Laser ablation of transition metal targets with concurrent 11 to 12 K condensation of CO2-Ar mixtures produces a sharp metal independent infrared absorption at 1657.0 cm^-1 due to CO2^-, which is formed from the capture of ablated electrons by CO2 molecules during the condensation process. Two additional metal independent absorptions are produced at 1856.7 and 1184.7 cm^-1 on matrix annealing to 25 K to allow diffusion and reaction of CO2 and CO2^-. Isotopic substitution (¹³CO2, C¹⁸O2, C^16,18O2 , and mixtures) shows that these two vibrations involve two equivalent CO2 subunits. The excellent agreement with frequencies, intensities, and isotopic frequency ratios from density-functional calculations supports assignment to the symmetrical C2O4^- anion with D2d symmetry. Photodissociation (470-580 nm) and failure to observe these absorptions in identical experiments doped with the electron trapping molecule CCl4 further support the molecular anion assignments. Although absorptions were observed for weak (CO2) (CO2^-) complexes, no evidence was found for the asymmetric O2C . OCO^- molecule-anion complex characterized by calculations.