Excitation and polarization of the 3³D state of helium by electron impact

Abstract

Electron impact excitation of the 33D state of helium has been studied experimentally and theoretically using the convergent close coupling (CCC) and R-matrix with pseudo-states (RMPS) methods. Experimental and theoretical CCC emission cross sections and polarizations of the 33D→23P line are reported. Theoretical cross sections for direct excitation of the 33D state and polarizations of corresponding emitted radiation are also reported using both CCC and RMPS methods. Particular attention has been given to making realistic estimates of the large cascade contributions to both the cross sections and the polarization fraction of the 33D→23P radiation, using the CCC approach. The self-consistency of the experimental and theoretical polarization data gives confidence in their quality. On the other hand differences remain between experimental and theoretical emission cross sections at higher energies.

Fluorescence polarization of helium negative-ion resonances excited by polarized electron impact

“…Cascade contributions from the 4p, 5p and 6p levels are responsible for ~50% of the observed 588 nm radiation in the 50-60 eV energy range [13]. However, the effects of the resonances on the emitted radiation from the 4p and 5p states are known to be small [10], and we expect the same for the 6p state.…”

“…The electrons are electrostatically defl ected by 90° to produce a beam with transverse polarization and steered into a 5 cm long target gas cell. Helium pressure in the cell was kept at 8 × 10 -2 Pa to minimize the effects of radiation trapping [13]. Electrons pass through the target cell and are collected in a Faraday cup.…”

“…We not only analyze our data, but also that of Defrance [10] and Cvejanović et al [13,14]. The data of Batelaan et al [12] is excluded because we could not obtain it.…”

“…Relative emission cross section Q em (in arbitrary units) and Stokes parameters P 1 , P 2 /P e and P 3 /P e . Our data are shown as empty and fi lled circles (two separate experimental runs), the values of Defrance [10] are displayed as black lines and the work of Cvejanović et al [13,14] is represented with gray lines. Resonance energies of 57.2 and 58.3 eV are indicated.…”

“…Another method of investigation consists of monitoring the fl uorescence of states that result from decay of these negative-ion resonances. In particular, the intensity and linear polarization of the 3 3 D → 2 3 P transition have been studied following excitation with unpolarized incident electron beams [9][10][11][12][13][14]. (The resonance features in this transition are particularly pronounced.)…”