Electron mobilities in gaseous, critical, and liquid xenon: Density, electric field, and temperature effects: Quasilocalization

Abstract

The mobility μ of electrons in xenon varies with the density n : μαn^x. For thermal electrons in the gas, x=−1.0 when n20 molecule/cm³, independent of temperature. This corresponds to pressures ≲20 atm at 300 K and is characteristic of simple scattering by a polarization field. At higher gas densities x becomes more negative, then passes through a minimum, and μ has a positive temperature coefficient at constant n. At these densities thermal electrons form quasilocalized states in, or suffer enhanced scattering by, microscopic regions of relatively high density. The localization effect maximizes in the critical fluid, at n=5×10²¹ molecule/cm³ and T=290 K, and appears to be negligible again at n?6.8×10²¹. In the liquid phase x?+9 up to n=1.2×10²² molecule/cm³, then changes to x?−8 at higher densities. The positive value of x is due to the formation of a conduction band, and the negative value to scattering by the repulsive cores of the closely spaced atoms. In the gas and the low density liquid (n22), μ at low electric field strengths E is independent of E; at higher E, μ increases with E then passes through a maximum and decreases. The maximum in μ reflects the Ramsauer–Townsend minimum in the scattering cross section σ_ε of the atoms as a function of electron energy ε. Quasilocalization is inhibited by the application of moderate electric fields, and this enhances the observed dependence of μ on E at 4×10²⁰<n21. The electron drift velocity v_d tends to become independent of E at high E in the gas, liquid, and solid phases. The ’’plateau’’ drift velocity was 1.0 km/s at n?2×10²¹, 1.2 at 5×10²¹, 3.0 in the normal liquid at 1.4×10²², and 5.5 km/s in the solid at 1.6×10²² molecule/cm³ (Miller, Howe, and Spear). The velocity plateau in all phases is attributed to the steep rise in σ_ε on the high energy side of the Ramsauer–Townsend minimum. The maxima in μ vs E curves for heavy ions are discussed.