No 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.

The angular momentum properties of incident transversely polarized electrons and radiated photons have been combined with scattering symmetries to reveal the effects of exchange and spin-orbit coupling as well as some propensity rules for excitation and decay of the 3p states of the neon atom.Integrated Stokes parameters, P 1 , P 2 and P 3 , have been measured for the decay radiations of wavelengths 616.4, 603.0, 626.7, 638.3 and 703.3 nm from the 2p 5 3p [1/2] 1 , 2p 5 3p [3/2] 1 , 2p 5 3p[3/2] 1 and 2p 5 3p[1/2] 1 states, respectively. While these states are intermediately (j l) coupled, their LS compositions partially explain some phenomena. For the states having a dominant triplet component, both the exchange and the spin-orbit interaction within the atom are important in the excitation, while the spin-orbit interaction plays the significant role for the state dominated by a singlet component. Decay photons from a common upper state with J = 1 have larger linear polarization than those with J = 0 and −1 while those from a 2 P 1/2 core upper state have larger linear polarizations than those from a 2 P 3/2 core upper state. The state multipoles are derived from the measured Stokes parameters. The influence of negative-ion resonances near threshold on the Stokes parameters and the state multipoles is very strong in some cases. The core state effects on the alignment of excited states are investigated.

No abstract

The two outgoing electrons from (e,2e) scattering processes are selected by two 180° electrostatic analyzers whose energy dispersed output spectra are detected by microchannel plates followed by resistive anodes. These detectors permit the identification of all multiple coincident events occurring within the dispersal width of the analyzers. It was found that the time difference between the two detectors for correlated events is strongly dependent on the energy of the incident electrons, which consequently causes the time coincidence window to move systematically. This behavior has been accounted for by a model that determines the time taken by an electron to go around a hemispherical analyzer. A description is given of a data collection system that can compensate for the shift in the coincidence spectra and that records all multiple coincidence events with an increase in the overall coincidence count rate of about two orders of magnitude relative to conventional non position sensitive detectors without position sensing. The performance of the system is shown for an (e,2e) spectrum of argon.