https://doi.org/10.1140/epjd/e2005-00222-6
Single molecule vibrationally mediated chemistry*
Towards state-specific strategies for molecular handling
Institut für Experimentalphysik, Freie Universität Berlin
Arnimallee 14, 14195 Berlin, Germany
Corresponding author: a pascual@physik.fu-berlin.de
Received:
4
April
2005
Revised:
26
May
2005
Published online:
9
August
2005
Tunnelling electrons may scatter inelastically with an adsorbate, releasing part of their energy through the excitation of molecular vibrations. The resolution of inelastic processes with a low temperature scanning tunnelling microscope (STM) provides a valuable tool to chemically characterize single adsorbates and their adsorption mechanisms. Here, we present a molecular scale picture of single molecule vibrational chemistry, as resolved by STM. To understand the way a reaction proceed it is needed knowledge about both the excitation and damping of a molecular vibration. The excitation is mediated by the specific coupling between electronic molecular resonances present at the Fermi level and vibrational states of the adsorbate. Thus, the two-dimensional mapping of the inelastic signal with an STM provides the spatial distribution of the adsorbate electronic states (near the Fermi level) which are predominantly coupled to the particular vibrational mode observed. The damping of the vibration follows a competition between different mechanisms, mediated via the creation of electron-hole pairs or via anharmonic coupling between vibrational states. This latter case give rise to effective energy transfer mechanisms which eventually may focus vibrational energy in a specific reaction coordinate. In this single-molecule work-bench, STM provides alternative tools to understand reactivity in the limit of low excitation rate, which demonstrate the existence of state-specific excitation strategies which may lead to selectivity in the product of a reaction.
PACS: 68.37.Ef – Scanning tunneling microscopy (including chemistry induced with STM) / 68.43.-h – Chemisorption/physisorption: adsorbates on surfaces / 63.22.+m – Phonons or vibrational states in low-dimensional structures and nanoscale materials
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag, 2005