Fine-structure excitation of C+ and Si+ by atomic hydrogen

We present calculations of cross sections for fine-structure excitation in collisions of carbon and silicon ions in the P-2 state with atomic hydrogen in the ground state. The results are based on accurate calculations of CH+ and SiH+ molecular potentials, including electronic core correlation and relativistic effects. We find that the energy dependence of the excitation cross sections is largely determined by shape resonances. Our work improves on the results of previous calculations with less accurate potentials. Analytical expressions for the cooling efficiency of C+(P-2(1/2)) and Si+(P-2(1/2)) are given for the temperature interval 15 - 2000 K.