Surface sum-frequency vibrational spectroscopy of nonpolar media

Abstract

Significance Sum-frequency vibrational spectroscopy (SFVS) has been developed into a viable surface analytical tool for media with inversion symmetry. It is considered surface specific because the bulk contribution to the signal vanishes under the electric-dipole approximation. However, there is always the worry that beyond the electric-dipole approximation, the bulk contribution may not be negligible. The problem is particularly acute if the medium under investigation does not have a strongly polar-oriented surface layer. We show here that for nonpolar media with well-separated infrared and Raman modes, it is possible to deduce bulk and surface spectra separately from combined transmission and reflection phase-sensitive SFVS measurements. The work also provides guidelines for evaluation of the importance of bulk contribution to SFVS in general.