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Transient absorption spectra following ionization of supercritical CO2 have been investigated using the pulse radiolysis technique. Absorption spectra measured from 400 to 800 nm suggest that at least two transient species absorb. We have previously reported that one species is (CO2)2 +. In the near UV region, we observed a transient species of which the lifetime and reactivity are different from the dimer cation. We assign this species to a dimer anion, (CO2)2 -, or an anion−molecule complex, (CO2 -)(CO2) x . Comparison with the photobleaching of CO2 anion clusters in solid rare gas matrixes and their reactivity with H2 and O2 confirm the assignment. Theoretical calculations, in which solvation is taken into account, are consistent with these assignments. It is well-established that the adiabatic electron affinity of CO2 is negative, but the adiabatic electron affinity of CO2 dimer has been calculated to be 0.89 eV for D 2 d symmetry (CO2)2 - in the gas phase. The calculations predict that CO2 - in a model continuum solvent is stable to autodetachment.

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