Sum-Frequency Generation Vibrational Spectroscopy of an Extramolecular Chemical Bond

Infrared-visible sum-frequency generation (SFG) vibrational spectroscopy is applied to evidence the extramolecular Pt-C chemical bond and deduce an experimental value of its Raman activity at the CO/Pt(110) interface. This measurement, performed on a flat metal surface, without surface roughening, does not involve any enhancement of the electromagnetic fields. We evidence that the experimental Raman polarizability of the extramolecular Pt-C stretching vibration mode is around 1.49 times smaller than that of the intramolecular CO stretching vibration mode. Furthermore, SFG data show that the infrared dynamic dipole moments differ in sign for the two vibrational modes, as suggested by density functional theory (DFT) calculations. The weak SFG intensity experimentally measured from the Pt-C chemical bond is accounted for by its low infrared activity. For surface chemistry, these results open the door to systematic investigation of extramolecular vibrational activity at the submonolayer level.