Pentacene is a planar aromatic molecule that was first used in the context of high-resolution AFM to observe intramolecular bonding ( 16). Recently, there have been several breakthrough studies of molecular characterization using functionalized AFM ( 18– 22), but as yet, observed molecules on surfaces are usually planar and the tip-molecule junction of C═O⋅⋅⋅H–C has never been investigated systematically. This offers a tool to finally image internal molecular structure because the onset of the Pauli repulsion and the flexibility of a CO-functionalized tip offer a highly local probe of chemical reactivity ( 17), with the promise of directly studying the bonding of hydrogen. In atomic force microscopy (AFM), chemical reactivity has been explored in pioneering work on silicon surfaces ( 14, 15), but extending this to molecules and quantitative bond studies still required innovations in functionalized tip imaging ( 16). Although these studies paved the way for molecular chemistry and engineering, the long range of the molecular orbitals prevents the direct observation of a hydrogen atom in molecules. Later studies looked intensively into the chemistry of hydrogen in molecules ( 5– 11), including detection of the vibration of the C–H bond via inelastic tunneling current ( 12) and of the spin-split state induced by the C–H bond on graphene ( 13). Even in the early years of the technique, hydrogen adsorbed on a Si(100) surface was observed via the modulated electronic state of the hydrides ( 4). In contrast, the strong charge redistribution induced by hydrogenation is readily detectable in scanning tunneling microscopy (STM) measurements.
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