A measurement of the P₁ coherence parameter in forward scattering for excitation of the ns'(1/2)₁⁰ state in Ar and Ne by electron impact

The authors report measurements of the P₁ coherence parameter for excitation of the spin-orbit coupled ns'(1/2)₁⁰(' ¹P₁') state of Ne (n=3) and Ar (n=4) by unpolarised electrons of intermediate energy (30-100 eV) which are subsequently scattered in the forward direction. The authors' experimental results verify the theoretical prediction of P₁=+1 when angular resolution effects are properly taken into account. This result clarifies the previous discrepancy between experiment and theory and demonstrates unambiguously that the excitation process under these conditions proceeds via the direct excitation of the LS-coupled singlet contribution of the excited states, i.e. via transfer of orbital angular momentum.