서강대학교

초록/요약

Although aqueous surfaces and interfaces are smaller in size in comparing with the size of bulk of aqueous, many important processes such as physical, chemical, and biological processes are originated from and governed by the surfaces and interfaces. Surface sum-frequency vibrational spectroscopy has been a well-known surface specific tool since the first sum-frequency generation spectroscopy was invented by professor Shen’s research group in 1989. Popularity and usefulness of surface sum-frequency vibrational spectroscopy can be seen through varieties of publications from many research groups such as prof. Y.R. Shen’s, prof. T. Tahara’s, prof. A. Yamaguchi’s, prof. H.C. Allen’s, prof. D. Kim’s, and prof. G.L. Richmond’s groups. In this thesis, the surface sum-frequency vibrational spectroscopy is constructed to study air/water interfaces. In sum-frequency generation spectroscopy (SFG), broadband band infrared (IR) light source or tunable IR source and narrow visible are required. Making use of second order nonlinear optical process of optical parametric amplification (OPA) followed by differential frequency generation (DFG), tunable broadband IR light is generated. Higher frequency and lower frequency outputs of OPA (signal and idler) are used as pump and seed for DFG system. Two beta barium borate (BBO) crystals and one silver gallium sulfide (AgGaS2) crystals are used in the OPA and DFG systems, respectively. By appropriately tuning the OPA output, IR outputs in 5500 nm – 7500 nm and 2700 nm - 3800 nm region which correspond to vibrational frequencies of C=O, COO- in the carboxylic group of the fatty acid molecule consisting the Langmuir monolayer, and OH (of water subphase) and CHx (of the terminal methyl group and the methylene chain of the fatty acid) were generated, respectively. To narrow down the bandwidth of the visible light, etalon is used. To show the validity of the SFG setup, SFG spectra in the ranges of CHx, OH, and COOH stretch vibration were measured. Langmuir monolayers of the fatty acid molecules (having an area per molecule corresponding to untilted condensed phase) were prepared in a petri dish. The spectra contained CH2 symmetric stretch and asymmetric stretch, CH3 symmetric stretch and asymmetric stretch, Fermi resonance between CH3 symmetric stretch and bending overtone (in CHx region: 2855-2962 cm-1), and interfacial water vibrational normal modes (3000 - 3800 cm-1). In the headgroup region (COOH) of the SFG spectrum, CO stretch mode of the protonated carboxylic headgroup and symmetric stretch peak of C-O bonds of the deprotonated headgroup could be distinguished clearly depending upon the pH of the water subphase.