Probing Either Side of Thin Films Using Vibrational Sum Frequency Spectroscopy in Multiple Beam Polarizations

Knowledge of the thickness of films, combined with the ability to control and ideally tune the angle of incidence, provides the possibility for vibrational sum-frequency generation experiments to selectively probe either interface of the film. This is of interest to studies targeting the adhesive properties of polymer films on substrates, or cases where the structure of the exposed side of the film is sought in relation to its environmental conditions, for example, solvent exposure. A straightforward theory is presented that is readily implemented for samples according to their thickness and refractive index. This results in a procedure for planning experiments that reveals whether there is a combination of film thickness and angle of incidence in different beam polarizations that enables multiple beam interference effects to be used to their advantage. The most complicated case where all beams are p-polarized is discussed in detail, as this is often a necessary experimental configuration for dielectric or metal samples.