Electron and cation transport in gaseous carbon dioxide: Density and temperature effects

In low density CO2 gas, the density normalized mobility of electrons is nμe =1.88×1024 molecule/m V s, independent of temperature. It is an order of magnitude lower than the value in ethene, a hydrocarbon of similar molecular size and anisotropic polarizability. Scattering of thermal electrons by carbon dioxide molecules might involve a transient anion state. At densities n>5×1025 molecule/m3 along the vapor/liquid coexistence curve, electron attachment to molecular clusters occurs, e−+(CO2)q⇄(CO2)−q, where q≊6. At n>14×1025 molecule/m3 in the coexistence vapor the negative and positive charges have the same mobility, so the electrons are permanently attached. Electron detachment can occur when the gas is heated at constant density; heating decreases the average cluster size and provides activation energy for detachment. At a constant density ≥30×1025 molecule/m3 the value of nμe can be changed 1000-fold by changing T. At n=31 and 56 (1025 molecule/m3) the values of nμe for quasifree (unattached) electrons are 1.65 and 1.50 (1024 molecule/m V s), respectively. The equilibrium between attachment and detachment has ΔH0a =−90 kJ/mol and ΔS0a =−270 J/mol K. The value of nμ+ of the cations in the coexistence vapor is larger at temperatures below the triple point than above it. Perhaps the clustered ions also undergo a structure transition near the triple point. Positron attachment to (CO2)q clusters occurs at similar temperatures and densities as electron attachment.