Theoretical Studies On Novel Schemes Of Cooling And Trapping Molecules And The Applications Of Stark Spectroscopy

Recently, cold molecule has been the focused subject for the researchers. Due to its rich internal energy levels, it can be applied to a large of research fields. Such as measuring the fimdamental physical constants precisely, studying the dipole-dipole interaction,and performing quantum computation. However,the complicated internal states of the molecules make them difficult to be cooled by laser directly. As the realization of laser cooling of some certain diatomic molecules,the prospect of study on cold molecules becomes more optimistic. In this thesis,we focus on the electrostatic trapping and laser cooling of diatomic molecules, and the applications of Stark spectroscopy in measuring electric field.First, we propose an optically accessible electrostatic trap for cold molecules.The electric field around the trap center is calculatednd the Monte-Carlo simulations of the dynamic processes for the cold ND3 molecules show that when the initial central velocity of the molecular beam is 8 m/s,andthe loading time is 1.24 ms,a maximum loading efficiency of-14% can be obtained,andthe 3D(three dimensional temperature of the trapped molecules is about 20 mK. Due to the completely opened optical accesses,three pairs of laser beams can be arranged in three mutually orthogonal directions, thus the proposed electrostatic trap can be used to study optical-potential eva porative cooling of cold molecules,even to realize the electro-optical trap if some polar molecules with proper energy levels are found.Then,to solve the problem of the shallow trap depth in x direction of the electrostatic trap described above,we proposed another electric trap with deeper depth as wel l as simpler structure. We can realize both the loading and trapping processes by applying different voltages on the two electrodes without any other additional electrode.Due to the symmetric structure of the trap, by selecting appropriate loadingvoltages, molecular beam with different central velocities slowed by a Stark decelerator can be loaded to the trap efficiently. For example, when Ui=3.75 kV,U2=25.0 kV, molecular beam with central velocity of 10 m/s can be loaded into the proposed trap with a loading efficiency of —67%. While Ui=7.50 kVU2=50.0 kV,molecular beam with 14 m/s can be loaded to the trap, and the loading efficiency is as high as-76%. Besides,we study the adiabatic cooling and the eva porative cooling of the trapped molecules due to the large trap volume, which is 3.6 cm. The simulated data indicate that Ihe temperature of the trapped ND3 molecules can be cooled from23.3 mK to 1.47 mK by reducing the trapping voltages applied to the electrodes from50.0kVtol0.0kV.Afterwards, we calculate some parameters related to the laser cooling of molecules,including the vibrational energy levels of some diatomic moleculesh evibrational line strength named Franck-Condon factors,the rotational energy levels,the fine and hyperfine structures. According the criteria of laser cooling of molecules^some possible candidates are listed. Furthermore, we calculate the Stark shift as well as Zeeman shift of some diatomic molecules in an applied external field,and studythe dependence of the parities of the energy levels on the electric field strength quantitatively. The calculated results show that as the electric field increases,the mixing of the levels withpositive parities and the negative parities becomes strong.When the electric field is large enough, theparities are mixed completely, and the parity selection rule in electric dipole transition is not valid any longer, as a resultiie forbidden lines occur.Finally,we measure the electric filed by the Stark spectroscopy of molecules.Since the mixing between the opposite parities of the A-doublets in an external electric field,the forbidden transitions in the absence of electric field are allowed.With thisprinciple, we choose CS molecule and excite the R(2)- and R(3)-transition,respectively, and calculate the dependence of the ratio of Q-branch line intensity to P-branch line intensity on the electric field strength. The calculated results are consistent with the experimental data,which proves that our calculation is not only correct, but also more accurate than the results calculated in the reference. And then we focus on the ZrO molecule, and calculate the intensity ratio of Q-branch to P-branch versus the field strength for the different excitations, respectively, and find ZrO molecule is more sensitive than CS. Besides, we propose a novel mothed to measure much weaker electric field strength by the intensity of Q-branch and the photon counting technology, and the limit of the electric field can be measured is about1fiV I cm, which is higher than the experimental results reported in the literature by about six orders of magnitude.