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

Abstract

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 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.

Refined theoretical study of radiative association: Cross sections and rate constants for the formation of SiN

“…The rate coefficient for the → transition increases steadily up to a maximum of 1.65 × 10 −19 cm 3 /s around 1500 K. The rate for → displays slow oscillations and its highest value is 1.01 × 10 −20 at about 80 K. Both rates decrease rapidly at temperatures above ∼2000 K as a consequence of the drop in the cross sections. The semiclassical calculation by Singh et al 10 for → is also displayed and it shows no drop at high temperature, as expected since it is based on the unrestricted transition probability. We also see a difference between their result and our direct component (green curve) at low temperatures.…”

“…In fact the bound state condition on the transition probabilities in Eqs. (10) and (14) assumes no penetration of the potential so the semiclassical treatment is more consistent in this respect.…”

“…Also shown is the magnitude of the difference between them (blue), and the X 2 + ↔ A 2 transition dipole moment D(R) (pink). The data is taken from Singh et al 10 transitions from an upper to a lower electronic state. This theory can thus not be applied to, e.g., X 2 + →X 2 + transitions.…”

“…Reaction (1) has been studied semiclassically by Singh et al 10 We use the same ab initio data as Singh et al for the potential energy curves and transition dipole moments, which they calculated at multireference configuration interaction (MRCI) level with correlated consistent polarized valence quadrupole zeta (cc-pVQZ) 11, 12 basis functions.…”

Dicarbon Formation in Collisions of Two Carbon Atoms

“…of rate coefficients corresponding to semi-classical cross sections (using different molecular data) were available for comparison (Dalgarno et al 1990;Singh et al 2002). Using the quantum-mechanical theory [our Eq.…”