Formation of Si_2, C_2, C^+_2 and N^+_2 by radiative association

Rate coefficients for the direct radiative association of silicon atoms to form Si_2, carbon atoms to form C_2, a carbon atom and a carbon ion to form the C^+_2 ion, and a nitrogen atom and a nitrogen ion to form the N^+_2 ion are estimated for temperatures in the range 300 to 14 700K. For Si_2, the rate coefficients (k_Si2) increase with an increase of temperature, and they can be expressed by k_Si2=2.19x10^-18 (T/300)^0.045exp (-258.79/T) cm^3 s^-1. At T≯7100K, the rate coefficient k_Si2 is ~2.5x10^-18 cm^3 s^-1 and is independent of temperature. The rate coefficients for C_2, C^+_2 and N^+_2 in the temperature range 300 to 14 700K are fitted to the relation where alpha, beta and gamma are: 4.36x10^-18 cm^3 s^-1, 0.35 and 161.31 for C_2, 4.01x10^-18 cm^3 s^-1, 0.17 and 101.52 for the C^+_2 ion, and 3.71x10^-18 cm^3 s^-1, 0.24 and 26.12 for the N^+_2 ion, respectively. In each case, the rate coefficients increase with an increase of temperature.