Metastable electrons, high-mobility solvent anions, and charge transfer reactions in supercritical carbon dioxide

Time-resolved de photoconductivity has been used to observe quasifree elect rons and solvent radical anions in supercritical CO2 (rho (c) approximate t o 0.468 g/cm(3); T-c approximate to 31 degreesC). The electrons have lifeti me tau (e) < 200 ps and mobility mu (e), > 10 cm(2)/(V s). For rho/rho (c) > 0.64, the product mu (e)tau (e) rapidly increases with the solvent densit y rho, reaching 2.5 x 10(-9) cm(2)/V at rho/rho (c) approximate to 1.82. Th e mobility of the solvent anions exponentially increases with p, being 2-10 times higher than the mobilities of other ions. The activation energy of t he 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 o f the solvent anion has been observed. The cross section of photodetachment Linearly increases with photon energy above 1.76 eV end exhibits a shoulde r 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 anio ns react with nonpolar electron acceptor solutes whose gas-phase electron a ffinity (EA(g)) is greater than 0.4 eV. At T = 41 degreesC and rho/rho (c) approximate to 1.77, the rates of the electron attachment and solvent anion scavenging correlate with each other and the solute EA,. Only solutes with EA(g) > 2 eV exhibit diffusion-controlled kinetics. For pip, r 0.85, the s cavenging radii of these diffusion-controlled reactions decrease with rho. For O-2, the scavenging of the solvent anion is reversible (DeltaH degrees approximate to -0.44 eV).