  Single-molecule junctions are found to show anomalous spikes in dI/dV
spectra. The position in energy of the spikes are related to local vibration
mode energies. A model of vibrationally induced two-level systems reproduces
the data very well. This mechanism is expected to be quite general for
single-molecule junctions. It acts as an intrinsic amplification mechanism for
local vibration mode features and may be exploited as a new spectroscopic tool.
