  Electron stimulated desorption of cyclopentene from the Si(100)-2x1 surface
is studied experimentally with cryogenic UHV STM and theoretically with
transport, electronic structure, and dynamical calculations. Unexpectedly for a
saturated hydrocarbon on silicon, desorption is observed at bias magnitudes as
low as 2.5 V, albeit the desorption yields are a factor of 500 to 1000 lower
than previously reported for unsaturated molecules on silicon. The low
threshold voltage for desorption can be attributed to hybridization of the
molecule with the silicon surface, which results in low-lying ionic resonances
within 2-3 eV of the Fermi level. These resonances are long-lived, spatially
localized and displaced in equilibrium with respect to the neutral state,
resulting, upon excitation, in symmetric (positive ion) or asymmetric (negative
ion) motion of the silicon dimer atoms. This study highlights the importance of
nuclear dynamics in silicon-based molecular electronics and suggests new
guidelines for the control of such dynamics.
