Inelastic Mg+H collision processes at low energies

Abstract

Full quantum scattering calculations of cross sections for low-energy near-threshold inelastic Mg + H collisions are reported, such processes being of interest for modelling of Mg spectral lines in stellar atmospheres. The calculations are made for three transitions between the ground and two lowest excited Mg states, Mg(3s2 1S0), Mg(3s3p 3P) and Mg(3s3p 1P). The calculations are based on adiabatic potentials and nonadiabatic couplings for the three low-lying 2Σ+ and the first two 2Π states, calculated using large active spaces and basis sets. Non-adiabatic regions associated with radial couplings at avoided ionic crossings in the 2Σ+ molecular potentials are found to be the main mechanism for excitation. Cross sections of similar order of magnitude to those obtained in Li + H and Na + H collisions are found. This, together with the fact that the same mechanism is important, suggests that as has been found earlier for Li and Na, processes such as ion pair production may be important in astrophysical modelling of Mg, and motivates continued study of this system including all states up to and including the ionic limit.