  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.
