Molecule-specific interactions of diatomic adsorbates at metal-liquid interfaces.

Ultrafast vibrational dynamics of small molecules on platinum (Pt) layers in water are investigated using 2D attenuated total reflectance IR spectroscopy. Isotope combinations of carbon monoxide and cyanide are used to elucidate inter-adsorbate and substrate-adsorbate interactions. Despite observed cross-peaks in the CO spectra, we conclude that the molecules are not vibrationally coupled. Rather, strong substrate-adsorbate interactions evoke rapid (∼2 ps) vibrational relaxation from the adsorbate into the Pt layer, leading to thermal cross-peaks. In the case of CN, vibrational relaxation is significantly slower (∼10 ps) and dominated by adsorbate-solvent interactions, while the coupling to the substrate is negligible.