Electron transport in liquids: Effect of unbalancing the sphere-like methane molecules by deuteration, and comparison with argon, krypton, and xenon

The mobility of electrons in liquid methane is reduced when D atoms are substituted for H atoms in CH4. The reduction is greatest in the least symmetrical system CH2D2. The mobilities decrease in the order CH4>CD4≈CHD3>CH3D>CH2D2. The fractional decrease is greatest at the density of the mobility maximum nμmax ≈11×1027 molecule/m3: μ(deuterated)/μ(CH4)=0.60 in CH2D2, 0.63 in CH3D, and 0.67 in CHD3 and CD4. In the low density liquid near the critical region (nc =6.1×1027 molecule/m3), the isotope effect is obscured by quasilocalization of the electrons in density fluctuations. The isotope effect is attributed to (1) disruption of the sphere-like symmetry of the methane molecule, which increases the potential fluctuations in the conduction band in the liquid, and (2) enhanced inelastic interaction of the electrons with the hindered rotation of the molecules in the liquid. Inelastic scattering is nearly negligible in the liquids xenon, krypton, and argon at nμmax, but it makes progressively larger contributions in methane and neopentane.