Photoionization of carbon dioxide

Abstract

The yields of photoions from CO₂ cooled to 150 °K have been measured at a resolution of 0.22 Å for CO₂⁺ and 0.4 Å for the O⁺ and CO⁺ fragments, in the photon energy region extending from onset for each species to approximately 20 eV. Most of the observed structure of the molecular ion yield curve recapitulates the well known features of the absorption spectrum, but a number of anomalies are reported. Autoionization peaks at photon energies just above the first molecular ionization limit of 13.773 ± 0.002 eV do not fit into a pattern characteristic of Rydberg series converging to excited vibrational levels of the ion. Although the Tanaka‐Ogawa series and Henning's sharp series can be assigned as ns Rydberg series on the basis of quantum defects, the expected 3s members could not be observed in the present study. The most prominent feature of the O⁺ yield curve is a step at 19.39 eV, suggesting that the principal mechanism for production of this fragment is predissociation of ${\mathrm{CO}}_2^{+}{\mathrm{(C\hspace{0.17em}}}^2{\Sigma }_g^{+})$ in its ground vibrational state. Completeness of this predissociation is inferred, although the proposed explanation involves doublet‐quartet mixing at large bending angles. The structure of the O⁺ yield curve between 19.07 and 19.39 eV suggests that Rydberg states converging to ${\mathrm{CO}}_2^{+}\mathrm{(C)}$ also undergo predissociation, with spontaneous ionization, to produce this fragment ion. Evidence pertaining to production of CO⁺ fragments by predissociation of excited vibrational states of ${\mathrm{CO}}_2^{+}\mathrm{(C)}$ is also presented.