  Molecule-surface interaction is key to many physical and chemical processes
at interfaces. Here, we show that the dynamics of single molecules on a surface
under ultrahigh vacuum can be resolved using fluorescence imaging. By adapting
oil-immersion microscopy to a thin vacuum window, we measure the surface
adsorption, translational and rotational diffusion of single perylene molecules
on a fused silica surface with high spatial and temporal resolutions.
Time-dependent measurements of the fluorescence signal allow us to deduce two
characteristic decay time scales, which can be explained through a simplified
model involving two adsorption states and five energy levels. The system
presented in this work combines fluorescence imaging with essential ingredients
for surface science and promises a platform for probing single molecule-surface
interactions in highly defined conditions.
