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Electron-atom collision studies using optically state selected beams. Progress report, 1 May 1985-30 April 1986

R. J. Celotta, Michael H. Kelley

Published Jan 1, 1986• 0 citations
OpenAlex

Abstract

Collisions studies using optically state selected beams is discussed. Measurements of spin resolved differential cross sections for superelastic scattering of electrons from excited sodium atoms (3P) are reported. The excited states are prepared by laser optical pumping with circularly polarized light thus giving a pure atomic state before the collision. A crossed beam arrangement of sodium atoms and electrons is used with a scattered electron detector. Scattered electron intensity as a function of scattering angle, incident electron energy, and relative orientation of the incident electronic and atomic spins is measured. 8 refs., 10 figs.

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An experimental investigation of electron - impact induced transitions from the laser - excited 3P state of sodium atom to the 4S and 3D excited states is presented, the main objective being measurement of the atomic collision parameters ( ACPs ) that are related to individual scattering amplitudes. These parameters are complementary to the differential excitation cross - sections and provide finer details on the collisional excitation process. The experiment involved the preparation of the 3P - state with coherent laser radiation of a particular polarisation and measuring the rate of electrons scattered from the laser - excited state as function of laser polarisations. The ACPs were then deduced from these measurements and optical pumping parameters, which account for the optical preparation of the 3P - state. To formulate different measurement schemes, a measurement theory based on the general description of transitions between states of different angular momentum using density matrix formalism is derived. The investigation of the 3P - 4S transition is very similar to the studies of the 3S - 3P excitation performed by superelastic method involving the 3P - 3S scattering in the 'time - inverse' geometry. 'Standard' ACPs are used and, similarly to the superelastic method, the measurements for the 3P - 4S excitation performed in the 'time - inverse' geometry also reveal ACPs for the 4S - 3P de -excitation process. For the more complex 3P - 3D transition, however, the 'standard' ACPs may be less informative. It is shown that certain sums of individual magnetic sublevel excitation probabilities can be measured which may be considered as atomic collision parameters for transitions between P and D - states.

•Jan 1, 1998•0 citations

Electron-atom collision studies using optically state selected beams. Progress report, 1 May 1984-30 April 1985

R. J. Celotta, Michael H. Kelley

This report discusses the progress made on the research program on collision studies using optically state selected beams. The objective of this work is to stimulate deeper theoretical understanding of the electron-atom interaction by providing more complete experimental measurements on colliding systems. In particular we concentrate on effects dependent on the electron and atomic spins. Recent years have seen a rapid growth in the number and complexity of experiments which include spin analysis. The additional information provided by such experiments has contributed greatly to our ability to model numerically collisions between electrons and atoms. Those measurements have typically been on systems where one of the incident particles has zero spin. Hence, one could derive no direct information about the exchange interaction. In our measurement, both collision partners have non zero spin so that we see directly the effects of the exchange as well as the spin-orbit interaction. Indeed, our experimental observable, the asymmetry in a scattering rate when the incident spins are parallel versus antiparallel, is directly related to the ratio of the collision cross sections for scattering via the triplet versus the singlet spin state. 19 refs., 12 figs.

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Superelastic Electron Scattering from Laser Excited States of Sodium

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This thesis presents the results of a series of experiments in which electrons are superelastically scattered from various laser excited states of sodium. The atoms, once in the optically prepared state, are forced to relax via the superelastic collision with an electron. The rate of detection of superelastically scattered electrons was measured as a function of the laser polarisation which enabled pseudo Stokes parameters to be determined. These pseudo Stokes parameters are functions of both optical pumping parameters and atomic collision parameters. The optical pumping parameters describe the laser-atom interaction and the atomic collision parameters describe the electron-atom collision process. Three different laser excitation mechanisms were used to optically pump the atoms into various excited states. The first of these used a single laser tuned to the 32S 112(F'=2 hyperfine state)-~32P312 transition. The excited atoms underwent a superelastic collision with an electron leaving the atom in the ground state and pseudo Stokes parameters were measured as a function of both scattering angle and incident electron energy. The second superelastic experiment, utilised a folded step excitation mechanism which employed two lasers tuned from the two hypethne states of the 32S112 ground state respectively to the 32P312 excited state. Power broadening effects in the single laser experiment cause the atoms to be optically pumped into the F= 1 hyperfine ground state. The laser powers used were not great enough to power broaden the hyperfine ground states and as such the F'= 1 sublevel effectively acted as a sink. The folded step excitation method enabled the excited state population to be increased so that data at larger scattering angles could be obtained. Stokes parameters from both of these experiments which had an incident energy range of 10eV to 30eV and an angular range of 5°-25° were compared to three current electron-atom scattering theories and previous experimental data. Overall, fair to good agreement was found between theory and experiments for the individual Stokes parameters. Losses of coherence was observed at small scattering angles (50.200) at 20eV and 25eV incident electron energies which were poorly modelled by the three different theories. The third superelastic experiment involved the use of two lasers of specified polarisation to stepwise excite the atoms to the 32D512 excited state. Superelastic collisions with incident electron energies of 20eV from the 32D512-*32P312~312 collision were studied at three different scattering angles and pseudo Stokes parameters for the case where the polarisations of the radiation from the lasers were parallel were measured. The single step and folded step laser-atom interactions for it excitation were modelled using a full quantum electrodynamical treatment so that the optical pumping parameters from the single and folded step experiments could be investigated. Equations of motion were derived in the Heisenberg picture and it is shown that for the single laser case 59 equations of motion are required to fully model the interaction and for the folded step ease 78 equations of motion are required. The results of calculations demonstrated that the optical pumping parameters were sensitive to laser intensity, laser detuning and the Doppler width of the atomic beam. The theoretical quantum electrodynamical calculation results were in good agreement with the experimental results.

