Non-Adiabatic Quantum Molecular Dynamics: Competing Reaction Mechanisms in Atom-Cluster Collisions Ulf Saalmann a) and Rüdiger Schmidt b) a) Atomfysik, Stockholms Universitet, Frescativägen 24, 10405 Stockholm, Sweden us@atom.msi.se b) Institut für Theoretische Physik, Technische Universität, 01062 Dresden, Germany schmidt@physik.tu-dresden.de Atom-cluster collisions are studied by means of a simultaneous and self-consistent treatment of electronic and atomic degrees of freedom involved. We examine the excitation mechanisms (electronic versus vibrational) and the related relaxation phenomena (phase transitions, fragmentation) for impact energies from eV to MeV. "Cluster transparency" is found in a small window of energies. In addition, the charge transfer competing with fragmentation in collisions with sodium clusters is investigated.