  The ultrafast response to an optical pulse excitation of the spin-spin
exchange interaction in transition metal anti-ferromagnets is studied within
the framework of the time-dependent spin-density functional theory. We propose
a formulation for the purely dynamical exchange interaction, which is non-local
in space, and it is derived starting from {\it ab initio} arguments. Then, we
investigate the effect of the laser pulse on the onset of the dynamical
process. It is found that we can distinguish two types of excitations, both
activated immediately after the action of the laser pulse. While the first one
can be associated to a Stoner-like excitation and involves the transfer of spin
from one site to another, the second one is related to the ultrafast
modification of a Heisenberg-like exchange interaction and can trigger the
formation of spin waves in the first few hundred femtoseconds of the time
evolution.
