Excitation of He and Ne 3p states by polarized electrons

Abstract

Integrated Stokes parameters for the transitions in helium (388.9 nm) and neon (640.2 nm) and (626.7 nm) have been measured over the electron energy range from threshold to 300 eV after excitation by polarized electrons. The measured zero values of for both the He 388.9 and Ne 640.2 nm photons are consistent with the LS coupling of the upper states for each transition. Similarly, the measured non-zero values of for the 626.7 nm photons provide clear evidence of the breakdown of LS coupling for the Ne state due to the spin - orbit interaction of the atomic electrons. Non-zero values of the Stokes parameter indicate strong electron exchange effects, particularly near threshold. Negative ion resonances show considerable effects on the polarizations of the subsequent radiations. The effect of different core configurations on the alignment of excited states is discussed.

Electron-Collision-Induced Alignment of Rare Gases near Threshold

“…With the exception of the work of the Münster group on Hg [13], none involved polarized electrons. Furst et al [5,14] made the first measurements using a polarized electron beam in the study of the heavy noble gases, and the Perth group has recently reported extensive measurements with neon using polarized electrons as well [7][8][9].…”

“…While they both have their excited-electron orbital angular momentum, their excitedelectron spins, and j c "lined up" to give the maximal J for the state, the 3 D 3 state has an alignable ( j c 3͞2) core while the 3 P 2 state does not ( j c 1͞2). The tendency of the atomic 3 D 3 alignment to remain high well above threshold has thus in the past been interpreted in terms of an alignment "flywheel" model [7,20]: the "storage" of alignment in the 3 D 3 core could reduce both the depolarizing influence of negative-ion decay and the ordinary fall of P 1 from its threshold value as sublevels with M L . 0 begin to be excited.…”

Computer stabilized spin polarized electron source

“…It is found that the electron polarization is not critically dependent on the conditions of the crystal surface and is very stable over long operating times. 4 For this source, as the emission per unit laser power decreases from 10 to 0.2 A/mW, the change in the measured absolute polarization is less than 0.5%, which is within the error of the polarization measurement technique. This is a surprising result and suggests that contaminants of the surface have little effect on the spin polarization for this surface orientation and for our operating procedures.…”

“…2 The spin polarized electron beam apparatus described here is a significant improvement of our earlier source 3 and has been used for the excitation of neutral helium and neon atoms with observations of spin orbit coupling and electron exchange phenomena. 4,5 The general principle and characteristics of the source, based on photoemission from a GaAs crystal, have been investigated in detail previously. [6][7][8] The significant improvements reported here have resulted in a high electron flux which is shown to be stable over long periods of time.…”

Spin and Electron Correlation Effects in Excitation of 3d Metal Atoms

“…The measurement of integrated Stokes parameters is an established method [11] based on observation of the polarization characteristics of photons emitted after the excitation of atoms by spin polarized electrons. Both types of measurements are based on crossed electron and atomic beams and the use of spin polarized electrons.…”