The mobility of CO chemisorbed on Pt(111) has been measured using a technique that synchronizes pulsed molecular‐beam dosing with fast scanning infrared interferometry. The method is based on the observation that molecules are generally more tightly bound at defects (e.g., steps) where they exhibit a different internal vibrational frequency to that associated with the same molecule adsorbed on the low‐index terrace site. On a stepped Pt(111) surface, a low coverage (θ∼0.006 monolayer) of CO is deposited in a random spatial distribution (in 600 μs). The time evolution of the infrared spectrum is then recorded with ≥5‐ms resolution as the CO diffuses to the steps. A simple kinetic model makes it possible to extract the statistical microscopic hopping rates for CO diffusion on the (111) plane over the range of temperature from 95 to 195 K.

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