Using Structural Information to Fit Vibrational Sum Frequency Spectra

Vibrational sum frequency spectra encode detailed information about molecules at surfaces and buried interfaces. However, fitting such spectra in order to retrieve vibrational mode amplitudes, resonance frequencies, and peak widths is more challenging than fitting infrared absorption or Raman spectra as a result of the bisignate nature of the amplitude in the sum frequency response. This is especially true when the signal-to-noise and spectral resolution are limited and when explicit phase information is not available. A general fitting method is presented that makes use of molecular structure using only the predicted signs of the vibrational mode amplitudes from associated electronic structure calculations. This leaves the sought amplitudes free to be determined from experimental spectra but with sign constraints imposed. Using this method, we illustrate that fitting intensity-only spectra can produce results that approach the accuracy of what is obtained by fitting phase-resolved spectra.