Theoretical And Experimental Spectroscopic Studies Of Lead Monofluoride (PbF) Molecules Towards The Measurement Of The Electron’s Electric Dipole Moment (eEDM)

The Standard Model of particle physics has achieved great success in predicting fundamental particles and their interactions.However,it has not been able to explain certain phenomena,such as the matter-antimatter asymmetry,the origin of dark matter and dark energy,and the fundamental mechanisms of gravitational interaction.This has led to the exploration of new theories and mechanisms beyond the Standard Model.Although these new theories and mechanisms can be directly measured through large scientific facilities such as the Large Hadron Collider,the precision measurement of the electron’s Electric Dipole Moment（eEDM）using the atom/molecule/ion as a carrier has significant technological and cost advantages as well as strong scientific complementarity,making it another important means of studying new physics beyond the Standard Model.Compared with other molecules or ions such as YbF,HfF⁺and ThO that have provided the experimental results of eEDM measurement,PbF molecules have the following characteristics:（1）its eEDM coherent state is the(X₁²Π_1/2 state,so its coherent measurement time is not limited by its ground state lifetime;（2）the effective electric field within the molecule is relatively large and easy to polarize,making it possible to obtain a certain degree of statistical sensitivity for the eEDM measurement;（3）the ground state of the PbF molecule is a g-2 type polar molecule,so its g factor can be obtained with smaller values under certain external electric fields,resulting in a lower sensitivity to external or stray magnetic fields than g-type polar molecules.The combination of these characteristics makes PbF another potential candidate for the eEDM measurement.In addition,experimental designs based on optical/radio frequency pumping transition scheme for preparing the eEDM coherent state,phase modulation using electro-optic modulation devices for obtaining eEDM interference fringes,and detection of PbF molecules using the resonance-enhanced multiphoton ionization method,have certain characteristics such as high detection sensitivity,low probability of systematic and/or statistical errors,and are expected to play a certain role in the eEDM measurement.Therefore,circumventing on the topic of the precise measurement of eEDM using PbF molecules,the experimental scheme for measuring eEDM using PbF molecules is discussed and analyzed firstly,and the upper limit of statistical sensitivity for eEDM measurement under this scheme is estimated.Secondly,the molecular energy levels and transition spectra are calculated via an effective Hamiltonian approach.Thirdly,the possible application of laser cooling in the eEDM measurement is discussed,while the suitability of laser-cooling PbF molecules to measure eEDM is also explored.Subsequently,a new generation of low-temperature beam source based on buffer-gas cooling was designed and built to enhance the system’s performance in vacuum sealing,low-temperature refrigeration,signal detection and other aspects.Buffered PbF molecules were successfully generated and relevant parameters affecting the properties of the molecular beam were optimized.Finally,fine and hyperfine spectroscopy between the(X₁²Π_1/2 and B²Σ⁺states of PbF molecules at cryogenic temperatures was obtained using the optimized molecular beam source.The resulting spectroscopy were assigned using theoretical methods and spectroscopic constants for the B²Σ⁺（v′=0）state of ²⁰⁸Pb¹⁹F molecules were updated.In summary,the work presented in this thesis provides a stable molecular beam source for the precise measurement of eEDM using heavy polar PbF molecules.It also provides experimental and theoretical references for the subsequent spectral scanning and analysis of the(X₁²Π_1/2→A²Σ⁺transition of PbF molecules,and establishes the necessary spectroscopic foundation to probe the eEDM phase based on this specific transition,which eventually paves the way to characterize the eEDM interference fringes.