  Antiproton proton annihilation reactions allow unique access to the moduli
and phases of nucleon electromagnetic form factors in the time like region. We
present the helicity amplitudes for the unequal mass single photon reaction $p
\bar{p}\to l^+ l^-$ in the s channel including the lepton mass. The relative
signs of these amplitudes are determined using simple invariance properties.
Helicity amplitudes for the annihilation reaction $p \bar{p} \to N \bar{N}$ are
also given, where $N$ is any spinor particle with structure. Crossing relations
between the $e p \to e p$ scattering and the $p \bar{p}\to l^+ l^-$
annihilation channels are discussed and the crossing matrix for the helicity
amplitudes is given. This matrix may be used to verify known expressions for
the space like helicity amplitudes due to one photon exchange.
