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 En>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 μT0.45. 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 σν=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, (En)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. (En)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 En 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. © 1979 The American Physical Society.