  In this work, we unveil the unique complex dynamics of multimode soliton
interactions in graded-index optical fibers through simulations and
experiments. By generating two multimode solitons from the fission of an input
femtosecond pulse, we examine the evolution of their Raman-induced red-shift
when the input pulse energy grows larger. Remarkably, we find that the output
red-shift of the trailing multimode soliton may be reduced, so that it
accelerates until it collides with the leading multimode soliton. As a result
of the inelastic collision, a significant energy transfer occurs between the
two multimode solitons: the trailing soliton captures energy from the leading
soliton, which ultimately enhances its red-shift, thus increasing temporal
separation between the two multimode solitons.
