  Raman scattering in the quarter-filled spin ladder system alpha'-NaV_2O_5
shows in the dimerized singlet ground state ($T \leq T_{SP}=35K$) an unexpected
sequence of three magnetic bound states. Our results suggest that the recently
proposed mapping onto an effective spin chain for $T > T_{SP}$ has to be given
up in favor of the full topology and exchange paths of a ladder in the
dimerized phase for $T < T_{SP}$. As the new ground state we propose a dynamic
superposition of energetically nearly degenerate dimer configurations on the
ladder.
