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

R. J. Celotta, Michael H. Kelley

Published Jan 1, 1985• 0 citations

Abstract

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

R. J. Celotta, Michael H. Kelley

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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Optical collisions in an atomic beam experiment

Joachim Großer, O. Hoffmann, Selina Klose

The optical collision Na(3s)+Kr+hν→Na(3p)+Kr has been investigated in an experiment with crossed atomic beams and with a differential detection of the excited collision products. Three groups of experimental results have been obtained, which all demonstrate unprecedented possibilities for the observation of atomic collisions. 1. The oscillatory structure of the differential cross sections allows a sensitive test of the molecular potentials. 2. Polarization experiments give direct information about the geometric characteristics of the collision complex. 3. State analysis of the collision products provides a tool for the observation of elementary nonadiabatic processes. A number of possible future experiments, which make use of the new ideas, is discussed, in addition.

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Atom-atom and atom-ion collisions. III. Collisions involving excitation transfer, spin exchange or depolarization, without internal energy change

H. S. W. Massey

Three further types of collision between atomic systems in which no change of internal energy takes place are considered. These are collisions in which transfer of excitation occurs between two similar atoms, spin exchange collisions, and collisions in which the direction but not the magnitude of the orbital angular momentum of an excited atom is changed on impact. An account is given of the theory of these processes in relation to that for other processes, elastic scattering and symmetrical charge exchange, discussed in the earlier articles, in which no change of internal energy occurs. Methods for measuring the relevant cross sections are described. These make use of optical pumping and nuclear magnetic resonance techniques. Comparison is made between observed and calculated cross sections.

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Theory of angular-correlation experiments in electron scattering, including fine structure

Gregory A. Parker, Thomas M. Miller, Massoud Mahgerefteh +1 more

A theory of electron-impact excitation of atoms in which fine structure is included as a perturbation is presented in detail. Hyperfine structure is included in the target atom but neglected in the electron-atom interaction. Expressions for electron-photon angular-correlation parameters and scattering cross sections are derived in terms of products of complex scattering amplitudes, with emphasis on electron-sodium excitation. Examples are given in which zero-field quantum beats may be observed in electron-photon angular-correlation and differential-cross-section measurements. Some of the interference terms are only observable in angular-correlation experiments. Examples are also given of partial cross sections for electron-impact excitation of state-to-state hyperfine levels in terms of direct- and exchange-scattering amplitudes plus additional terms due to the inclusion of fine-structure effects. The expressions given allow a comparison between the various possible scattering experiments.

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Alkali-Alkali Differential Spin-Exchange Scattering. I

David E. Pritchard, Gary M. Carter, Frank Y. Chu +1 more

We present measurements of differential elastic cross sections for various alkali-alkali pairs, as well as the angular dependence of the probability of electron spin exchange in these collisions. The resolution is sufficient to reveal interference structure in the angular dependence of both quantities. We describe our apparatus, which polarizes one incident beam and analyzes the polarization of the scattered atoms, and discuss the measurement and data-reduction procedures. Results are presented in the range 0.1-0.2 eV for the systems Na-Cs, Na-Rb, Na-K, K-Cs, and K-Rb.

Physical Review A•Nov 1, 1970•25 citations

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.

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.

The optical collision Na(3s)+Kr+hν→Na(3p)+Kr has been investigated in an experiment with crossed atomic beams and with a differential detection of the excited collision products. Three groups of experimental results have been obtained, which all demonstrate unprecedented possibilities for the observation of atomic collisions. 1. The oscillatory structure of the differential cross sections allows a sensitive test of the molecular potentials. 2. Polarization experiments give direct information about the geometric characteristics of the collision complex. 3. State analysis of the collision products provides a tool for the observation of elementary nonadiabatic processes. A number of possible future experiments, which make use of the new ideas, is discussed, in addition.

This talk will review electron-atom scattering experiments using recoil techniques, wherein observation of the collision process is made on the recoil-scattered atom rather than on the electron. Using this technique it is possible to obtain absolute cross sections without normalization or knowledge of the atom density in the interaction region; atomic state selection and analysis is also more readily performed. Comparisons with theory and with other crossed beam experiments, including those using polarized electrons, will be made. Details of ongoing experiments are presented in companion papers at this conference. A discussion of how laser cooling, and velocity and spatial compression can be employed to improve dramatically spatial and energy resolution and sensitivity will be presented.

Three further types of collision between atomic systems in which no change of internal energy takes place are considered. These are collisions in which transfer of excitation occurs between two similar atoms, spin exchange collisions, and collisions in which the direction but not the magnitude of the orbital angular momentum of an excited atom is changed on impact. An account is given of the theory of these processes in relation to that for other processes, elastic scattering and symmetrical charge exchange, discussed in the earlier articles, in which no change of internal energy occurs. Methods for measuring the relevant cross sections are described. These make use of optical pumping and nuclear magnetic resonance techniques. Comparison is made between observed and calculated cross sections.

A theory of electron-impact excitation of atoms in which fine structure is included as a perturbation is presented in detail. Hyperfine structure is included in the target atom but neglected in the electron-atom interaction. Expressions for electron-photon angular-correlation parameters and scattering cross sections are derived in terms of products of complex scattering amplitudes, with emphasis on electron-sodium excitation. Examples are given in which zero-field quantum beats may be observed in electron-photon angular-correlation and differential-cross-section measurements. Some of the interference terms are only observable in angular-correlation experiments. Examples are also given of partial cross sections for electron-impact excitation of state-to-state hyperfine levels in terms of direct- and exchange-scattering amplitudes plus additional terms due to the inclusion of fine-structure effects. The expressions given allow a comparison between the various possible scattering experiments.

We present measurements of differential elastic cross sections for various alkali-alkali pairs, as well as the angular dependence of the probability of electron spin exchange in these collisions. The resolution is sufficient to reveal interference structure in the angular dependence of both quantities. We describe our apparatus, which polarizes one incident beam and analyzes the polarization of the scattered atoms, and discuss the measurement and data-reduction procedures. Results are presented in the range 0.1-0.2 eV for the systems Na-Cs, Na-Rb, Na-K, K-Cs, and K-Rb.