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Physical Review Letters

Scattering of Low-Energy Electrons by Excited Sodium Atoms Using a Photon and Electron Atomic Beam Recoil Technique

Natarajan D. Bhaskar, Bernardo Jaduszliwer, Benjamin B. Bederson

Published Jan 3, 1977• 37 citations
Publisher

Abstract

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 $3^{2}P_{\frac{3}{2}}({m}_{F}=3)$ state of sodium at 4.4 eV is presented.

DOI
10.1103/physrevlett.38.14

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Two techniques have been used to study low energy electron scattering processes in the alkali metals. A transmission experiment was used to search for resonance structure in the total scattering cross section of sodium, potassium, rubidium, and cesium. A shape resonance was observed in sodium at 0.08 (+OR-) 0.03 eV. In rubidium, evidence was found for the existence of a shape resonance below 0.05 eV. No shape resonance structure was observed in potassium or cesium for energies above 0.1 eV. For energies near the first excited state to near the lowest ionization potential, a progression of Feshbach resonances was observed in each atom. The graphical analysis technique of Spence was used to group most of these resonances into seven families. An analysis is given of the mechanisms affecting the rejection of scattered electrons in the transmission spectrometer. A quantitative description of two modes of operation of the spectrometer is presented. The second portion of this dissertation concerns the measurement of the near-threshold excitation and ionization cross sections in sodium, potassium, and rubidium. A trapped-electron experiment was used to make a relative measurement of these cross sections. The shapes of the excitation cross sections are, in general, somewhat different from other experimental work but are in good agreement with the close-coupling calculations of Moores and Norcross. The shapes of the ionization cross sections are in reasonable agreement with previous work. The ratio of the excitation cross section at 2.5 eV in sodium, 2.0 eV in potassium, and 1.8 eV in rubidium to the respective ionization cross section at the first peak was measured and found to be 3.9 (+OR-) 13% in Na, 5.8 (+OR-) 10% in K, and 6.4 (+OR-) 13% in Rb. These ratios are generally larger than those derived from independent values of the individual cross sections. An analysis of several systematic effects associated with the measurement of the cross section ratio was made. Results from this analysis were used to correct the measured excitation cross section for effects of the elastically scattered electrons.

•Jan 1, 1983•0 citations
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.
An electron and photon atomic-recoil technique has been used to measure absolute total cross sections for electrons scattered by 32{3}^{2}32P3/2{P}_{3/2}P3/2​ sodium atoms in the 6--25-eV energy range, with the use of a new, high-resolution crossed-beam apparatus. A single-mode tunable ring dye laser is used to selectively excite the F=3 hyperfine level. Excited-state populations are determined by using the atomic recoil in resonant photon absorption. At 6 eV there is very good agreement with a previous measurement [B. Jaduszliwer, R. Dang, P. Weiss, and B. Bederson, Phys. Rev. A 21, 808 (1980)].
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.
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.
Two techniques have been used to study low energy electron scattering processes in the alkali metals. A transmission experiment was used to search for resonance structure in the total scattering cross section of sodium, potassium, rubidium, and cesium. A shape resonance was observed in sodium at 0.08 (+OR-) 0.03 eV. In rubidium, evidence was found for the existence of a shape resonance below 0.05 eV. No shape resonance structure was observed in potassium or cesium for energies above 0.1 eV. For energies near the first excited state to near the lowest ionization potential, a progression of Feshbach resonances was observed in each atom. The graphical analysis technique of Spence was used to group most of these resonances into seven families. An analysis is given of the mechanisms affecting the rejection of scattered electrons in the transmission spectrometer. A quantitative description of two modes of operation of the spectrometer is presented. The second portion of this dissertation concerns the measurement of the near-threshold excitation and ionization cross sections in sodium, potassium, and rubidium. A trapped-electron experiment was used to make a relative measurement of these cross sections. The shapes of the excitation cross sections are, in general, somewhat different from other experimental work but are in good agreement with the close-coupling calculations of Moores and Norcross. The shapes of the ionization cross sections are in reasonable agreement with previous work. The ratio of the excitation cross section at 2.5 eV in sodium, 2.0 eV in potassium, and 1.8 eV in rubidium to the respective ionization cross section at the first peak was measured and found to be 3.9 (+OR-) 13% in Na, 5.8 (+OR-) 10% in K, and 6.4 (+OR-) 13% in Rb. These ratios are generally larger than those derived from independent values of the individual cross sections. An analysis of several systematic effects associated with the measurement of the cross section ratio was made. Results from this analysis were used to correct the measured excitation cross section for effects of the elastically scattered electrons.