Vibrational sum frequency scattering in absorptive media: A theoretical case study of nano-objects water

The structures of interfaces of nano- and microscale objects in aqueous solution are important for a wide variety of physical, chemical and biological processes. Vibrational sum frequency 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 sum frequency scattering process. Here, we have simulated vibrational sum frequency 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 sum frequency 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 introducing of a new high frequency peak. This last feature is enhanced by non-resonant interactions.