  We have studied the radiative decay of the 2s state of one-electron and
one-muon ions, where the two-photon mechanism plays an important role. Due to
the nuclear size corrections the radiative decay of the 2s state in the
electron and muon ions is qualitatively different. Based on the accurate
relativistic calculation, we introduced a two-parameter approximation, which
makes it possible to describe the two-photon angular-differential transition
probability for the polarized emitted photons with high accuracy. The emission
of photons with linear and circular polarizations was studied separately. We
also investigated the transition probabilities for the polarized initial and
final states. The investigation was performed for ions with atomic numbers 1 <
Z < 120.
