Real-time observation of molecular motion on a surface

The laser-induced movement of CO molecules over a platinum surface is followed in real time using ultrafast vibrational spectroscopy. Because the CO molecules bound on different surface sites exhibit different C^_O stretch vibrational frequencies, the site-to-site hopping, triggered by excitation with a laser pulse, can be determined from sub-picosecond changes in the vibrational spectra. The unexpectedly fast motionM^_characterized by a 500-femtosecond time constant reveals that a rotational motion of the CO molecules, rather than pure translation, is required for this diffusion process. This conclusion is corroborated by density functional theory calculations.