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Physical Review

New Process of Excitation Transfer in Helium

Robert M. St. John, Richard Fowler

Published Jun 15, 1961• 72 citations
Publisher

Abstract

Precise observations have been made on light emission from low triplet levels of helium gas traversed by a monoenergetic electron beam, with varying helium pressure and electron energy. In the past it has been customary to assign this triplet excitation to direct transfer from $^{1}P$ states in flagrant violation of the Wigner rule and seemingly excessive cross sections for the transfer have been inferred.A new process of excitation transfer is proposed which minimizes the conflict with the Wigner rule by reducing the sizes of the cross sections required to values close to the gas-kinetic cross section. It is hypothesized that many $^{1}P$ states, including those with large quantum number $n$, transfer excitation energy to neighboring triplet states having closely corresponding principal quantum numbers. The triplet states thus formed in turn populate low-level triplet states by radiative transitions. It is found that states lying between $n=4$ and $n=15$ would play the dominant role in the transfer process.Satisfying qualitative explanations of several additional excitation-transfer-process phenomena are derived from this new multiple-state-transfer process.

DOI
10.1103/physrev.122.1813

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Inelastic collisions between selectively excited rubidium 6D2 -state atoms and noble-gas atoms: Fine-structure-state mixing

Byron George Zollars, Hans A. Schuessler, Joel Wm. Parker +1 more

Inelastic collisional processes in fine-structure-state mixing have been studied by selective stepwise excitation and observation of the resulting fluorescence during collisions of excited rubidium $6^{2}D_{\frac{5}{2}}$ state atoms with various noble-gas atoms. It is shown that for intermediately excited rubidium atoms the fine-structure-state mixing cross sections are generally increasing from helium to xexon. The cross sections for the process $\mathrm{Rb}(6^{2}D_{\frac{5}{2}})+A(n^{1}S_{0})\ensuremath{\rightarrow}\mathrm{Rb}(6^{2}D_{\frac{3}{2}})+A(n^{1}S_{0})+\ensuremath{\Delta}E$ have the values ${\ensuremath{\sigma}}_{\mathrm{fs}}^{*}=2.4\ifmmode\pm\else\textpm\fi{}0.9, 3.1\ifmmode\pm\else\textpm\fi{}1.2, 2.6\ifmmode\pm\else\textpm\fi{}1.0, 5.8\ifmmode\pm\else\textpm\fi{}2.2, \mathrm{and} 9.3\ifmmode\pm\else\textpm\fi{}5.6$ in units of ${10}^{\ensuremath{-}14}$ ${\mathrm{cm}}^{2}$, where $A$ denotes in sequence any one of the noble gas atoms He, Ne, Ar, Kr, and Xe, respectively. Transfer cross sections for the transfer of excited Rb$6^{2}D$-state atoms out of the doublet were also determined and are about an order of magnitude smaller than the fine-structure-mixing cross sections.

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Effect of the Imprisonment of Resonance Radiation on Excitation Experiments

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The theory of the imprisonment of resonance radiation developed by Holstein and by Bieberman is applied to the analysis of experimental measurements of excitation cross sections for the resonance ($n^{1}P$) states of helium. The transport equation for the density of atoms in the resonance state is solved numerically for the case of a thin sheet of exciting electrons between parallel plane electrodes. The fraction of the resonance atoms producing visible radiation is then calculated for parallel plane geometry and estimated to within five percent for cylindrical geometry with an axial electron beam. Predictions of the theory are compared with the available experimental data for helium. The theory shows that at the helium pressures commonly used (${10}^{\ensuremath{-}3}$ to ${10}^{\ensuremath{-}1}$ mm of Hg) the observed visible radiation may easily be a factor of ten greater than that expected when imprisonment effects are neglected. As a result, the cross sections for the excitation of the $n^{1}P$ states given in the literature are much too large. For example, our analysis of the available experimental data suggests that the cross section for excitation by electrons to the $3^{1}P$ state of helium at 100 electron volts is 3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}18}$ ${\mathrm{cm}}^{2}$ instead of 4\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}17}$ ${\mathrm{cm}}^{2}$ as given in the literature.

