Progress Towards The Magneto-Optical Trapping Of MgF

During the past three decades,studies of cold and ultra-cold atoms have been developed into a very mature field with tremendous achievements.Cold molecules,owing to their rich internal degrees of freedom,large electric dipole moments and the unique chemical characteristics,provide new possibilities for precision measurement,quantum information science,condensed-matter physics,and controlled chemistry.The development of the cold molecule physics has progressed from traditional methods by taming molecules with electric and magnetic fields via the Stark decelerator and the Zeeman decelerator to the laser cooling technique.Direct laser cooling,as a relatively new way to achieve ultracold molecules,has indeed progressed rapidly.This thesis focuses on the theoretical and experimental studies on laser cooling and magneto-optical trapping(MOT)of magnesium fluoride(MgF).After creating the MgF radicals,another problem we faced is to optimize the molecular beam and measure the characterstics of buffer-gas-cooled molecules.Then,we also need to obtain the exact frequencies of cycling for laser cooling,along with the construction of quasi-closed transitions.Finally,the feasibility of MgF magneto-optical trap and the relevant selection of polarization and detuning are discussed.Firstly,we introduced the mechanism for laser cooling of atoms,including the doppler and sub-doppler cooling,followed by the scheme of constructing the quasi-closed cycling of MgF molecule based on the high diagonal Franck-Condon factors of its vibrational levels and the selection rule of the rotational states.Secondly,we describe the structures of our cryogenic apparatus and the creation of MgF molecule by chemical reaction of Mg with SF6.The parameters,which would affect the molecular beam source,were tested and verified to ensure a better performance of signal.We also compare the characteristics of two various structures of cell in preparing MgF.Using the rectangular cell,we measured the collision cross section between MgF and helium.Then,based on the optimized signal,the high-resolution P,Q,and R branches in the electronic transition of the X2?+ to A2?1/2 are measured unambiguously by in-cell absorption spectra.For the first time,we show that the A2? state of MgF is actually a normal state,not an inverted one.The laser cooling relevant transitions X2??(v= 0,1,N = 1)?A2 ?1/2(v'= 0,J' 1/2)are also identified,along with the hyperfine structure of the X2E+(v 0,1,N = 1)states.We also give a set of new physical constants related to MgF by fitting the measured spectra.In Chapter IV,we demonstrate a new approach with fabrication of anti-reflective coating to substantially reduce the scattering light in an utra-high vacuum during laser induced fluorescence(LIF)detection.To do so,the surface of the vacuum chamber in the detection region was blackened and coated with the special solar heat absorbing nanomaterials.We demonstrate that more than 97.5%of the stray light in the chamber spanning from near infrared to ultraviolet can be absorbed which effectively improves the signal to noise(S/N)ratio.With this technique,the LIF signal from the cold MgF molecules has been observed with an S/N ratio of?4 times better than without that.Then on the basis of this,we verified the influence on LIF signal from sidebands,magnetic field,and the X2??(v = 1,N = 1)?A2?1/2(v' = 0,J' = 1/2)transition laser,respectively.With the above measured two cycling laser frequencies,we also verified the transfer of vibrational population between v=0 and v =1 states.Finally,we present a theoretical study of MOT force exerted on MgF with three-dimensional rate equations,in which we have considered the complex vibrational and rotational levels and the effects of small internal splittings and degeneracies,including fine and hyperfine structures and the magnetic quantum numbers.We investigate the feasibility of MOT for MgF with a very small excited-state g factor(ge=-0.0002)and a large radiative decay rate(r = 2 ?× 22 MHz)for the electronic transition of X2?+ to A2?1/2 states.We also optimize the MOT with reference to the three-,four-,and more-frequency component models with various polarization configurations and detunings.By applying the dual-frequency arrangement to more than one hyperfine level,we suggest a configuration of the(3 + 1)-frequency components for achieving the MOT of MgF.