  Vibronic effects in resonant electron transport through single-molecule
junctions are analyzed. The study is based on generic models for molecular
junctions, which include electronic states on the molecular bridge that are
vibrationally coupled and exhibit Coulomb interaction. The transport
calculations employ a master equation approach. The results, obtained for a
series of models with increasing complexity, show a multitude of interesting
transport phenomena, including vibrational excitation, rectification, negative
differential resistance (NDR) as well as local cooling. While some of these
phenomena have been observed or proposed before, the present analysis extends
previous studies and allows a more detailed understanding of the underlying
transport mechanisms. In particular, it is shown that many of the observed
phenomena can only be explained if electron-hole pair creation processes at the
molecule-lead interface are taken into account. Furthermore, vibronic effects
in sytems with multiple electronic states and their role for the stability of
molecular junctions are analyzed.
