Inelastic Mg plus H collision processes at low energies

International audienceFull 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(3s(2) (1)S(0)), Mg(3s3p (3)P) and Mg(3s3p (1)P). The calculations are based on adiabatic potentials and nonadiabatic couplings for the three low-lying (2)Sigma(+) and the first two (2)Pi states, calculated using large active spaces and basis sets. Non-adiabatic regions associated with radial couplings at avoided ionic crossings in the (2)Sigma(+) 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