| Optical activity(OA)spectroscopy,as a research tool for polarization spectroscopy,can reveal complex information by measuring the polarization optical response characteristics of substances,such as the absolute configuration of chiral molecules or the fine structure and external field response of non chiral molecules,which provides a powerful spectroscopic detection tool for pharmacology,asymmetric synthesis,analytical chemistry,and atomic and molecular physics in strong field.Commercially available OA spectrometers consist of two independent devices including circular dichroism(CD)spectrometer and optical rotatory dispersion(ORD)ellipsometer.They rely on a photoelastic modulator(PEM)to periodically obtain leftand right-circularly polarized light,and detect small absorption or refractive index differences through a phase locked amplifier.The spectral resolution of the instrument is about several wavenumbers,and it is only suitable for low-resolution optical activity analysis(such as liquid/solid phase chiral molecules),with a measurement time of up to several hours.Moreover,the wavelength dependence of PEM accumulates a large amount of artifacts during long measurement periods,which requires additional correction for distorted OA spectra.Traditional high-resolution OA spectroscopy experiments are completed by using a Fourier spectrometer with a long range scanning,which trades for spectral resolution of hundreds of megahertz at the cost of several meters of mechanical scanning optical path.The single scanning time can be as long as several seconds,which requires a long sampling time to accumulate sufficient detection sensitivity.In this thesis,the dual-comb OA spectroscopy was proposed for the first time.This technology is based on a novel dual-comb spectroscopy with high resolution,high sensitivity,and broadband spectral measurement capabilities,combined with cross-polarization detection schemes.The phase and amplitude of the vibrational OA free-induced decay(FID)electric field are obtained through optical asynchronous sampling,where vibrational CD(VCD)and ORD can be simultaneously inverted.This thesis proposes and validates the dual-comb OA spectroscopy technology.Based on the self-developed mid-infrared dual-comb system,it has successively carried out the researches on magnetic vibrational OA spectroscopy of gas-phase paramagnetic molecules and chiral OA spectroscopy of liquid-phase chiral molecules,which comprehensively analyzed the principles and technical advantages of dual-comb OA spectroscopy.The dual-comb OA spectroscopy provided a solution to solve the current difficulties in OA spectroscopy measurement.The specific content and research innovation of this paper are as follows:1.The dual-comb OA spectroscopy(DC-OAS)was proposed for the first time.And experimental verification of the dual-comb OA spectroscopy measurement was completed based on the self-developed mid-infrared dual-comb system.Taking the magnetic OA of the paramagnetic molecules NOx in a longitudinal magnetic field as an example,the dual-comb magnetic OA spectroscopy(DC-MOAS)combined with an cross-polarization detection scheme,measured the amplitude and phase of the MOA-FID electric field to simultaneously invert the MOA spectra(including magnetic VCD and magnetic ORD).The refresh time of a single spectrum is as short as 0.833 ms5 the spectral resolution is 108.4 MHz,and the linewidth of comb teeth is 90 kHz,The detection sensitivity is ΔA～1.31×10-4@1000s(Δφ～0.6×10-4 rad@1000 s).This result has created a record for MOAS measurements in terms of high resolution,high sensitivity,and rapid measurement.The high-resolution and high-sensitivity measurement of magneto-optical spectrum with DC-MOAS provides an experimental basis and application potential for comprehensive study on the magneto-optical effects of substances.2.Researches on the measurement and theoretical simulation of the rotationally MOA spectra of paramagnetic NOx.The measured high-resolution,high-sensitivity,and broadband MOAS includes rich magneto-optical information,which provides experimental evidence for studying the fine structure of paramagnetic molecules and revealing magneto-optical interactions,such as spin-rotation coupling,spin-orbit coupling,and so on.We determine the Zeeman sub-transitions(including σ+,σ-andπ transition)to obtain the numerical simulation results of the MO AS of paramagnetic molecules.The simulation results are highly consistent with the experimental results,verifying the reliability of MO AS,which explains the relationship between the MVCD transition spectral lines and the fine structure of the spin-rotation coupling in experimental measurements and reveals molecular structure information hidden under Doppler broadening.3.Based on the dual comb OA spectroscopy,the chiroptical activity(COA)measurement with several-second-sampling time has been realized for the first time.The experiment combines external trigger time-domain apodization technology and dual-comb coherence averaging to achieve the measurement of the FID signal of the COA of limonene.The signal intensity of COA is～10-5,the spectral resolution is 72 GHz,and the total sampling time is 5.4 seconds.This technique reduces the measurement time of the COA spectra from hours to seconds,improving the measurement speed by more than three orders of magnitude.Based on this technology,chiral optical activity spectroscopy measurements of limonene enantiomers and racemic have been completed,verifying their high sensitivity and high-speed detection capabilities.It is expected to fulfill time-resolved VCD experiments to study the ultrafast dynamics of secondary structures of biomolecules. |
