  Using inelastic electron tunneling spectroscopy with the scanning tunneling
microscope (STM-IETS) and density functional theory calculations (DFT), we
investigated properties of a single H2 molecule trapped in nanocavities with
controlled shape and separation between the STM tip and the Au (110) surface.
The STM tip not only serves for the purpose of characterization, but also is
directly involved in modification of chemical environment of molecule. The bond
length of H2 expands in the atop cavity, with a tendency of dissociation when
the gap closes, whereas it remains unchanged in the trough cavity. The
availability of two substantially different cavities in the same setup allows
understanding of H2 adsorption on noble metal surfaces and sets a path for
manipulating a single chemical bond by design.
