  Creation of charged massless fermion pair by a photon in external constant
crossed electromagnetic field is considered. For this we use the expression of
elastic scattering amplitude (EAS) of photon in the one-loop approximation of
massive quantum electrodynamics obtained earlier and calculate its massless
limit. We assume that the imaginary part of EAS of photon describes the total
probability of charged massless fermion pair creation in external
electromagnetic field. Photon emission by a charged massless fermion is also
studied in constant crossed electromagnetic field. We obtain the total
probability of photon emission calculating elastic scattering amplitude of
charged massive fermion in the electromagnetic field in the massless limit.