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Excitation-Transfer Collisions between Rubidium and Helium Atoms

T. J. Beahn, W. J. Condell, Hirsch I. Mandelberg

An experiment is described which measures the collision cross section for transfer of excitation in rubidium by helium. Rubidium atoms are pumped to either the $^{2}P_{\frac{1}{2}}$ or $^{2}P_{\frac{3}{2}}$ state by irradiation with one of the resonance lines. Some excited rubidium atoms are transferred to the other $P$ state by collisions with helium atoms. The rubidium atoms then radiate both resonance lines. Measurements of the intensities of both resonance lines as a function of helium pressure are used to calculate the collision cross section. The cross section for $^{2}P_{\frac{1}{2}}$ to $^{2}P_{\frac{3}{2}}$ is 1.0\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}17}$ ${\mathrm{cm}}^{2}$\ifmmode\pm\else\textpm\fi{}10% and for $^{2}P_{\frac{3}{2}}$ to $^{2}P_{\frac{1}{2}}$ is 1.2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}17}$ ${\mathrm{cm}}^{2}$\ifmmode\pm\else\textpm\fi{}10%.

Physical Review•Jan 7, 1966•28 citations
Precise observations have been made on light emission from low triplet levels of helium gas traversed by a monoenergetic electron beam, with varying helium pressure and electron energy. In the past it has been customary to assign this triplet excitation to direct transfer from 1P^{1}P1P states in flagrant violation of the Wigner rule and seemingly excessive cross sections for the transfer have been inferred.A new process of excitation transfer is proposed which minimizes the conflict with the Wigner rule by reducing the sizes of the cross sections required to values close to the gas-kinetic cross section. It is hypothesized that many 1P^{1}P1P states, including those with large quantum number nnn, transfer excitation energy to neighboring triplet states having closely corresponding principal quantum numbers. The triplet states thus formed in turn populate low-level triplet states by radiative transitions. It is found that states lying between n=4n=4n=4 and n=15n=15n=15 would play the dominant role in the transfer process.Satisfying qualitative explanations of several additional excitation-transfer-process phenomena are derived from this new multiple-state-transfer process.
We have measured the effect of the transfer of excitation on the population of the 33D3^{3}D33D state of helium. Through careful diagnosis of a proton beam passing through a long target chamber containing helium gas, we have been able to separate the various contributions to the light intensity from the 33D3^{3}D33D state. Our measurements were performed at target-gas pressures roughly two orders of magnitude lower than those of previously reported measurements. We measure the summation of the excitation-transfer cross sections ${\ensuremath{\sigma}}_{c}(n)$ for the transfer from an n1Pn^{1}Pn1P state to the corresponding n3Fn^{3}Fn3F state times the branching ratio Bn{B}_{\mathrm{n}}Bn​ for radiative cascade to the 33D3^{3}D33D state, and obtain $\ensuremath{\Sigma}{B}_{n}{\ensuremath{\sigma}}_{c}(n)=2.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ cm2{\mathrm{cm}}^{2}cm2. This measurement enables us to relate the excitation transferred to the 33D3^{3}D33D state QT(33D){Q}_{T} (3^{3}D)QT​(33D) to the excitation cross section of the 41P4^{1}P41P state $\ensuremath{\sigma}(4^{1}P)$. We obtain ${Q}_{T} (3^{3}D)=(1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15} {\mathrm{cm}}^{3})N\ensuremath{\sigma}(4^{1}P)$, an expression, which for the low pressures used in this experiment, is linear in the number density NNN.
The interaction of an intense optical-maser field with excited atoms in a gas discharge leads to a considerable change in the population of those levels which are resonant at the maser frequency. This will cause a change in the population of other levels that are connected to the maser levels through collision-induced nonradiative as well as radiative transitions. Using the 1.15-\ensuremath{\mu} optical maser, these effects have been studied in detail for the 2s2s2s levels of neon where the changes are completely due to collisions. By analyzing the intensity changes in spontaneous emission originating from a gas-discharge cell placed within the maser resonator, we have observed the approach to a partial thermalization of two closely spaced levels. In this way we have been able to obtain the atomic-collision cross section for excitation transfer between the 2s22{s}_{2}2s2​ and 2s32{s}_{3}2s3​ levels. In pure neon this has been found to be ${\ensuremath{\sigma}}_{23}=(2.3\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$ cm2{\mathrm{cm}}^{2}cm2. For a helium-neon mixture the measured cross section is ${\ensuremath{\sigma}}_{23}=(1.8\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$ cm2{\mathrm{cm}}^{2}cm2. The experimental method used to determine these cross sections eliminates the uncertainties caused by the effects of electron-atom collisions and radiative cascade. These techniques also yield the ratio of the Einstein coefficients for the transitions $2{s}_{2}\ensuremath{\rightarrow}2{p}_{10}$ and $2{s}_{3}\ensuremath{\rightarrow}2{p}_{10}$. The measured ratio is $\frac{{A}_{2}}{{A}_{3}}=3.0\ifmmode\pm\else\textpm\fi{}0.4$.
