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

Abstract

Time-resolved dc photoconductivity has been used to observe quasifree electrons and solvent radical anions in supercritical CO2 (ρc ≈ 0.468 g/cm3; T c ≈ 31 °C). The electrons have lifetime τe < 200 ps and mobility μe > 10 cm2/(V s). For ρ/ρc > 0.64, the product μeτe rapidly increases with the solvent density ρ, reaching 2.5 × 10-9 cm2/V at ρ/ρc ≈ 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 (CO2) 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 (EAg) is greater than 0.4 eV. At T = 41 °C and ρ/ρc ≈ 1.77, the rates of the electron attachment and solvent anion scavenging correlate with each other and the solute EAg. Only solutes with EAg > 2 eV exhibit diffusion-controlled kinetics. For ρ/ρc > 0.85, the scavenging radii of these diffusion-controlled reactions decrease with ρ. For O2, the scavenging of the solvent anion is reversible (ΔH° ≈ −0.44 eV).

Multimer Radical Ions and Electron/Hole Localization in Polyatomic Molecular Liquids: A Critical Review

“…For reduced solvent density ρ r > 1.2 (defined as the ratio of the solvent density ρ and the critical density ρ c = 0.468 g/cm 3 ) the product ρ r µ e τ e exponentially increases with ρ r . The onset for the formation of rapidly migrating quasifree electrons coincides with the emergence of the CB in the solvent [18]. Exactly the same behavior was observed by Holroyd and co-workers for sc saturated hydrocarbons.…”

“…If the lifetime were this short, the electrons would contribute negligibly either to the conductivity or the product formation. However, this extrapolation is not supported by experiment [18,20]. In low-temperature solid matrices (e.g., Ne and Ar at 5-11 K), the CO 2 -monomer, (CO 2 -)(CO 2 ) n multimer anion (n = 1, 2), and the C 2 O 4 -dimer were observed using IR spectroscopy; the latter species was the only one stable at 25-31 K [26].…”

“…At a given temperature, the ion mobility decreases rapidly with ρ r for 0.2 < ρ r <1 and then decreases slowly for 1 < ρ r < 2 [18,21]. In contrast to this behavior, the mobility of the solvent anion exponentially increases with ρ r , being 2-10 times greater than the mobilities of all other ions in sc CO 2 [18]. The activation energy of the solvent anion migration is 0.46 eV (for constant ρ r ) whereas for the solute ions (Fig.…”

“…1/12/04 conduction band (CB) electron and a rapidly hopping multimer anion (a self-trapped electron) [18,20]. Both of these species exhibit unusual properties that account for many oddities observed in radiolysis of sc CO 2 .…”

“…Both of these species exhibit unusual properties that account for many oddities observed in radiolysis of sc CO 2 . Quasifree electrons in sc CO 2 have lifetime τ e < 200 ps and mobility µ e > 100 cm 2 /Vs [18]. For reduced solvent density ρ r > 1.2 (defined as the ratio of the solvent density ρ and the critical density ρ c = 0.468 g/cm 3 ) the product ρ r µ e τ e exponentially increases with ρ r .…”