Field ionization measurements of CH 3 I and C 2 H 5 I dopant high-n molecular Rydberg states in argon and krypton perturbers are presented as a function of perturber number density along various isotherms up to the density of the triple point liquid. Using these data, a new local Wigner-Seitz model for the density-dependent energy V 0 ͑ P ͒ of a quasifree electron in argon and krypton is developed. This model, which contains only one adjustable parameter, uses a local Wigner-Seitz radius derived from the local number density rather than from the bulk number density, includes a statistical mechanical calculation of both the ion/medium polarization energy and the electron/medium polarization energy, and includes the thermal kinetic energy of the quasifree electron. Using this model, V 0 ͑ P ͒ and the perturber-induced energy shift of the dopant ionization potential ⌬ D ͑ P ͒ are calculated to within ±0.1% of experiment. Previously reported V 0 ͑ P ͒ data for xenon are also shown to be interpretable within this new model.

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Field ionization of C2H5I doped into argon is presented as a function of argon number density along the argon critical isotherm. These data exhibit a decrease in the argon-induced shift of the C2H5I ionization energy near the critical point similar to that observed in recent field ionization measurements of CH3I/Ar. We show that this decrease is due to the interaction between argon and the quasi-free electron arising from dopant field ionization and is, therefore, independent of the dopant. The energy of the quasi-free electron is calculated in a local Wigner–Seitz model containing no adjustable parameters to within ±0.2% of experiment.