Eighth Condensed Phase and Interfacial Molecular Science (CPIMS)

Program Scope:
Spectroscopic Imaging Toward Space-Time Limit
Wilson Ho
Department of Physics & Astronomy and Department of Chemistry
University of California, Irvine
Irvine, CA 92697-4575 USA
wilsonho@uci.edu
This project is concerned with the experimental challenge of reaching single molecule
sensitivity with sub-molecular spatial resolution in optical spectroscopy and photochemistry.
These experiments would lead to an understanding of the inner machinery of single molecules
that are not possible with other approaches. Results from these studies will provide the scientific
basis for understanding the unusual properties, processes, and phenomena in chemical and
physical systems at the nanoscale. The experiments rely on the combination of the unique
properties of lasers and scanning tunneling microscopes (STM). By using a low temperature
scanning tunneling microscope (STM) and optics in the nano-junction, it has become possible to
probe optical phenomena with sub-atomic resolution. Specific examples of such capabilities
include the spatial dependence of multi-electron induced light emission and photo-induced
desorption of single molecules, and the primary steps of electron transfer to a single molecule.
In the conversion of sun light to energy and in optoelectronics, a promising scheme involves the
use of nanoscale objects as the active media. The investigation of the fundamental mechanisms
of how light can be efficiently coupled to molecules and nanostructures not only can lead to new
scientific phenomena but also form the basis for new technology.
Recent Progress:
Bond-selective chemistry, which involves selectively breaking and forming specific
bonds, is one of the ultimate goals of chemistry. Achieving bond-selectivity in a complex
functionalized molecule is particularly important for advancing the molecule-based technologies,
such as molecular electronics, organic solar cells, and nanomachines. However, due to the rich
internal degrees of freedom and the intramolecular energy redistribution in functionalized
molecules, bond-selective control has so far been difficult to realize. We succeeded to induce a
sequence of target-selective bond dissociation (Fig. 1) and formation (Fig. 2) steps in a single
thiol-based π-conjugated molecule adsorbed on a NiAl(110) surface, using a scanning tunneling
microscope (STM). By locally injecting the energy-tunable tunneling electrons into the resonant
states derived from the functional groups in the molecule, we were able to selectively abstract
different functional groups from the molecule step by step and monitor the evolution of the
molecular electronic structure both in energy and real space at each reaction step. Furthermore,
the bond-selective dissociations allowed us to activate the sulfur functional groups and form
different types of Au-S bonds by manipulating and attaching a single gold atom to the sulfur
atom at each end of the molecule. The microscopic geometry of the Au-S bond at the single
molecule level and its influence on the electronic structure of the molecule were determined by
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