The excitation transfers through collision between hydrogen atoms and between other simple identical atoms are considered theoretically by an impact parameter method. A rotating atom approximation and a fixed atom approximation (concerned with the direction of electronic polarization) are applied to the problem. The cross-section for the transfer of the excitation (2 p 2 P ) of a hydrogen atom with thermal velocity (kinetic energy of 0.025 ev) to the other hydrogen atom in the ground state is estimated to be 3065 Å 2 with the rotating atom approximation and 680 Å 2 with the fixed atom approximation. In the case of helium atoms, the cross-section for the transfer of the excitation (2 p 1 P ) is also estimated to be 606.7 Å 2 with thermal velocity.
Inelastic collisional processes in fine-structure-state mixing have been studied by selective stepwise excitation and observation of the resulting fluorescence during collisions of excited rubidium 62D526^{2}D_{\frac{5}{2}}62D25​​ state atoms with various noble-gas atoms. It is shown that for intermediately excited rubidium atoms the fine-structure-state mixing cross sections are generally increasing from helium to xexon. The cross sections for the process $\mathrm{Rb}(6^{2}D_{\frac{5}{2}})+A(n^{1}S_{0})\ensuremath{\rightarrow}\mathrm{Rb}(6^{2}D_{\frac{3}{2}})+A(n^{1}S_{0})+\ensuremath{\Delta}E$ have the values ${\ensuremath{\sigma}}_{\mathrm{fs}}^{*}=2.4\ifmmode\pm\else\textpm\fi{}0.9, 3.1\ifmmode\pm\else\textpm\fi{}1.2, 2.6\ifmmode\pm\else\textpm\fi{}1.0, 5.8\ifmmode\pm\else\textpm\fi{}2.2, \mathrm{and} 9.3\ifmmode\pm\else\textpm\fi{}5.6$ in units of ${10}^{\ensuremath{-}14}$ cm2{\mathrm{cm}}^{2}cm2, where AAA denotes in sequence any one of the noble gas atoms He, Ne, Ar, Kr, and Xe, respectively. Transfer cross sections for the transfer of excited Rb62D6^{2}D62D-state atoms out of the doublet were also determined and are about an order of magnitude smaller than the fine-structure-mixing cross sections.
The theory of the imprisonment of resonance radiation developed by Holstein and by Bieberman is applied to the analysis of experimental measurements of excitation cross sections for the resonance (n1Pn^{1}Pn1P) states of helium. The transport equation for the density of atoms in the resonance state is solved numerically for the case of a thin sheet of exciting electrons between parallel plane electrodes. The fraction of the resonance atoms producing visible radiation is then calculated for parallel plane geometry and estimated to within five percent for cylindrical geometry with an axial electron beam. Predictions of the theory are compared with the available experimental data for helium. The theory shows that at the helium pressures commonly used (${10}^{\ensuremath{-}3}$ to ${10}^{\ensuremath{-}1}$ mm of Hg) the observed visible radiation may easily be a factor of ten greater than that expected when imprisonment effects are neglected. As a result, the cross sections for the excitation of the n1Pn^{1}Pn1P states given in the literature are much too large. For example, our analysis of the available experimental data suggests that the cross section for excitation by electrons to the 31P3^{1}P31P state of helium at 100 electron volts is 3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}18}$ cm2{\mathrm{cm}}^{2}cm2 instead of 4\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}17}$ cm2{\mathrm{cm}}^{2}cm2 as given in the literature.
An experiment is described which measures the collision cross section for transfer of excitation in rubidium by helium. Rubidium atoms are pumped to either the 2P12^{2}P_{\frac{1}{2}}2P21​​ or 2P32^{2}P_{\frac{3}{2}}2P23​​ state by irradiation with one of the resonance lines. Some excited rubidium atoms are transferred to the other PPP state by collisions with helium atoms. The rubidium atoms then radiate both resonance lines. Measurements of the intensities of both resonance lines as a function of helium pressure are used to calculate the collision cross section. The cross section for 2P12^{2}P_{\frac{1}{2}}2P21​​ to 2P32^{2}P_{\frac{3}{2}}2P23​​ is 1.0\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}17}$ cm2{\mathrm{cm}}^{2}cm2\ifmmode\pm\else\textpm\fi{}10% and for 2P32^{2}P_{\frac{3}{2}}2P23​​ to 2P12^{2}P_{\frac{1}{2}}2P21​​ is 1.2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}17}$ cm2{\mathrm{cm}}^{2}cm2\ifmmode\pm\else\textpm\fi{}10%.