Metastable Electrons, High-Mobility Solvent Anions, and Charge Transfer Reactions in Supercritical Carbon Dioxide

Time-resolved dc photoconductivity has been used to observe quasifree electrons and solvent radical anions in supercritical CO 2 (ρ c ≈ 0.468 g/cm 3; T c ≈ 31 °C). The electrons have lifetime τ e < 200 ps and mobility μ e > 10 cm 2 /(V s). For ρ/ρ c > 0.64, the product μ e τ e rapidly increases with the solvent density ρ, reaching 2.5 × 10 -9 cm 2 /V at ρ/ρ c ≈ 1.82. The mobility of the solvent anions exponentially increases with ρ, being 2−10 times higher than the mobilities of other ions. The activation energy of the solvent anion migration is 0.46 eV, whereas for solute ions, this energy is less than 20 meV. Electron detachment upon 1.0−3.5 eV photoexcitation of the solvent anion has been observed. The cross section of photodetachment linearly increases with photon energy above 1.76 eV end exhibits a shoulder at 2.82 eV; the spectrum is similar to photoelectron spectra of (CO 2 ) n - clusters ( n = 6−9) in the gas phase. Both the electrons and solvent anions react with nonpolar electron acceptor solutes whose gas-phase electron affinity (EA g ) is greater than 0.4 eV. At T = 41 °C and ρ/ρ c ≈ 1.77, the rates of the electron attachment and solvent anion scavenging correlate with each other and the solute EA g . Only solutes with EA g > 2 eV exhibit diffusion-controlled kinetics. For ρ/ρ c > 0.85, the scavenging radii of these diffusion-controlled reactions decrease with ρ. For O 2, the scavenging of the solvent anion is reversible (Δ H ° ≈ −0.44 eV).