  Small fluctuations around a constant electric or constant magnetic field F
are analyzed in a theory with pseudo scalar [phi] with a coupling g[phi]F~F. It
is found that a magnetic external field leads to mass generation for the small
perturbations, while an electric field suffers from a tachyonic mass generation
in the case in which the field strength is higher than a critical value
(related to the pseudo scalar mass). The vacuum energy can be exactly evaluated
and it is found that an imaginary part is present when the external electric
field exceeds its critical value.
