Polarization Effects In Circularly Polarized Stimulated Raman Scattering And The Weak Intermolecular Interaction In 2-Propanol/Water Mixture

Raman spectroscopy is a powerful tool to study molecular structure and intermolecular interaction.It is widely used in many fields such as physics,chemistry,biology,materials and industry.Stimulated Raman scattering(SRS)is a third-order nonlinear optical effect based on Raman resonance enhancement,whose nature is the interaction between light field and thirdorder nonlinear polarizability χ(3)of the medium.Unlike spontaneous Raman scattering,stimulated Raman scattering(SRS)requires the use of two light fields(pump light and Stokes light).Compared with spontaneous Raman,the sensitivity of SRS can be increased by 104～106,which makes SRS possible to quickly respond to the medium.In addition to the high sensitivity,the polarization state of the two lasers in SRS providesan additional degree of freedom for regulating the SRS process,because intrinsically,the thirdorder nonlinear polarizability χ(3)of the medium is closely related to the molecular symmetry.In previous studies,only the SRS with linear polarization of two beams has been reported,while there is no report on that the excitation light field is circularly polarized.As a matter of fact,circularly polarized SRS is more suitable for investigating anisotropic samples such as chiral molecules,crystals and birefringent materials.In this end,the SRS spectra excited by circularly polarized light field are investigated in detail theoretically and experimentally in this paper.Additionally,the concentration-dependent Raman spectra of deuterated isopropanol(CD3CHOHCD3)aqueous solution is investigated by using spontaneous Raman spectroscopy and quantum chemistry theory,and the cooperative effect between weak C-H...O and traditional hydrogen bond O-H...O is revealed.The results are summarized as follows:1.polarization effects in circularly polarized stimulated Raman scattering spectroscopy(SRS)(1)Based on the nonlinear coupled wave equation,the expressions of SRS intensity excited by circularly polarized light field and linearly polarized light field are derived theoretically.The results show that the SRS intensity depends not only on the polarization direction of the two beams,but also on the symmetry of molecular vibrational modes.(2)Taking the spherically symmetric methane molecule as an example,the SRS spectra of υ1 and υ3 vibrational modes under the combination of circular polarization and linear polarization of pump light and Stokes light were measured by stimulated Raman photoacoustic spectroscopy(PARS).(3)The experimental measurement is completely consistent with the theoretical analysis.The results show that the circularly polarized SRS has unique properties different from the linearly polarized SRS.When the handedness of circular polarization state of pump light and Stokes light is the same(both are left-handed or right-handed),the SRS intensity of symmetric vibrational mode remains unchanged,whereas that of antisymmetric vibrational mode is suppressed.When the handedness of circular polarization state of pump light and Stokes light is opposite(one is left-handed and the other is one righthanded),the SRS intensity of the symmetric vibration mode is suppressed,whereas that of the antisymmetric vibrational mode is enhanced.These results will provide new insight into the regulation of SRS process.2.Cooperative effect between weak C-H...O hydrogen bond in isopropanol aqueous solution(1)The concentration-dependent C-H stretching Raman spectrum is measured for deuterated isopropanol-water solution by spontaneous Raman scattering spectroscopy.It is found that the C-H stretching spectrum blue-shifted to high wavenumber with the increase of the concentration of water.(2)The geometric structure of hydrated cluster PWn(n=1～6)containing an isopropanol molecule is calculated by using quantum chemical calculation software Gaussian 16,in order to simulate the variation of C-H stretching vibration spectrum in aqueous solution at different concentrations.The hydrogen bonding networks between isopropanol and water molecules are characterized by atoms in molecules(AIM)theory.It is shown that the weak hydrogen bond C-H...O is a real hydrogen bond in isopropanol-water solution.(3)Theoretical and experimental analysis show the C-H blue shift is caused by both O-H...O and C-H...O hydrogen bonds in aqueous solution.When the concentration of isopropanol is relatively high,the blue shift of C-H stretching vibration spectrum is mainly caused by the traditional hydrogen bond O-H...O,whereas with the increase of water concentration,the blue shift of C-H spectrum is caused by the synergistic effect of weak C-H...O andOH...O hydrogen bonds.The innovation points are summarized as follows:(1)The SRS intensity excited by circularly polarized pump light and Stokes light is investigated theoretically and experimentally.The results show that circularly polarized SRS has unique properties different from linearly polarized SRS.(2)By explaining the observed blue shift of C-H stretching vibration spectrum in isopropanol aqueous solution in the concentration-dependent Raman spectra,the cooperative effect between weak C-H...O and traditional hydrogen bond O-H...O hydrogen bonds was revealed.