Study Of Isotopic Spectroscopy And Radiative Deflection Of Magnesium Fluoride (MgF) Molecules

Since the idea of laser cooling of atoms was put forward in 1975,a series of important achievements in laser cooling,trapping and manipulation of neutral atoms have been made theoretically and experimentally.Recently,the research on laser cooling of neutral molecules has aroused great interest.The rich internal structure,permanent electric dipole moments,long range interaction,and anisotropic characteristics of molecules make cold molecules have many exciting applications,such as ultracold chemistry,physical research beyond standard model,simulation of multi-body quantum system and quantum information processing.The main difference between molecules and atoms lies in the complex internal structures,such as vibration and rotational energy levels,which brings many difficulties to laser cooling of molecules.Therefore,laser cooling of molecules is research full of opportunities and challenges.This thesis mainly focuses on the theoretical and experimental researches of MgF molecular spectroscopy and radiative deflection,making necessary preparations for laser cooling of MgF molecules.We perform the deep researches on the detection and optimization of molecular beam source,the energy level structure of MgF isotopes,the rovibrational and hyperfine spectra,the theoretical calculation,and experimental measurement of Franck-Condon factor,the quasi-cycling transition of laser cooling and the deflection resulting from radiative force.Firstly,we introduced the source system for producing MgF cold molecular beam,the lasers and frequency stabilization used in the experiment,and the molecular detection methods.Based on laser-induced fluorescence(LIF)detection,the measurements of the average forward velocity,distribution of forward velocity and rotational temperature of molecular beam are introduced.According to these properties of molecular beam,the structure of source cell is optimized and the effects of different structural parameters on molecular beam are analyzed.Finally,the cell structure is selected to produce cryogenic MgF beam which meets the experimental requirements.Secondly,the spectra of MgF isotopes are studied theoretically and experimentally.The energy level structures of three isotopes are analyzed and calculated.Their rovibrational spectra of the~2?_1/2-(3~2?_1/2 transition are measured,then the molecular constants are obtained.The hyperfine spectra of low-lying rotational states of three isotopes are measured by LIF detection method while the hyperfine energy structures of(3~2?_1/2 are calculated by effective Hamiltonian method.The peak relative intensity in spectra is obtained by solving the density matrix equation and calculating the number of scattered photons.The experimental spectra are in good agreement with the theoretical spectra.Thirdly,the research on Franck-Condon(FC)factors of the A-X transition of MgF is introduced.The FC factors are calculated by Rydberg-Klein-Rees(RKR)inversion method theoretically and measured by dispersed LIF spectrum are f₀₀=0.972 and f₀₁=0.028,both of which are in good agreement.When one main pump laser and two repump lasers are applied,50,000 scattered photons can be obtained before entering the dark state.To obtain the FC factors and spontaneous emission rate of molecules,an experimental scheme by studying the saturation of laser-induced fluorescence and the associated power broadening of spectral lines is also introduced.The preliminary results are obtained.It is verified that MgF molecule has a highly diagonalized FC factor and is a good candidate molecule suitable for laser cooling.Finally,the optical cycling of MgF is constructed with the~2?_1/2(^?=0,?=1/2,+)-(3~2?⁺_1/2(^?=0,=1,-)transition,and radiative deflection is realized.The transition cycling is closed by adding v₁₀ repump laser,sideband modulation and magnetic field.The feasibility of the optical cycling is proved by the enhancement of LIF signal.The experimental results are consistent with the theoretical prediction by solving the density matrix equation of 4+13 multilevel model.By increasing interaction time between molecules and lasers,deflection resulting from radiative force of MgF molecular beam is observed.Each molecule scatters 200 photons,leading to a deflection of?4.9 mm.The scattering rate is estimated to be 9 MHz.This is an important step for laser cooling of molecules,which lays an experimental foundation for subsequent experiments.