A b i n i t i o study of the locations and intensities of the lowest-lying electronic transitions of the C3 and C2O molecules

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Abstract
A series of ab initio SCF and CI calculations is reported for the C3 and C2O molecules in their lowest excited states. A large AO basis containing two primitive d functions on each atom and s- and p-type bond functions at the center of each bond is employed for C3 and CI secular equations of order 10 000 are solved based on a well-known configuration selection procedure. The location of the A 1Πu–X 1Σ+g transition is computed within an error of 0.07 eV relative to experiment and an oscillator strength of 0.052 is obtained for absorption, corresponding to a radiative lifetime for the A 1Πu state of 95 ns. The C 1Πg is found to lie only 0.838 eV above the A 1Πu state with a computed lifetime of 0.938 μs. Analogous calculations for the C2O molecule are carried out employing only a single primitive d function on each atom and geometrical optimization is undertaken for the (linear) X 3Σ− ground state. The A 3Πi, 1Σ+, 1Δ, and lowest two 1Π states are treated at the ground state equilibrium conformation, with the A–X transition energy again found to agree well with available experimental data; the lifetime of the A 3Πi state is calculated to be 0.773 μs. The relationship of the calculated results to problems of astrophysical interest is also discussed.