  One- and two-photon transitions in the hydrogen atom are analytically
evaluated in the absence and in the presence of an external electric field. The
emission probabilities are different for the hydrogen and anti-hydrogen atoms
due to the existence of contributions, linear in electric field. The magnitude
of these contributions is evaluated within the nonrelativistic limit. The
Coulomb Green function method is applied. Different nonrelativistic "forms" for
the decay probabilities in combination with different gauge choices are
considered. The three-photon E1E1E1 2p-1s transition probability is also
evaluated and possible applications of the results are discussed.
