Collection efficiency of photoelectrons injected into near- and supercritical argon gas

Injection of photoelectrons into gaseous or liquid dielectrics is a widely used technique to produce cold plasmas in weakly ionized systems for investigating the transport properties of electrons. We report measurements of the collection efficiency of photoelectrons injected into dense argon gas for T = 152.7 K, close to the critical temperature T(c) ≈ 150.9 K, and for T = 200.0 K. The high-field data agree with the Young-Bradbury model and with previous measurements below T(c) and at an intermediate temperature above T(c). The effective, density-dependent electron-atom momentum transfer scattering cross section can be deduced. However, the weak-field data near T(c) show large deviations from the theoretical model. We show that the electron behavior at weak field is influenced by electrostriction effects that are only important near the critical point.