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Electron scattering by excited sodium atoms has been carried out in a crossed-beam experiment. The atoms have been optically pumped into a well defined hyperfine level (3 2 P 3 2/F=3) by a tunable CW dye laser. For two of the most prominent scattering processes, the 3 2 P to 3 2 S and 3 2 P to 4 2 S transitions, the effect of the anisotropy of the initial state introduced by the laser light is studied in detail and is interpreted in terms of multipole moments describing the optical excitation and the collision process. Possibilities and limitations of the experiment are discussed. Electron collisions may be seen as a test case for the understanding of more involved interaction processes.

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Connection between superelastic and inelastic electron-atom collisions involving polarized collision partners

Klaus Bartschat, D. H. Madison

It is shown how the results of a recent experiment by Jiang, Zuo, Vu\ifmmode \check{s}\else \v{s}\fi{}kovi\ifmmode \acute{c}\else \'{c}\fi{}, and Bederson [Phys. Rev. Lett. 68, 915 (1992)], who investigated low-energy electron scattering from laser-excited polarized sodium atoms in the initial (3p) $^{2}$P${\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}_{3/2}$ (F=3, ${\mathit{M}}_{\mathit{F}}$=3) state, can be related to the inelastic 3S\ensuremath{\rightarrow}3P transition involving initially unpolarized electron and atom beams. Hence, this method can provide an independent check of the traditional electron-scattering experiment with unpolarized beams.

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Coherence, exchange and superelastic electron scattering from sodium

P. J. O. Teubner, Robert E Scholten

A series of superelastic scattering experiments have been carried out on optically pumped sodium atoms in the 3 2 P 3/2 state. The total polarization P + has been observed at electron energies of 12.1 eV and 22.1 eV referred to the ground state, over a wide range of electron scattering angles. The connections between this parameter, the coherence of the excitation process, and exchange are discussed. Significant departures from P + =1 are observed. A qualitative explanation of this phenomenon is described which provides some insight into the collision process.

