The radiative association of Si and N atoms, C and O atoms, and C atoms and S+ ions

Potential energy curves for the SiN.X2Σ+ and A2Πi electronic states, and the SiN(A2Πi--X2Σ+) transition moment function (TMF) are obtained at the MRCI level of theory, with a SA-CASSCF zero-order wavefunction. These potential energy curves and TMF are used to compute radiative association rate coefficients of Si and N atoms for temperatures in the range 300--14 700 K. Rate coefficients for radiative association of carbon and oxygen atoms for the formation of CO, and carbon atoms and sulphur ions for the formation of CS+, are also calculated for temperatures in the range 300--14 700 K. These radiative association rate coefficients (k) are fitted to the standard equation (1) where A=2.02×10−19 cm3 s−1, α=0.029 and β=83.9 for the SiN molecule. The corresponding values for the CO molecule are 3.09×10−17 cm3 s−1, 0.325 and 1629, and those for the CS+ ion are 2.01×10−18 cm3 s−1, 0.066 and 300.9. Radiative association of C and O atoms occurs with emissions of the first overtone band at 2.3 μm and the fundamental band at 4.6 μm found in the ejecta of Supernova 1987A and novae.