The ultrafast non-radiative relaxation processes between electronic states of Hg2 trapped in a cryogenic argon matrix after photon excitation at 266 nm is studied by mol. dynamics with quantum transitions (MDQT). Population transfer by matrix-induced couplings and vibrational energy relaxation compete on the same time scale during the dynamics. In spite of multiple nonadiabatic crossing events between the 14 different electronic excited states involved in the dynamics, our simulations show that the vibrational coherence of the Hg2 oscillator is retained during the first few picoseconds. These results demonstrate that vibrational coherence of a solute can be preserved even after several solvent-induced curve-crossing events. [on SciFinder (R)]