Validation of broadband infrared normalization in sum-frequency generation vibrational spectroscopy through simultaneous chiral terms on α-quartz crystal

Abstract

Sum-frequency generation vibrational spectroscopy (SFG-VS) has been widely used for characterizing various interfaces. However, obtaining SFG signals with a high signal-to-noise ratio can be challenging for certain interfaces, such as those involving powder particles, which scatter the SFG light and make it difficult to obtain accurate spectra. To address these challenges, we developed a new approach using a z-cut α-quartz crystal as the substrate loaded with a very small amount of powder sample. This approach not only amplifies the SFG signal from particles through the interference of the electric field from the quartz crystal, but also allows for phase reference and normalization of the broadband infrared SFG spectrum. By distinguishing the different polarizations of the SFG light, we were able to separate and simultaneously collect the achiral and chiral SFG signals. We used the chiral SFG signal to normalize the achiral SFG intensity, thereby avoiding any potential changes to the interface caused by loading substances onto the quartz, as well as coincidence differences resulting from the instability of light at different moments. We demonstrated our method by measuring the adsorption of CH3OD on a quartz substrate loaded with MoC nanoparticles. Our approach produced a high signal-to-noise ratio SFG spectrum, regardless of the interface situation.