Controlling Photo-and Magneto-association Process Of Ultracold Atoms Using External Electric And Magnetic Fields

The formation and application of ultracold molecules is a hot topic in the field of atomic and molecular physics.This dissertation investigates theoretically the process of photo-and magneto-association in the preparation of ultracold molecules and their quantum modulation via external electric and magnetic fields.The main works are summarized as follows.(1)We propose a scheme to prepare ultracold Cs2 molecules in the lowest vibrational level of the ground electronic state by using four laser pulses and study the photoassociation efficiency using time-dependent quantum wave packet method.Firstly,we use a negative chirped laser pulse to produce the unstable photoassociated molecules in the excited elec-tronic state.Then a dump pulse is utilized to transfer the population of molecules in the excited state into a high vibrational level ?" = 18 of the ground electronic state.By taking advantage of the double-well structure of the excited state potential function,we transfer the population to the ?" = 0 level of the ground state via a stimulated Raman adiabatic passage and form the stable molecules in the ground state.By using the optimized laser parameters,a high population transfer efficiency up to 96.2%is obtained from ?" = 18 to ?" = 0 in the ground electronic state.(2)We study the one-photon photoassociation process of the 7Li and 133Cs atoms con-trolled by electric and magnetic fields via inducing a Feshbach resonance.We calculate the distribution of probability density of LiCs molecules in the magnetic field and find the dra-matic enhancement of probability density in the short internuclear distance near the Feshbach resonance.By adding an electric field,we find that the electric field can induce the coupling of different partial wave components,and can change the positions and widths of the Fesh-bach resonances.The numerical results show that the combination of electric and magnetic fields can induce the scattering resonances and significantly increase the photoassociation rate.Moreover,we could directly produce ultracold molecules in the lowest rotational state via one-photon emission photoassociation due to the coupling of different partial waves.In the presence of magnetic field,only a relatively weak electric field is required to induce the scattering resonance for heteronuclear alkali-mental atom pairs.(3)We investigate the photoassociation process of ultracold 40K and 87Rb atoms in the vicinity of Feshbach resonance using short pulses.The total probability density of the coupled hyperfine states can be enhanced at short interatomic separations via magnetically tuned Feshbach resonances.By giving the wave functions of different scattering hyperfine channels,we find that the probability density distribution in the interatomic separation varies with different Feshbach resonances.The route of photoassociation process and the choice of the intermediate state are chosen appropriately in accordance with the probability density distribution.Taking advantage of the spin-orbit coupling of the excited states,we can transfer the population to the singlet ground state X1?+ from the initial triplet state.By comparing the final population in the vicinity of Feshbach resonances at B0 = 546.80 and 659.34 G,we obtain that a Feshbach resonance corresponding to a deeply bound vibrational level can enhance the photo-and magneto-association efficiency of ultracold atoms.