  Probing optical Stark effect at the single-molecule or atomic scale is
crucial for understanding many photo-induced chemical and physical processes on
surfaces. Here we report a study about optical Stark effect of single atomic
defects on TiO2(110) surface with photo-assisted scanning tunneling
spectroscopy. When a laser is coupled into the tunneling junction, the mid-gap
state of OH-O2 defects changes remarkably in the differential conductance
spectra. As laser power gradually increases, the energy of the mid-gap state
shifts away from the Fermi level with increase in intensity and broadening of
peak width. The observation can be explained as optical Stark effect with the
Autler-Townes formula. This large optical Stark effect is due to the
tip-enhancement and the strong dipole moment in the transient charged state
during electron tunneling. Our study provides new aspects in exploring
electron-photon interactions at the microscopic scale.