Journal of Physics B Atomic Molecular and Optical Physics•Jun 28, 1992•15 citations
Collisions studies using optically state selected beams is discussed. Measurements of spin resolved differential cross sections for superelastic scattering of electrons from excited sodium atoms (3P) are reported. The excited states are prepared by laser optical pumping with circularly polarized light thus giving a pure atomic state before the collision. A crossed beam arrangement of sodium atoms and electrons is used with a scattered electron detector. Scattered electron intensity as a function of scattering angle, incident electron energy, and relative orientation of the incident electronic and atomic spins is measured. 8 refs., 10 figs.
An experimental investigation of electron - impact induced transitions from the laser - excited 3P state of sodium atom to the 4S and 3D excited states is presented, the main objective being measurement of the atomic collision parameters ( ACPs ) that are related to individual scattering amplitudes. These parameters are complementary to the differential excitation cross - sections and provide finer details on the collisional excitation process. The experiment involved the preparation of the 3P - state with coherent laser radiation of a particular polarisation and measuring the rate of electrons scattered from the laser - excited state as function of laser polarisations. The ACPs were then deduced from these measurements and optical pumping parameters, which account for the optical preparation of the 3P - state. To formulate different measurement schemes, a measurement theory based on the general description of transitions between states of different angular momentum using density matrix formalism is derived. The investigation of the 3P - 4S transition is very similar to the studies of the 3S - 3P excitation performed by superelastic method involving the 3P - 3S scattering in the 'time - inverse' geometry. 'Standard' ACPs are used and, similarly to the superelastic method, the measurements for the 3P - 4S excitation performed in the 'time - inverse' geometry also reveal ACPs for the 4S - 3P de -excitation process. For the more complex 3P - 3D transition, however, the 'standard' ACPs may be less informative. It is shown that certain sums of individual magnetic sublevel excitation probabilities can be measured which may be considered as atomic collision parameters for transitions between P and D - states.
This report discusses the progress made on the research program on collision studies using optically state selected beams. The objective of this work is to stimulate deeper theoretical understanding of the electron-atom interaction by providing more complete experimental measurements on colliding systems. In particular we concentrate on effects dependent on the electron and atomic spins. Recent years have seen a rapid growth in the number and complexity of experiments which include spin analysis. The additional information provided by such experiments has contributed greatly to our ability to model numerically collisions between electrons and atoms. Those measurements have typically been on systems where one of the incident particles has zero spin. Hence, one could derive no direct information about the exchange interaction. In our measurement, both collision partners have non zero spin so that we see directly the effects of the exchange as well as the spin-orbit interaction. Indeed, our experimental observable, the asymmetry in a scattering rate when the incident spins are parallel versus antiparallel, is directly related to the ratio of the collision cross sections for scattering via the triplet versus the singlet spin state. 19 refs., 12 figs.
This thesis presents the results of a series of experiments in which electrons are superelastically scattered from various laser excited states of sodium. The atoms, once in the optically prepared state, are forced to relax via the superelastic collision with an electron. The rate of detection of superelastically scattered electrons was measured as a function of the laser polarisation which enabled pseudo Stokes parameters to be determined. These pseudo Stokes parameters are functions of both optical pumping parameters and atomic collision parameters. The optical pumping parameters describe the laser-atom interaction and the atomic collision parameters describe the electron-atom collision process. Three different laser excitation mechanisms were used to optically pump the atoms into various excited states. The first of these used a single laser tuned to the 32S 112(F'=2 hyperfine state)-~32P312 transition. The excited atoms underwent a superelastic collision with an electron leaving the atom in the ground state and pseudo Stokes parameters were measured as a function of both scattering angle and incident electron energy. The second superelastic experiment, utilised a folded step excitation mechanism which employed two lasers tuned from the two hypethne states of the 32S112 ground state respectively to the 32P312 excited state. Power broadening effects in the single laser experiment cause the atoms to be optically pumped into the F= 1 hyperfine ground state. The laser powers used were not great enough to power broaden the hyperfine ground states and as such the F'= 1 sublevel effectively acted as a sink. The folded step excitation method enabled the excited state population to be increased so that data at larger scattering angles could be obtained. Stokes parameters from both of these experiments which had an incident energy range of 10eV to 30eV and an angular range of 5°-25° were compared to three current electron-atom scattering theories and previous experimental data. Overall, fair to good agreement was found between theory and experiments for the individual Stokes parameters. Losses of coherence was observed at small scattering angles (50.200) at 20eV and 25eV incident electron energies which were poorly modelled by the three different theories. The third superelastic experiment involved the use of two lasers of specified polarisation to stepwise excite the atoms to the 32D512 excited state. Superelastic collisions with incident electron energies of 20eV from the 32D512-*32P312~312 collision were studied at three different scattering angles and pseudo Stokes parameters for the case where the polarisations of the radiation from the lasers were parallel were measured. The single step and folded step laser-atom interactions for it excitation were modelled using a full quantum electrodynamical treatment so that the optical pumping parameters from the single and folded step experiments could be investigated. Equations of motion were derived in the Heisenberg picture and it is shown that for the single laser case 59 equations of motion are required to fully model the interaction and for the folded step ease 78 equations of motion are required. The results of calculations demonstrated that the optical pumping parameters were sensitive to laser intensity, laser detuning and the Doppler width of the atomic beam. The theoretical quantum electrodynamical calculation results were in good agreement with the experimental results.
Electron scattering by excited sodium atoms has been carried out in a crossed-beam experiment. The atoms have been optically pumped into a well defined hyperfine level (3 2 P 3 2/F=3) by a tunable CW dye laser. For two of the most prominent scattering processes, the 3 2 P to 3 2 S and 3 2 P to 4 2 S transitions, the effect of the anisotropy of the initial state introduced by the laser light is studied in detail and is interpreted in terms of multipole moments describing the optical excitation and the collision process. Possibilities and limitations of the experiment are discussed. Electron collisions may be seen as a test case for the understanding of more involved interaction processes.
It is shown how the results of a recent experiment by Jiang, Zuo, Vu\ifmmode \check{s}\else \v{s}\fi{}kovi\ifmmode \acute{c}\else \'{c}\fi{}, and Bederson [Phys. Rev. Lett. 68, 915 (1992)], who investigated low-energy electron scattering from laser-excited polarized sodium atoms in the initial (3p) 2^{2}2P${\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}_{3/2}$ (F=3, MF{\mathit{M}}_{\mathit{F}}MF​=3) state, can be related to the inelastic 3S\ensuremath{\rightarrow}3P transition involving initially unpolarized electron and atom beams. Hence, this method can provide an independent check of the traditional electron-scattering experiment with unpolarized beams.
A series of superelastic scattering experiments have been carried out on optically pumped sodium atoms in the 3 2 P 3/2 state. The total polarization P + has been observed at electron energies of 12.1 eV and 22.1 eV referred to the ground state, over a wide range of electron scattering angles. The connections between this parameter, the coherence of the excitation process, and exchange are discussed. Significant departures from P + =1 are observed. A qualitative explanation of this phenomenon is described which provides some insight into the collision process.