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Journal of Physics B Atomic Molecular and Optical Physics

Stokes parameters from neon states excited by polarized electrons

Dehong Yu, P A Hayes, Jim Williams, Clayton R. Locke

Published Jul 14, 1997• 6 citations
Publisher

Abstract

For neon 3p states excited by transversely polarized electrons, the integrated Stokes parameters, and , have been measured from threshold to 300 eV for the decay photons of 692.9, 667.8 and 717.4 nm wavelengths from the and states, respectively. The and parameters for the 692.9 and 667.8 nm transitions indicate similar excitation mechanisms consistent with their similar ` L S component states'. The non-zero indicates the breakdown of L S -coupling and the large values near threshold suggest a significant exchange process. For the 717.4 nm transition, the three Stokes parameters are much smaller than those of the other two lines. The low is surprising since the large (75.4%) -coupled component state was expected to cause large exchange effects.

DOI
10.1088/0953-4075/30/13/002

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For neon 3p states excited by transversely polarized electrons, the integrated Stokes parameters, and , have been measured from threshold to 300 eV for the decay photons of 692.9, 667.8 and 717.4 nm wavelengths from the and states, respectively. The and parameters for the 692.9 and 667.8 nm transitions indicate similar excitation mechanisms consistent with their similar ` L S component states'. The non-zero indicates the breakdown of L S -coupling and the large values near threshold suggest a significant exchange process. For the 717.4 nm transition, the three Stokes parameters are much smaller than those of the other two lines. The low is surprising since the large (75.4%) -coupled component state was expected to cause large exchange effects.
Spin-orbit and exchange effects are separately identified through the measurement of integrated Stokes parameters P2{P}_{2}P2​ and P3{P}_{3}P3​ for the decay radiation from spin-polarized electron impact excited states. The observations are made for the radiations from the transitions ${3p}^{\ensuremath{'}}[\frac{1}{2}{]}_{1}\ensuremath{\rightarrow}{3s}^{\ensuremath{'}}[\frac{1}{2}{]}_{0}$ (616.4 nm) and $3p[\frac{1}{2}{]}_{1}\ensuremath{\rightarrow}3s[\frac{3}{2}{]}_{2}$ (703.3 nm) in neon just above threshold.
In an electron-polarized photon coincidence experiment the Stokes' parameters P 1 , P 2 , P 3 and P 4 have been determined for four Ne * (3p) to Ne * (3s) transitions at a fixed electron energy of 50 eV and in the electron angular scattering range 30 degrees -70 degrees . The results indicate a tendency towards positive values for P 3 at the lowest scattering angles studied and a strong tendency towards zero and even negative values for P 4 . In all cases a drop in the total polarization degree between 30 degrees and 50 degrees electron scattering angle is observed, indicating that there is a decrease of coherence at these scattering angles.
For the mercury 6s6p 1 P 1 and 6s6p 3 P 1 states excited by electron impact, the linear polarization of the light emitted in the transition to the ground state has been measured from threshold to intermediate energy for the decay photons of 254 and 185 nm wavelength, respectively. The results for both lines are in surprisingly good agreement with a calculation based on the Born-Ochkur approximation, neglecting spin-orbit terms for the continuum electron but allowing for electron exchange effects and intermediate coupling.
The Stokes parameters P1{P}_{1}P1​, P2{P}_{2}P2​, P3{P}_{3}P3​, and P4{P}_{4}P4​ have been measured in a scattered electron-polarized cascade photon correlation study of the 41F4^{1}F41F state of helium excited by 29.6 eV electron impact. This is the first detailed experimental study of electron impact excitation leading to the transfer of three units of angular momentum. The data are expected to be influenced by the breakdown of LS\mathrm{LS}LS coupling for $L\ensuremath{\ge}3$ states, even in a simple system like helium, providing a further challenge for theory.
A beam of unpolarized electrons is crossed at right angles with a beam of partially polarized sodium or potassium atoms (degree of polarization P A approximately=21%) and resonance lines from the excited 2 P 3/2, 1/2 states are observed in a direction perpendicular to both the atom and electron beams for electron energies from threshold to 200 eV. The polarization of the emitted photons is measured in the form of the Stokes parameters P 1 , P 2 and P 3 . The present measurements for sodium atoms are in better agreement with the theoretical values than the previously published experimental results. No other experimental or theoretical results are available for potassium atoms.
The 3s-3p excitation of spin-polarized Na atoms by Ne+{\mathrm{Ne}}^{+}Ne+ ions has been studied for impact energies Elab=200{E}_{\mathrm{lab}=200}Elab=200​ eV to 6 keV, i.e., in the adiabatic regime. The total excitation cross section and the three Stokes polarization parameters of the fluorescence light have been measured. The linear polarization of the light shows a preferential excitation of the \ensuremath{\Vert}ml{m}_{l}ml​\ensuremath{\Vert}=1 magnetic substates. The circular polarization probes the spin orientation of the excited 3p state. At the highest impact energies investigated the experimental data are compatible with conservation of spin orientation during the collision. With decreasing impact energy, the spin polarization of the final 3p state becomes smaller than the spin polarization of the initial 3s state. This apparent spin depolarization is attributed to the exchange interaction between the Na valence electron and the unfilled Ne+{\mathrm{Ne}}^{+}Ne+ 2p5{p}^{5}p5 core in the quasimolecule formed during the collision.