Angular distributions and polarization fractions of helium resonance radiation (nP1−1S1) in the extreme ultraviolet

We present the first experimentally determined angular intensity distributions of helium ($n{}_{}{}^1{}_{}{}^{}P-1{}_{}{}^1{}_{}{}^{}S$) resonance photons with respect to the exciting electron beam. The angular intensity distributions were measured at selected electron-impact energies from 25 eV (near threshold) to 150 eV. Polarization fractions (II) were obtained by analyzing the data in terms of the theoretical relation between angular intensity distribution and II, i.e., $I\left(\theta \right)=I\left(90\right)\mathrm{}\times (1-\Pi {cos}^2\theta )$. The experimental values for II are compared with recent theoretical results and with previous experimental values for the ($3{}_{}{}^1{}_{}{}^{}P-1{}_{}{}^1{}_{}{}^{}S$) transition. Our results are slightly higher than the theoretical results for electron-impact energies from 80 to 150 eV, e.g., 0.36 vs 0.32 at 100 eV. Our values for II reach a maximum at ∼40 eV where II=0.55 and then decrease for lower electron energies, reflecting the increasing importance of cascade ($m{}_{}{}^1{}_{}{}^{}S-n{}_{}{}^1{}_{}{}^{}P$) transitions at lower electron energies.