Vibrational spectroscopy is a valuable quantitative tool for the determination of structure at surfaces. Various techniques may be applicable and useful, depending on what is available, the transparency of the substrates, the need for in situ probes, and the degree of interfacial specificity required. We examine and compare signals in infrared absorption, Raman scattering, and vibrational sum-frequency generation spectroscopy to the underlying molecular response. In all of these experiments, varying the beam polarizations enables the orientation of specific chemical functional groups to be determined. However, the sensitivity of each technique is directly connected to the manner in which the molecular response manifests itself in the measured signal. Starting with simple distributions of a single vibrational mode, leading up to multiple vibrational bands in more complex orientation distributions, we compare these three techniques in terms of their sensitivity to features of the molecular orientation distribution. This review is aimed at guiding planned experiments when multiple techniques are available for surface structural analysis.

No abstract

The optimum experimental geometry for visible-infrared sum-frequency generation experiments depends rather sensitively on the molecules adsorbed at the surface, their orientation, and the nature of the adjacent bulk media. We consider the commonly encountered case of methyl groups situated at air-water, air-gold, and polymer-water interfaces. We provide expressions that may be used to determine the optimal visible and IR beam incident angles, considering the symmetric and antisymmetric modes separately and then together. The analysis is carried out for co-propagating (collinear and non-collinear geometries) and counter-propagating configurations. We first consider that one or more vibrational modes are of interest, and the goal is to study them quantitatively under a single polarization scheme; our results enable the user to set the beam angles for such an experiment. In the second case, molecular orientation information is desired, and so the calibrated response is required in all accessible polarization schemes for full characterization of the nonlinear susceptibility tensor.

No abstract

Linear programming was used to assess the ability of polarized infrared absorption, Raman scattering, and visible–infrared sum-frequency generation to correctly identify the composition of a mixture of molecules adsorbed onto a surface in four scenarios. The first two scenarios consisted of a distribution of species where the polarity of the orientation distribution is known, both with and without consideration of an arbitrary scaling factor between candidate spectra and the observed spectra of the mixture. The final two scenarios have repeated the tests, but assuming that the polarity of the orientation is unknown, so the symmetry-breaking attributes of the second-order nonlinear technique are required. The results indicate that polarized Raman spectra are more sensitive to orientation and molecular identity than the other techniques. However, further analysis reveals that this sensitivity is not due to the high-order angle dependence of Raman, but is instead attributed to the number of unique projections that can be measured in a polarized Raman experiment.