  Understanding current-induced bond rupture in single-molecule junctions is
both of fundamental interest and a prerequisite for the design of molecular
junctions, which are stable at higher bias voltages. In this work, we use a
fully quantum mechanical method based on the hierarchical quantum master
equation approach to analyze the dissociation mechanisms in molecular
junctions. Considering a wide range of transport regimes, from off-resonant to
resonant, non-adiabatic to adiabatic transport, and weak to strong vibronic
coupling, our systematic study identifies three dissociation mechanisms. In the
weak and intermediate vibronic coupling regime, the dominant dissociation
mechanism is stepwise vibrational ladder climbing. For strong vibronic
coupling, dissociation is induced via multi-quantum vibrational excitations
triggered either by a single electronic transition at high bias voltages or by
multiple electronic transitions at low biases. Furthermore, the influence of
vibrational relaxation on the dissociation dynamics is analyzed and strategies
for improving the stability of molecular junctions are discussed.
