Hot band excitation of CO/Ir{1 1 1} studied by broadband sum frequency generation

The first measurement of the vibrational hot band of the C–O stretch of carbon monoxide adsorbed on Ir{1 1 1} has been obtained by sum frequency generation (SFG) with intense broadband (ca. 150 cm−1) infrared pulses. At 0.07 ML we see an anharmonic frequency shift of 26.8 ± 0.6 cm−1. The υ = 1 → 2 hot band is visible as a distinct band at coverages as high as 0.11 ML and its contribution to the SFG spectra can be followed up to 0.25 ML. Both the fundamental and hot band frequencies increase with coverage due to dipole–dipole coupling with a stronger increase for the hot band due to the higher dynamic dipole moment of the υ = 1 → 2 transition. The variation of line width with coverage reflects the change in homogeneity of the layer.