Transient Negative Species in Supercritical Carbon Dioxide:  Electronic Spectra and Reactions of CO₂ Anion Clusters

Transient absorption spectra following ionization of supercritical CO 2 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 (CO 2 ) 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, (CO 2 ) 2 -, or an anion−molecule complex, (CO 2 - )(CO 2 ) x . Comparison with the photobleaching of CO 2 anion clusters in solid rare gas matrixes and their reactivity with H 2 and O 2 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 CO 2 is negative, but the adiabatic electron affinity of CO 2 dimer has been calculated to be 0.89 eV for D 2 d symmetry (CO 2 ) 2 - in the gas phase. The calculations predict that CO 2 - in a model continuum solvent is stable to autodetachment.