Electron scattering in dense He-Ar gas mixtures: A pressure shift study

The dependence of the energies of high-n Rydberg states of CH₃I on the molar composition of helium-argon mixtures (in the number density range 1.3×{}10²⁰⁻⁻5.6×{}10²⁰ cm^-3) is reported. The energy shifts, when normalized to a given density value, are found to vary linearly with the mole fraction of either component of the binary, rare-gas mixture. The observed change in sign of the energy shift is attributable to the different signs of the electron scattering lengths for the two rare-gas components. As a result, there exists a mixture composition, at a mole ratio [He]/[Ar]=2.0, at which the shift is null. The experimental results for the gas mixture agree with the Fermi formula, as modified to include the Alekseev-Sobel'man polarization term. Effective electron scattering lengths and cross sections, polarizabilities, and thermal velocities are used to characterize the effects of the binary gas perturber system.