  Nuclear stopping in the heavy ion collisions over a beam energy range from
SIS, AGS up to SPS is studied in the framework of the modified UrQMD transport
model, in which mean field potentials of both formed and "pre-formed" hadrons
(from string fragmentation) and medium modified nucleon-nucleon elastic cross
sections are considered. It is found that the nuclear stopping is influenced by
both the stiffness of the equation of state and the medium modifications of
nucleon-nucleon cross sections at SIS energies. At the high SPS energies, the
two-bump structure is shown in the experimental rapidity distribution of free
protons, which can be understood with the consideration of the "pre-formed"
hadron potentials.
