Resonance transfer of excitation in fast symmetric atomic collisions

We consider the transfer of excitation in fast symmetric atom-atom collisions. We show that the process can be very strongly influenced by the resonance coupling to the radiation field and that the reason for this lies in the very long range of the latter. Further, we demonstrate that, because of the Doppler effect, the role of this channel in the transfer of excitation in symmetric collisions decreases when the collision energy increases. Since the resonance coupling represents an effect, which does not appear in the nonrelativistic treatment, we observe in such collisions the interesting situation: relativistic effects can be very profound at nonrelativistic collision velocities v, where (v/c)(2)< 1 (c is the speed of light), but tend to disappear in the extreme relativistic limit v -> c.