Understanding the Role of Nonresonant Sum-Frequency Generation from Polystyrene Thin Films

The nonresonant (NR) portion of sum-frequency generation (SFG) spectra of polystyrene (PS) thin films is shown to contain physical information and affect the analysis of the surface structure. When the NR signal is suppressed, PS thin films on three different substrates produce the same spectrum, suggesting the same structure at the free surface. Annealing or aging of PS films on silicon causes the NR signal to increase significantly compared with a fresh sample, indicating that the substrate is not the sole source of NR signal. A method is proposed for improved analysis of SFG spectra. First, spectra obtained with NR suppression are used to determine the resonant parameters. After these are constrained, the NR amplitude and phase parameters are determined more uniquely from unsuppressed spectra. NR-SFG must be properly handled for the analysis of SFG spectra to be mathematically unique and physically meaningful