Centre-of-mass corrections on atomic binding energies in a magnetic field

Centre-of-mass corrections on the total energies of general atomic systems are investigated in the presence of a homogeneous magnetic field. Except for an accidental symmetry of the neutral hydrogen atom, the internal good quantum numbers are not conserved when the system dissociates into an atomic subsystem and an electron. Threshold energies can therefore easily be defined from the ground-state energy of the subsystem. The general form of binding energies with respect to single-electron ionisation is then established as a function of binding energies obtained in an approximation where the nucleus mass is considered as infinite. A transverse collective motion of the atomic system is shown to reduce the binding energies, especially in the case of negative ions. Cyclotron excitation and mass polarisation effects may also significantly affect these energies. Expressions for ionisation energies of one-electron and two-electron atoms are derived as examples. A detailed quantitative study of the H- and He atoms in very strong magnetic fields is presented. The H- binding energies are very sensitive to centre-of-mass effects. In particular, the H- bound spectrum becomes finite, in opposition to its properties in the infinite-proton-mass approximation. (author)