Study Of Transverse Cooling And Longitudinal Slowing Of Magnesium Monofluoride Molecules

Ultracold molecules exhibit promising prospects for various applications in fields such as quantum chemistry,many-body physics simulations,and precision measurements,among others.To enable these applications,it is essential to prepare ultracold molecular samples,and currently,laser cooling is one of the primary techniques used to achieve this.Nevertheless,the intricate internal state structure of molecules poses a challenge in manipulating the center-of-mass motion and altering their internal state.Even with advanced cooling methods such as constructing dualfrequency effects in magneto-optical trapping or Raman dark states in gray viscosity to enhance cooling efficiency,the number and density of prepared ultracold molecular samples remain significantly lower than those of ultracold atomic samples.In order to prepare a larger quantity of cold molecular samples,laser cooling experiments were conducted on the 24Mg19F molecule,with a comprehensive optimization of beam density,leap cycle closure,longitudinal deceleration efficiency,and trapping efficiency.The research endeavors encompassed the following advancements:(1)Combining a cryogenic beam source system,cooled by buffer gas,with a vacuum stepper motor,resulting in the development of an internationally acclaimed molecular beam source exhibiting a longitudinal center velocity of 203(4)m/s and a beam density of 1.2(1)× 1011/pulse/sr/state.(2)By laser-induced fluorescence signal,which is separated by pumping and detection,the vibronic spectrum of A2Π1/2(v’=1)-X2Σ1/2(v=2)was precisely determined,thereby constructing a more tightly closed leap cycle within the vibronic energy level.This breakthrough effectively expanded the upper limit of photon number scattering from 1400 to 38000,achieving the pioneering transverse cooling of MgF molecules.(3)Through the innovative implementation of the positive and negative sideband sweep technique via electro-optical modulation,we successfully achieved the first-ever efficient longitudinal deceleration of MgF molecules,resulting in a reduction of longitudinal velocity and temperature to 47 m/s and 0.32 K,respectively,while attaining a deceleration efficiency of 8.9%.(4)The dual-frequency effect of MgF magneto-optical trapping was further optimized using an artificial intelligence algorithm,exponentially augmenting the magnitude of the trapping force.Additionally,we delved into the exploration of sub-Doppler cooling induced by the intensity gradient in the hollow optical trap of MgF molecules,ultimately enabling the cooling of molecular samples to a low temperature of 68.7 μK.The theoretical and experimental approaches covered in this thesis,such as beam source optimization,positive and negative sideband sweeping for efficient deceleration,and artificial intelligence algorithm to enhance captivity efficiency,are essential in enhancing the number of supercooled samples of MgF molecules and provide crucial reference for efficiently preparing other supercooled molecules.Furthermore,this research plays a significant role in promoting the early realization of molecular BoseEinstein condensation.