We present new measurements of the perturber-induced shift of the CH3I 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. Published by Elsevier B. V.