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Probing Single Molecule Chemistry with a Femtosecond Laser Scanning Tunneling Microscope

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The goal of the studies presented in this dissertation is to continuously expand the capability of a scanning tunneling microscope (STM) by improving its chemical sensitivity and temporal resolution. It has been demonstrated that the combination of STM with other techniques gains insight into the physical and chemical properties of single molecules. Single molecule rotational spectroscopy and microscopy is demonstrated using STM inelastic tunneling spectroscopy (IETS). We conduct real-space measurements of rotational transitions of gaseous hydrogen molecules physisorbed on surfaces at 10 K. The j=0 to j=2 rotational transition for para-H2 and HD were observed by STM-IETS. It is also found that the rotational energy is very sensitive to its local environment, we could precisely investigate how the environmental coupling modifies the structure, including the bond length, of a single molecule with sub-Ångström resolution. Due to this high sensitivity, the spatial variation in the potential energy surface can be quantified by the rotational and vibrational energies of the trapped H2. The ability of the tip to drag along a hydrogen molecule as it scans over another adsorbed molecule combined with the sensitivity of the hydrogen rotational excitation recorded by IETS to its immediate environment lead to the implementation of rotational spectromicroscopy, which helps us reveal the intermolecular interaction and charge transfer between H2 and a large molecule. Furthermore, we demonstrate that the H2 in the STM junction can be dissociated by the mechanical motion of the STM tip. Hydrogen rotational spectroscopy and microscopy provides novels approach toward visualizing and quantifying the local potential energy surface as well as the potential landscape of chemical reactions.

Joint Ångström-femtosecond resolution is achieved by the combination of an STM with a femtosecond laser. We demonstrate the bond-selected, photo-assisted activation of a single C-H bond in an azulene molecule adsorbed on a Ag(110) surface. When the junction is illuminated by femtosecond laser, the electrons in the tip can be photo-excited into higher energy states and dissociate the molecule through a photo-assisted tunneling process. The photon-electron coupling at the junction enables the investigation of coherence molecular transformation with joint fs-Å resolution. We also show the band bending and laser induced band flattening at a molecule-semiconductor interface. More importantly, we observe the photo-induced, reversible conformational change between two structures for a single pyrrolidine molecule on a Cu(001) surface. The conductance changes of the STM junction associated with the structural transitions exhibits oscillates in time with periods corresponding to specific molecular vibrations. The vibrational frequencies and decay time are observed in real-space and real-time. Our laser-STM technique enables the investigation of inhomogeneous environmental effect on the molecular dynamics. We have found that the intermolecular interaction between two pyrrolidine molecules can increase the vibrational period while shortening its decay time. We anticipate that this novel technique would lead to a broad impact in physics and chemistry through direct visualization of coherently driven reactions resolved in space and time.

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