Electron transport in low density alkane gases: Effects of chain length and flexibility

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Abstract
The Ramsauer–Townsend (RT) minimum in the electron momentum transfer cross section (σm) occurs at the same energy (ξRT=0.12±0.01 eV) in all n-alkanes from C2H6 to n-C10H22. The lack of dependence on chain length indicates that the electron being scattered interacts with a chain segment that contains only two or three carbon atoms. The value of ξRT increases with increasing sphericity of the molecules, being 0.17 eV for i-butane, 0.22 eV neo-pentane, and 0.25 eV for methane. At high fields the electron drift velocity attains a ‘‘saturation’’ value (vsat) in the C1 to C5 hydrocarbons: 100 km/s in CH4, 54 km/s in C2H6, 51 km/s in C3H8, 49 km/s in n-C4H10, and 43 km/s in n-C5H12. Increasing the molecular sphericity increases vsat: 55 km/s in i-C4H10 and 57 km/s in neo-C5H12, to be compared with the values 49 and 43 in the n-isomers, and 100 km/s in CH4. The value of σm averaged over the thermal velocity distribution, σav, is an order of magnitude smaller than that expected from the simple polarization interaction, and is similar in magnitude to the physical cross section of the molecule. The scattering cross sections of the deuterated methanes CHxD4−x, x=0–3, are the same as those of CH4, in the low density gases.