  The dynamics of a molecular junction consisting of a PTCDA molecule between
the tip of a scanning tunneling microscope and a Ag(111) surface have been
investigated experimentally and theoretically. Repeated switching of a PTCDA
molecule between two conductance states is studied by low-temperature scanning
tunneling microscopy for the first time, and is found to be dependent on the
tip-substrate distance and the applied bias. Using a minimal model Hamiltonian
approach combined with density-functional calculations, the switching is shown
to be related to the scattering of electrons tunneling through the junction,
which progressively excite the relevant chemical bond. Depending on the
direction in which the molecule switches, different molecular orbitals are
shown to dominate the transport and thus the vibrational heating process. This
in turn can dramatically affect the switching rate, leading to non-monotonic
behavior with respect to bias under certain conditions. In this work, rather
than simply assuming a constant density of states as in previous works, it was
modeled by Lorentzians. This allows for the successful description of this
non-monotonic behavior of the switching rate, thus demonstrating the importance
of modeling the density of states realistically.
