Second Harmonic Generation Study Of The Air/Liquid Interface Of Different Solutions

In recent years, study on the interface, especially the interface of the molecular level, has been well recognized to be very important. Understanding molecular information at interfaces is of theoretical and practical importance. Molecular information can be particularly important in applications on intermolecular energy transfer, chemical and biomolecular reactions occurring at interfaces. However, there are still some difficulties, which lie in the fact that only a few surface techniques are effective to probe interfaces at molecular level. For example, techniques to probe interfaces include scanning tunneling microscopy (STM), atomic force microscopy (AFM), X-ray spectroscopy, neutron reflectivity, attenuated total reflection (ATR), second harmonic generation (SHG) and sum frequency generation (SFG). STM and AFM may apply to thin films of liquids on solid substrates, while X-ray spectroscopy and neutron reflectivity are more effective and informative. However, STM and AFM are limited to molecular movement at liquid interfaces in the general cases, and X-ray spectroscopy and neutron reflectivity are not highly surface-specific. The attenuated total reflection (ATR) method has been used to probe buried liquid interfaces, but it is also not very surface-specific.Recently, second harmonic generation (SHG) has developed to be a well-established surface technique for studying the air/liquid and liquid/liquid interfaces. Compared with the other interface detective method, SHG has stronger interface sensitivity. It is forbidden in centrosymmetric media but necessarily allowed at a surface or interface. It has submonolayer sensitivity and, with the output highly directional, can be used for in situ remote sensing of any interface.This paper introduces the nonlinear optical technology-Second harmonic technique applying at the interface scientific research. Taking advantage of the basic principles of second harmonic signals generated only at the interface, the second harmonic detective system was built. The molecular structure and properties of the monolayer at the interface were detected from the molecular scale and the microscopic molecular behavior of the surfactant molecules was studied at the air/liquid interface. In addition, the reason why the SHG signal at the air/liquid interface changed with the solution concentration and the external environment was explored. The SHG signals from the air/liquid interface of Rhodamine B and Sodium Dodecyl Benzene Sulfonate (SDBS) aqueous solutions were obtained and analyzed, which demonstrate that the SHG signal intensity of Rhodamine B is stronger than that of SDBS. Compared with one single solution, the SHG signal intensity of the mixed aqueous solution of Rhodamine B and SDBS decreases. From the UV-VIS absorption spectrum of the two aqueous solutions, it can be seen that Rhodamine B has an absorption peak closer to the second harmonic frequency. Therefore the resonance of the second harmonic frequency with the frequency of the dipole transitions of the chromophore considerably enhances the signal intensity. Furthermore, the hyperpolarizabilities of the molecules of Rhodamine B and SDBS are calculated from first-principles, which reveal that the hyperpolarizability of Rhodamine B molecule is greater than that of SDBS molecule. When they are mixed, molecules of Rhodamine B and SDBS gather together because Rhadamine B molecule carries positive surface charge and SDBS is anionic surfactant, causing the decrease of the SHG intensity of the mixed solution. Moreover, the SHG signals from the air/liquid interface of SDBS solutions of different concentrations and at different environment temperatures were measured, exploring the law of interfacial molecules changing with the solution concentrations and the external environment temperatures. The process of second harmonic generation was simulated with the finite element method, not only providing important theoretical basis for the study of surfactant, but also providing important technical means to achieve cost efficiency and energy conservation.