Dissociative Recombination in Helium Afterglows

Ferguson, E. E.; Fehsenfeld, F. C.; Schmeltekopf, A. L.

doi:10.1103/physrev.138.a381

Cited by 68 publications

(18 citation statements)

References 16 publications

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“…Strong evidence for this assumption is provided by recent studies of the helium afterglow. 6 We shall see, furthermore, that the analysis then results in an agreement with the theory of collisional-radiative recombination so that an internal consistency is achieved. Mass spectrometer measurements in the afterglow would allow a more definitive discussion than the above.…”

Section: Analysis Of the He+ Ion Recombinationsupporting

confidence: 55%

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Recombination ofHe+andHe++in the Afterglow of

Mosburg

1966

Phys. Rev.

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Studies of the time dependence of atomic and molecular light intensities, electron density, atomic metastable densities, and electron temperature have allowed the determination of the He + -electron and He ++electron recombination coefficients as functions of electron density and temperature. The results are in reasonable agreement with the theory of collisional-radiative recombination. The mechanisms controlling metastable densities and heating of the electron gas are discussed. In particular, the disappearance of the 2 3 5 metastable atom seems best explained in terms of collisional de-excitation by electrons.TN earlier experiments, 1 using pulsed microwave A cavities or waveguides, the observed time dependency of the electron density n was used to ascertain the degree to which the loss processes were dominated by electron-ion volume recombination and, simultaneously, to determine the value of the recombination coefficient itself from a plot of \/n{t) versus the time t. It has since been shown 2 that, in the presence of diffusion, linearity of a l/»(/)-versus-$ plot is neither a guarantee of recombination control nor a good measure of the recombination coefficient, unless the region of linearity is at least a factor of approximately 10 in the electron density. Even in cases where this criterion is met, there are other uncertainties. In general, the radial distribution of electron density was surmised rather than measured and was probably time-dependent. This introduces uncertainty into the interpretation of the frequency shift of the cavity in terms of the spacial average of the electron density. There are, moreover, serious limitations to the electron density region over which the cavity technique can be applied. 3 Purity of the gas was an additional problem and, for a while, the measured recombination coefficients became progressively lower as the purity was improved. In all of these early experiments, a dependence of the assumed two-body recombination coefficients on electron density was neither allowed for nor found experimentally. For helium, the theoretical picture was further obscured by the assumption that electron loss proceeded primarily by dissociative recombination of the He 2 + ion, 4 a belief that has persisted until very recently.

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Section: Analysis Of the He+ Ion Recombinationsupporting

confidence: 55%

“…6 Other recent work 7 -8 is in reasonable agreement 9 with the theory of collisional-radiative recombination, but further experimental confirmation is clearly needed.…”

Section: Introductionmentioning

confidence: 56%

Recombination ofHe+andHe++in the Afterglow of

Mosburg

1966

Phys. Rev.

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“…Then the lifetime of excited atoms τ is equal by their quenching on the cathode owing to their diffusion τ = l 2 2D * , N * = N e N a k ex l 2 2D * , (13.1.16) where D * is the diffusion coefficient of excited atoms, and we consider the depth of the cathode layer l to be small compared with a current radius that corresponds to the normal regime of glow discharge. The reduced diffusion coefficient for metastable helium atoms He(2 3 S) in helium is D * N a = (1.5 ± 0.1) × 10 19 cm −1 s −1 according to experimental data [306][307][308][309][310][311][312][313][314][315][316], for excited argon atoms in states 3 P 2 and 3 P o the reduced diffusion coefficient in argon is D * N a = (1.9 ± 0.3) × 10 18 cm −1 s −1 according to measurements [321][322][323][324][325][326], and the above data corresponds to the statistical average of indicated measurements. In addition, as it is obtained in §6.6, the rate constant of ionization of metastable helium atoms He(2 3 S) by electron impact in strong electric fields is k * ion (He) = 2.5 × 10 −7 cm 3 /s, and the rate constant of ionization of metastable argon atoms Ar (3 P 2 ) by electron impact in strong electric fields is k * ion (Ar ) = 3.6 × 10 −7 cm 3 /s.…”

Section: Capillary Dischargementioning

confidence: 65%

“…The diffusion coefficient D of excited atoms in a gas is inversely proportional to the number density N a of gas atoms and usually is reduced to the normal number density of atoms N a = 2.69×10 19 cm −3 . The diffusion coefficients of excited helium atoms and molecules in helium averaged over measurements [167,[305][306][307][308][309][310][311][312][313][314][315][316][317][318][319] is [194] 0.59 cm 2 /s for a metastable atom He(2 3 S) in helium, 0.52 cm 2 /s for a metastable atom He(2 1 S) in helium, and 0.45 cm 2 /s for a metastable molecule He(2 3 u ) in helium. The diffusion coefficient of metastable argon atoms Ar ( 3 P 2 ), Ar ( 3 P 0 ) in argon at room temperature averaged over measurements [317][318][319][320][321][322][323][324][325] is [194] 0.071 cm 2 /s.…”

Section: Transport Phenomena In Gasesmentioning

confidence: 99%

“…We assume that molecular ions He + 2 are located in ionized helium, the temperature of atoms and ions is room one, and take the diffusion coefficient of metastable atoms in helium to be D m ≈ 0.59 cm 2 /s at the normal number density of atoms, and from the sum of experiments [167,[304][305][306][307][308][309][310][311][312][313][314][315]317] we have D m N a = (1.6 ± 0.1) × 10 19 cm −1 s −1 . For the diffusion coefficient of molecular ions He + 2 in helium we have D i ≈ 0.46 cm 2 /s [41,190] at the normal number density of atoms.…”

Section: Stepwise Ionization In Helium Positive Columnmentioning

confidence: 99%

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Theory of Gas Discharge Plasma

Smirnov

2015

Springer Series on Atomic, Optical, and Plasma Physics

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The Springer Series on Atomic, Optical, and Plasma Physics covers in a comprehensive manner theory and experiment in the entire field of atoms and molecules and their interaction with electromagnetic radiation. Books in the series provide a rich source of new ideas and techniques with wide applications in fields such as chemistry, materials science, astrophysics, surface science, plasma technology, advanced optics, aeronomy, and engineering. Laser physics is a particular connecting theme that has provided much of the continuing impetus for new developments in the field, such as quantum computation and Bose-Einstein condensation. The purpose of the series is to cover the gap between standard undergraduate textbooks and the research literature with emphasis on the fundamental ideas, methods, techniques, and results in the field.More information about this series at

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