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. \newline \emph{The experimental measurements reported here were performed at the University of Wisconsin Synchrotron Radiation Center (NSF DMR-0537588). This work was supported by grants from the Petroleum Research Fund (45728-B6), from the Professional Staff Congress -- City University of New York (60074-34 35) and from the Louisiana Board of Regents Support Fund (LEQSF(2006-09)-RD- A33).}