Vibrational Sum Frequency Scattering in Absorptive Media: A Theoretical Case Study of Nano-objects in Water

The structures of interfaces of nano- and microscale objects in an aqueous solution are important for a wide variety of physical, chemical, and biological processes. Vibrational sum frequency (SF) scattering has emerged as a useful and unique probe of the interfacial structure of nano- and microscale objects in water. However, the full surface vibrational stretch mode spectrum has not been measured yet, even though it would be extremely informative to do so. The reason for this is that probing the vibrational modes of interfacial water requires a full understanding of how the linear absorptive properties of the bulk aqueous medium influence the SF scattering process. Here, we have simulated vibrational SF scattering spectra of the interface of nanoscale objects dispersed in water. We analyzed the effect of the infrared pulse absorption on the outcome of surface vibrational SF scattering measurements. We find that both infrared absorption as well as the type of optical detection can drastically modify the measured vibrational interfacial spectrum. The observed changes comprise spectral distortion, frequency shifting of the main vibrational stretch mode, and the introduction of a new high-frequency peak. This last feature is enhanced by nonresonant interactions.