Transient species in supercritical carbon dioxide. Does the dimer anion form in SC CO2?

The ion-molecule reactions in supercritical CO2 have been investigated using the pulse radiolysis technique. Absorption spectra measured in a visible wavelength region suggest that, at least, two transient species exist. We have previously reported that one species is (CO2)2+. This assignment was made by comparing the spectrum with the photodissociation cross section of (CO2)2+. In near UV region, we observed relatively long-lived transient species that is different from the dimer cation. We tentatively assigned this species to a dimer anion (CO2)2-. We measure reation rates of (CO2)2+ and (CO2)2- with several molecules. (author)