Phase-sensitive Sum Frequency Generation Spectroscopy Methods And Applications In Interface Research

The characterization of the properties of the interface molecules is helpful for understanding the nature of physical,chemical and even life processes.However,the traditional techniques are lack of the interface selectivity to probe the interface,the signal will be interfered by the signal of the bulk.Owing to the unique interface sensitivity,sub-monolayer sensitivity and non-invasive in-situ detection ability of the second-order nonlinear optical effect,sum frequency generation(SFG)has become one of the conventional techniques for interface characterization.In addition to the intensity information of the spectra,the new developed phase-sensitive sum frequency generation(PS-SFG)can also directly acquire the phase information of the spectra.The PS-SFG can characterize the structure,orientation and distribution of the interface molecules more objectively and clearly,and has provided a new opportunity for the study of interface molecules.This thesis focuses on the PS-SFG technology and its application in interface characterization,and the research contents are listed as follows:(1)The PS-SFG was applied on the characterization of the interfacial chirality.In the first part,the air/liquid interfaces of chiral carvone liquid were probed by noncollinear heterodyne-detected phase-sensitive sum frequency generation(HD-PS-SFG).The experimental results showed that there were four chiral vibrational peaks were with opposite signs in the chiral SFG spectra of the R-and S-carvone molecules,while the achiral SFG spectra of the S-carvone,R-carvone and their racemic mixture were similar.The characteristics of chiral PS-SFG spectra can directly characterize the intrinsic chirality of carvone molecule.In the second part,the Cd S/Cd Te superstructure(SPs)was characterized by collinear HD-PS-SFG.The experimental results showed that there were obvious chiral SFG signals in Cd S/Cd Te SPs while no identified chiral SFG signals in monodispersed Cd S/Cd Te nanoparticles.Therefore,it can be identified that the surface chirality of Cd S/Cd Te SPs should originate from the highly ordered chiral arrangement of the Cd S/Cd Te nanoparticles.The chiral PS-SFG can be used to effectively characterize the collective chirality of the Cd S/Cd Te SPs.(2)The PS-SFG was applied on the characterization of the structure of water molecules at liquid interface.Using the PS-SFG spectra of pure water as reference,the experimental results showed that the surface negative charge of the hydrophobic bitumen and hydrophilic silica could force the water molecules to form an orderly arrangement at interface.The addition of the hydrophilic ionic would destroy the ordered interfacial structures of water molecules at air/water and water/bitumen interface,but strengthen the ordered arrangement of water molecules at water/silica interface.The PS-SFG is helpful to understand the arrangement of water molecules at different interfaces.(3)The intrapulse interference broad-band sum frequency generation(II-BB-SFG).A new PS-SFG method named II-BB-SFG was theoretically proposed,which used the intrapulse interference phenomenon of the ?-step phase modulated femtosecond visible pulse.By recording the spectra with different time-delay between the IR pulse and the?-step phase modulated visible pulse,a spectral map with rich frequency information can be obtained.The basic principle and technical characteristics of the II-BB-SFG were introduced in detail.The innovation points of this thesis are listed as follows:(1)In this thesis,the phase information of the interfacical molecular vibration were directly acquired by PS-SFG,which can directly relate with the absolute orientation of the molecular group.The interface intrinsic chirality and collective chirality were directly characterized by PS-SFG respectively,which is difficult to characterize by traditional chiral characterization techniques such as circular dichroism and Raman rotation spectroscopy.(2)In this thesis,the structure and the ordered degree of the molecular arrangement of the water molecules at different interface and the effects of surface charge and ionic groups on the structure and arrangement of interfacial water molecules were directly measured by PS-SFG at the molecular scale,which was helpful to understand the microcharacteristics of the interface at the molecular scale.(3)In this thesis,a new PS-SFG method named II-BB-SFG was theoretically proposed.It used the intrapulse interference phenomenon of the ?-step phase modulated femtosecond visible pulse to obtain high spectral resolution without narrowband filtering of visible pulse.It can overcome the effects of the instability of the incident pulses and the non-resonant part of the interfacial molecular vibration on the spectra lineshape.