Quantum surface diffusion of vibrationally excited molecular dimers.

We consider the thermally activated quantum diffusion of a molecular dimer in a periodic surface potential by means of a time-dependent wave packet method. We show that the potential energy surface resulting from the interplay of intradimer and dimer-surface interactions can lead to resonant states and predict high tunneling probabilities at specific, below barrier, energies that depend also on the initial vibrational state of the dimer. For soft molecular bonds, we show that the chaotic dynamical regime of classical dimers is mirrored, in the quantum case, by the tunneling induced mixing of vibrational states. The knowledge of the transmission coefficient is used to formulate an approximate description of quantum thermal diffusion by defining an effective temperature-dependent activation energy that can be compared to the classical case.