Speed Control of Single Molecules on a Surface.

Laws of motion are of fundamental importance for the understanding of dynamic systems. At the atomic scale, the kinetic energy of molecules controls chemical reactions. The velocity of individual molecules, however, has never been measured, because it is difficult to determine time-distance relationships. Here, we show how the real speed of a single molecule moving across a Ag(111) surface is measured at the microsecond time scale. We found that atomic defects extend the travelling time by orders of magnitude, even at distances of few nanometers from the molecule and with high sensitivity to their precise location. Statistical measurements, obtained by repetitive translation of a single molecule in an oscillating electric field, give insight into the time scales of molecular motion in a confined environment. The strong influence of the defects underscores the need for their investigation during chemical reactions or collision experiments at variable velocities and geometries on a surface.