Vibrational spectroscopy of CO used as a local probe to study the surface morphology of Pt on Ru(0 0 1) in the submonolayer regime

Bimetallic layers of Pt/Ru(0 0 1) and the adsorption of CO on these have been investigated with infrared absorption spectroscopy (IRAS) and scanning tunneling microscopy (STM) in the submonolayer regime (θPt<1 ML). While the morphology of the Pt deposit is pictured by STM images, IRAS allows us to correlate the shift of the internal CO vibrating frequency sensitively with the local binding configuration. Thereby CO on Ru(0 0 1) (ω=1990 cm−1), CO on terraces of Pt monolayer islands on Ru(0 0 1) (ω=2085 cm−1) and CO at the various Pt–Ru step sites (ω=2033–2050 cm−1) can be distinguished. By annealing the sample from the deposition temperature of 80 K to ∼350 K, the binding energy of CO to Pt–Ru step sites is reduced significantly. This is explained by the transition of the Pt island shape and size from single atoms and small clusters at low temperatures to larger Pt islands at room temperature.