  As a step towards a more realistic modeling of vibrations in single-molecule
devices, we investigate the effects of charge-dependent vibrational frequencies
and anharmonic potentials on electronic transport. For weak phonon relaxation,
we find that in both cases vibrational steps split into a multitude of
substeps. This effectively leads to a bias-dependent broadening of vibrational
features in current-voltage and conductance characteristics, which provides a
fingerprint of nonequilibrium vibrations whenever other broadening mechanisms
are secondary. In the case of an asymmetric molecule-lead coupling, we observe
that frequency differences can also cause negative differential conductance.
