Hot-band excitation of CO chemisorbed on Ru(001) studied with broadband-IR sum-frequency generation

Broadband-IR sum-frequency generation spectroscopy was used to investigate the CO-stretch vibration of CO chemisorbed on a Ru(001) surface at coverages as low as 0.001 monolayers (ML). Due to the high intensity of the broadband-IR pulses the v=1→2 hot band of the CO-stretch vibration of CO on pure and oxygen-covered Ruthenium is observed for the first time. The simultaneous detection of the fundamental and hot-band transition allows the determination of the anharmonicity constant xeωe directly. For a coverage of 0.004 ML one obtains 13.6 cm−1 for 12C16O and 14.5 cm−1 for 12C16O co-adsorbed with O.