Mg-H+ charge transfer and Mg line intensities in gaseous nebulae

Cross-sections have been computed for the charge-transfer reaction Mg(H+,M)Mg+. The electronic wavefunction was expanded as a linear combination of molecular orbitals, incorporating a common translation factor to account for the relative nuclear motion. In the domain of energies relevant to gaseous nebulae, charge transfer occurs into the 4s and 3p states of Mg+. Transfer into the 3p state proceeds via coupling to an intermediate molecular state, which correlates with H+Mg+(4s) at long range. As a consequence, there is pronounced 'resonance' structure in the variation of the charge-transfer cross-section with the collision energy. Rate coefficients are presented for thermal energies between 5000 and 30000K. The reaction is rapid enough to ensure that only very weak Mg I lines are seen in planetary nebulae. (author)