Density and temperature effects on electron mobility in fluid methane

The mobility μ electrons in methane at n<6×10²⁰ molecule/cm³ is field dependent at E/n>0.012 Td (1 Td = 1×{}10^-17 V cm²/molecule). The temperature dependence of the thermal-electron mobility in the low-density vapor is μn∝T0.45. The threshold field and temperature dependence are much lower than reported previously. In the electron energy region 0.002<ε<0.2 eV the momentum-transfer cross section of methane molecules is well approximated by σ_ν=0.014v^-1.9 cm². In the low-density gas the electron drift velocity vd becomes superlinear with field strength at threshold velocity vdᵗʰʳ=c₀, the speed of low-frequency sound. At n>6×10²⁰ the value of μn in the coexistence gas decreases slightly, while d(μn)dT, (E/n)ₜₕᵣ, and vdᵗʰʳc₀ increase. The effects are due to quasilocation of the electrons by density fluctuations in the dense gas. The entropy ΔS^' and enthalpy ΔH^' of activation of electron transport correlate with the structure factor $S(0)$ of the dense gas: ΔS^'S(0)=19 J/mol K and ΔH^'≈TΔS^' near the vapor-liquid coexistence region. At n>6×10²¹ there is a rapid increase in μn due to conduction-band formation. (E/n)ₜₕᵣ passes through a cusp >0.04 Td at n=8×10²¹, where dμdE changes sign, then decreases again to 0.01 Td at n>1.0×10²² molecule/cm³. The threshold E/n in the normal liquid is similar to that in the low-density gas. At low field strengths the electron energy is moderated mainly by elastic collisions in the low-density gas, and by inelastic collisions in the liquid.