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Bulletin of the American Physical Society

Electron-atom scattering using atomic recoil

Benjamin B. Bederson

Published Dec 1, 1992• 1 citation
OpenAlex

Abstract

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.

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Optically trapped atoms are in many ways ideal replacements for atomic beams for scattering experiments. In a typical such experiment, a particle beam ejects atoms from the trap at a rate {Gamma} = {sigma}{phi}, where {phi} is the particle flux density and {sigma} the scattering cross section. {Gamma} is easily measured from trap decay transients. Since the trapped atoms have neV kinetic energies, the method is sensitive to extremely small momentum transfers. Since the beam flux is measured, the absolute target density is not required, removing an important source of error for most atomic beam experiments. At Wisconsin the authors have integrated an electron beam with a magneto-optical trap and performed electron-scattering experiments on Rb atoms in the range of 10-1000 eV. In the first experiment they measured total cross sections for scattering from the 5s state by lowering the capture velocity of the trap below the recoil velocity of the scattered atoms. In the second experiment they isolated the total ionization cross section by operating the trap such that virtually all elastically and inelastically scattered atoms were captured by the trap. These experimental results will test new theoretical methods for calculating total and ionization cross sections. Straightforward extensions ofmore » these methods should allow similar measurements using excited-state atoms (5p, 5d) as well. The techniques should also be readily adaptable to the important case of the metastable noble gases.« less

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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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The development of the theory of the effect of a laser on electron-atom scattering is continued by the derivation of explicit relations between the observed electron-atom scattering cross sections in the presence of a laser and exact electron-atom scattering cross sections with no laser present. No approximation concerning the scattering interaction is made. The only approximations concerning the laser are that (1) the laser-atom interaction energy is small compared to atomic energies, (2) the Rabi frequency times the collision time is small, and (3) the laser intensity in appropriate units is small.

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L. J. Uhlmann, R. G. Dall, Andrew G. Truscott +3 more

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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.
A new method for measuring cross sections for the scattering of electrons by laser-excited atoms is described. It is a generalization of the atomic-beam recoil technique, taking advantage of the recoil of atoms during resonant photon interactions to spatially separate excited from nonexcited atoms. A preliminary value for the total cross section for the scattering of electrons by the 32P32(mF=3)3^{2}P_{\frac{3}{2}}({m}_{F}=3)32P23​​(mF​=3) state of sodium at 4.4 eV is presented.
Optically trapped atoms are in many ways ideal replacements for atomic beams for scattering experiments. In a typical such experiment, a particle beam ejects atoms from the trap at a rate {Gamma} = {sigma}{phi}, where {phi} is the particle flux density and {sigma} the scattering cross section. {Gamma} is easily measured from trap decay transients. Since the trapped atoms have neV kinetic energies, the method is sensitive to extremely small momentum transfers. Since the beam flux is measured, the absolute target density is not required, removing an important source of error for most atomic beam experiments. At Wisconsin the authors have integrated an electron beam with a magneto-optical trap and performed electron-scattering experiments on Rb atoms in the range of 10-1000 eV. In the first experiment they measured total cross sections for scattering from the 5s state by lowering the capture velocity of the trap below the recoil velocity of the scattered atoms. In the second experiment they isolated the total ionization cross section by operating the trap such that virtually all elastically and inelastically scattered atoms were captured by the trap. These experimental results will test new theoretical methods for calculating total and ionization cross sections. Straightforward extensions ofmore » these methods should allow similar measurements using excited-state atoms (5p, 5d) as well. The techniques should also be readily adaptable to the important case of the metastable noble gases.« less
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 development of the theory of the effect of a laser on electron-atom scattering is continued by the derivation of explicit relations between the observed electron-atom scattering cross sections in the presence of a laser and exact electron-atom scattering cross sections with no laser present. No approximation concerning the scattering interaction is made. The only approximations concerning the laser are that (1) the laser-atom interaction energy is small compared to atomic energies, (2) the Rabi frequency times the collision time is small, and (3) the laser intensity in appropriate units is small.
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.