Single Molecule Desorption and Dissociation
at Room Temperature

Bond-selective molecular manipulation is one of the frontiers of atomic manipulation with the STM. Traditionally such experiments are conducted in the stable, low temperature regime; room temperature manipulation is much more challenging. Here we demonstrate room temperature, bond selective manipulation (“molecular dissection”) in a polyatomic molecule - chlorobenzene (C6H5Cl) anchored to the Si(111)-7x7 surface by chemisorption. Recently we showed that the mechanism of electron (or hole) injection from the STM tip into the p* LUMO (or p HOMO) orbitals of the benzene ring leads to controlled molecular desorption beyond a threshold voltage of +2.5V (-1.5V) [1]. The desorption yield is linearly proportional to the STM junction current, indicating a one electron process. In this work we explore C-Cl bond dissociation in the chemisorbed chlorobenzene molecule. Detailed STM images allow us to identify the azimuthal orientation of the individual chlorobenzene molecules on the surface and thus to correlate the final location of the liberated chlorine “daughter” atom with the position and orientation of the parent molecule [2]. We identify Cl atoms up to 50Å from the parent molecules. We find that both the radial and azimuthal distributions of Cl atoms depend sensitively on the tunnelling current [3] and that a wide range of surface sites is populated by the (energetic) Cl atoms (probably anions). This behaviour can be explained in terms of an energetic, two-electron dissociation process, as implied by the measured quadratic dependence of the dissociation rate on tunnelling current. We propose a mechanism based on dissociative electron attachment (DA) of the “second” electron to a molecule vibrationally excited by the “first” electron [3]. Such a mechanism explains how one can overcome the symmetry barrier to C-Cl dissociation via electron attachment to the ring states.