Energy of the quasi-free electron in low density Ar and Kr: Extension of the local Wigner–Seitz model

We present new measurements of the perturber-induced shift of the CH 3 I ionization energy at low perturber number densities and analyze these data within the local Wigner–Seitz model previously used at high density. We compare the local Wigner–Seitz model with the scattering approach developed by Fermi, and modified by Alekseev and Sobel’man, and show that the local Wigner–Seitz model extends smoothly into the low density region and accurately predicts the temperature dependence of the shift in ionization energy.