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Second harmonic generation study of the air/liquid interface of different solutions

As an intrinsically surface-specific technique, Second Harmonic Generation (SHG) is widely used in the study of interface in recent years. 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.