions for the first-order corrections in terms of parabolic quantum numbers of initial and final states. Simple forms of these expressions as functions of the upper-state quantum numbers are presented explicitly for the Lyman and Balmer series. The selective action of the dc electric field on the intensity of Stark components of hydrogen lines is revealed in numerical calculations for both and types of emitted radiation. This selectivity may cause, in particular, enhancement of the high-frequency and damping of the low-frequency Stark -lines of the Lyman series.