Infrared Free Induction Decay (IR-FID) of Non-Interfacial Origin Observed in the Interfacial Sum-Frequency Generation Vibrational Spectroscopy (SFG-VS)

We report the observation of infrared free induction decay (IR-FID) signal of the antisymmetric modes around 2350 cm–1 of the gaseous CO2 molecules in air in the sum-frequency generation vibrational spectroscopy (SFG-VS) measurement from the gold surface. These signals appeared with a time-dependent interference pattern in the 15–73 ps range and beyond after the time zero of the SFG-VS process. The interference pattern was found to reflect the rotational coherence of the gaseous CO2 molecules. Similar IR-FID and rotational coherence were also observed for the symmetric and asymmetric stretching modes of gaseous H2O molecules in air. The gold surface in this case serves as the up-conversion agent with the visible pulse as the time gate for the ultrafast IR-FID emissions. We tested this hypothesis by replacing the gold surface with a β-BBO (β-barium borate, β-BaB2O4) crystal and found 5 orders of magnitude increase of the signal in the reflecting geometry. The up-conversion of the IR-FID radiation of non-interfacial origin into the SFG-VS signal is a new mechanism that provides a mechanistic understanding of the "abnormal spectral bands" observed in broad-band SFG-VS induced by the bulk absorption and refraction reported in the literature.