Positronium in crossed electric and magnetic fields: the existence of a long-lived ground state

It was earlier reported [PRL 78 199, (1997)] that long-lived excited states of positronium can be formed in crossed electric and magnetic fields at laboratory field strengths. Unlike the lower-lying states that are localized in the magnetically distorted Coulomb well, these long-lived states which can possess a lifetime up to many years are localized in an outer potential well that is formed for certain values of the pseudomomentum and magnetic field. The present work extends the original analysis and studies the dependence of the spectrum as a function of field strength and pseudomomentum over a wide range of parameters. We predict that in the limit of large pseudomomentum, the ground state of positronium atom in a magnetic field will become delocalized; for strong fields, the binding energy of this state is quite large, resulting in a ground state that is both stable against direct annihilation and against ionization by low frequency background radiation.