Knowledge Bank

High accuracy electronic structure calculations (contracted MRCI wavefunctions with the quality of several $107$ uncontracted configurations) for the energies of $CO2$ in the low. est states in each of $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Sigma g+.1\Pi g,1\Delta u$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Sigma u-$ symmetries, as a function of the symmetric stretching coordinate, will be reported. The positions of the electronically excited states have been determined with an expected accuracy of about 0.05 eV. The geometries and the harmonic force constants for the symmetric stretch modes at the barriers to linearity for the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Sigma u-,1\Delta g$, states have been calculated. The results show that the electronically excited states $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Sigma u-$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Delta u$, of linear $CO2$ cross in the Franck-Condon region of the absorption spectrum with the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Pi g$, state. It has been found that the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Delta u$ state is separated by a barrier from the lowest dissociation asymptote and has a double minimum potential energy function. Implications for the interpretation of the absorption spectrum and the dynamics of the $CO2$ photodissociation will be discussed. P.J.Knowles and P.Rosmus: University Chemical Laboratory, University of Cambridge, Cambridge, U.K. H.J.Werner: Department of Chemistry, University of Bielefeld, Bielefeld, Germany