Vibrational effects in the photoionization shape resonance leading to the $C{}_{}{}^2{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$ state of C${\mathrm{O}}_2^{+}$

The effect of vibrational averaging on the photoionization cross section and photoelectron asymmetry parameter for photoionization of C${\mathrm{O}}_2$ leading to the $C{}_{}{}^2{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$ state of C${\mathrm{O}}_2^{+}$ has been studied. We have computed the photoionization continuum wave function in the frozen-core Hartree-Fock approximation using the iterative Schwinger variational method. Averaging over the symmetric stretching mode of C${\mathrm{O}}_2$ reduced the peak resonant cross section by about 15%. This is in contrast to the earlier published results using the continuum multiple-scattering method where a much larger reduction in the peak cross section was obtained. A similar difference between these two theoretical methods was seen in their predicted asymmetry parameters. The present results have also been compared to available experimental data.