Interface-Specific χ(4) Coherent Raman Spectroscopy in the Frequency Domain

The Topmost Water Structure at a Charged Silica/Aqueous Interface Revealed by Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy

Despite recent significant advances in interface-selective nonlinear spectroscopy, the topmost water structure at a charged silica surface is still not clearly understood. This is because, for charged interfaces, not only interfacial molecules at the topmost layer but also a large number of molecules in the electric double layer are probed even with second-order nonlinear spectroscopy. In the present study, we studied water structure at the negatively charged silica/aqueous interface at pH 12 using heterodyne-detected vibrational sum frequency generation spectroscopy, and demonstrated that the spectral component of the topmost water can be extracted by examining the ionic strength dependence of the Imχ⁽²⁾ spectrum. The obtained Imχ⁽²⁾ spectrum indicates that the dominant water species in the topmost layer is hydrogen-bonded to the negatively charged silanolate at the silica surface with one OH group. There also exists minor water species that weakly interacts with the oxygen atom of a siloxane bridge or the remaining silanol at the silica surface, using one OH group. The ionic strength dependence of the Imχ⁽²⁾ spectrum indicates that this water structure of the topmost layer is unchanged in a wide ionic strength range from 0.01 to 2 M.

Phonon mode of TiO2 coupled with the electron transfer from N3 dye

Low frequency vibrational spectra of submonolayer N3 dye (Ru(4,4(')-dicarboxy-2,2(')-bipyridine)2(NCS)2) adsorbed on TiO2 (110) were reported by using fourth-order coherent Raman spectroscopy, which is interface-sensitive vibrational spectroscopy. Most of the peaks observed in the experiment were at the same frequency as that of Raman and infrared spectra of the dye and TiO2. Two interfacial modes at 640 and 100 cm(-1) and one resonantly enhanced phonon at 146 cm(-1) appeared in addition to the pure TiO2 and N3 spectra. Adsorption of N3 dye on TiO2 contributed to the enhancement of 100 and 146 cm(-1) mode. The results not only reported interfacial low-frequency vibrations of TiO2 (110) with N3 dye adsorption but also suggested the coupling between the surface vibrations of TiO2 and charge transfer between N3 dye and TiO2 on the surface.

Sum Frequency Generation Studies on Bioadhesion: Elucidating the Molecular Structure of Proteins at Interfaces

The study of bioadhesion is significant to applications in a variety of scientific fields. Techniques that are surface sensitive need to be utilized to examine these kinds of systems because bioadhesion occurs at the interface between two surfaces. Recently, Sum Frequency Generation (SFG) has been applied to investigate different bioadhesive processes because of its intrinsic surface specificity, excellent sensitivity and its ability to perform experiments in situ. SFG studies on the bioadhesion of fibrinogen, factor XII and mefp-3 on various surfaces will be discussed in this review.

Fourth-order coherent Raman spectroscopy of liquid-solid interfaces: near-surface phonons of TiO2 (110) in liquids

The fourth-order coherent Raman response of a TiO2 (110) surface covered by HCl aqueous solution, neat octanol, acetic acid, or carbon tetrachloride layers is acquired. Four fourth-order optical responses were identified at 837-826, 452-448, 371-362, and 184-183 cm(-1) and assigned to near-surface phonons of TiO2. A third-order response produced in the bulk liquid layer was superimposed on the fourth-order response, when coherent vibrations are efficiently excited in the layer.

Determining electronic spectra at interfaces by electronic sum frequency generation: One- and two-photon double resonant oxazine 750 at the air/water interface

The second-order nonlinear electronic spectra were measured for a dye oxazine 750 (OX750) adsorbed at the air/water interface using the multiplex electronic sum frequency generation (ESFG) spectroscopy recently developed by our group. The excitation-wavelength dependence of the ESFG spectrum was investigated, and a global fitting analysis was performed to separate contributions of one- and two-photon resonances. The analysis yielded linear interface electronic spectra in the one- and two-photon resonance regions, which can be directly compared to bulk absorption spectra. A two-dimensional plot of the linear interface electronic spectra is newly proposed to graphically represent all the essential information on the electronic structure of interfacial molecules. On this new analytical basis of the ESFG spectroscopy, the spectroscopic properties of OX750 at the interface are discussed.

Molecular Vibrations at a Liquid−Liquid Interface Observed by Fourth-Order Raman Spectroscopy

Interface-selective, Raman-based observation of molecular vibrations is demonstrated at a liquid-liquid interface. An aqueous solution of oxazine 170 dye interfaced with hexadecane is irradiated with pump and probe light pulses of 630-nm wavelengths in 17-fs width. The ultrashort pulses are broadened due to group velocity dispersion when traveling through the hexadecane layer. The dispersion is optically corrected to give an optimized instrumental response. The pump pulse induces a vibrational coherence of the dye via impulsive stimulated Raman scattering. The probe pulse generates second-harmonic light at the interface. The efficiency of the generation is modulated as a function of the pump-probe delay by the coherently excited molecules. Fourier transformation of the modulated efficiency presents the frequency spectrum of the vibrations. Five bands are recognized at 534, 557, 593, 619, and 683 cm(-1). The pump-and-probe process induces a fourth-order optical response that is forbidden in a centrosymmetric media. The contribution of an undesired, cascaded optical process is quantitatively considered and excluded.