Whole-Molecule Approach for Determining Orientation at Isotropic Surfaces by Nonlinear Vibrational Spectroscopy

Abstract

Nonlinear vibrational spectroscopies such as visible-infrared sum-frequency spectroscopy may serve as powerful probes of interfacial structure. Obtaining quantitative orientation information, however, has been limited by the required knowledge of the corresponding molecular-level nonlinear optical properties. We provide a general scheme for calculating the vibrational hyperpolarizability of any infrared- and Raman-active mode, regardless of the molecular symmetry or complexity of the structure. Our method involves all atoms and therefore does not rely on making any local mode approximations. We show how this information is used together with experimental data to arrive at the tilt and twist angles of a surfactant headgroup at the air/water interface. Since our approach is completely general, it may be used for the analysis of any adsorbate at an isotropic interface.

An empirical approach to the bond additivity model in quantitative interpretation of sum frequency generation vibrational spectra

“…[10][11][12][16][17][18][19][20][21][22][23][24][25][26] The bond additivity model can also be called the bond polarizability derivative model. As Hore et al pointed out recently, 14 the key to the bond additivity model is that through symmetry analysis the polarizability tensors of the individual bond stretches ͑C ϱv ͒ are coupled to produce the normal mode coordinate of a particular molecular group ͑C 3v , C 2v , etc.͒. 16,17 There are two ways to determine the polarizability tensor ratios of the individual bond stretches, i.e., r = ␤ / ␤ = ␤ / ␤ , where ͑͒ are the single bond fixed coordinates with as the primary axis of the single bond.…”

“…11,12 The key for such quantitative interpretation lies on the ability to obtain or estimate the ratios between different microscopic polarizability ␤ i Ј j Ј k Ј tensor elements of a chemical group in a molecule. 10,13,14 So far, almost all of the efforts have focused on the stretching vibrational modes. The approaches employed to obtain the ␤ i Ј j Ј k Ј tensor ratios are all based on the following general relationship between the second order hyperpolarizability tensors ͑␤ i Ј j Ј k Ј q ͒ for a particular vibrational mode q to the Raman polarizability derivative tensors ‫␣ץ͑‬ i Ј j Ј / ‫ץ‬Q q , usually denoted as ␣ i Ј j Ј Ј ͒ and dipole moment derivative tensors ‫ץ͑‬ k Ј / ‫ץ‬Q q , usually denoted as k Ј Ј ͒ of the qth mode: 10,13,15 …”

Experimental evidence for an optical interference model for vibrational sum frequency generation on multilayer organic thin film systems. I. Electric dipole approximation

“…47 Vibrational sum frequency generation spectroscopy (VSFG) is a second-order spectroscopy that has been demonstrated as a sensitive probe of molecular structure at interfaces. [48][49][50][51] In addition to interfacial sensitivity, it also meets the second requirement of the ideal experiment with its ability to make nondestructive measurements with few limits on the types of systems that can be probed. In principle, it can be used to study any interface accessible by light.…”

Sodium Dodecyl Sulfate Adsorption onto Positively Charged Surfaces: Monolayer Formation With Opposing Headgroup Orientations

“…5, 8, 9, 20–34 While most of these techniques can accurately determine the amount of surfactant adsorbed at the interface, SFG spectroscopy is the only technique capable of providing insight into the molecular conformation, orientation, and order of surfactant layers in situ . The adsorption kinetics and energetics of a variety of surfactants including SDS have been studied in great detail, but the number of spectroscopic studies is still very limited.…”