Theoretical And Experimental Research Of Stark Velocity Filtering Of Polar Molecules And Buffer Gas Cooling

This thesis reports some experimental results of cold polar molecular beams produced by Stark velocity filter, which filters cold, slow molecules from either room temperature reservoir, or buffer gas cooled molecular beam.The filtering process exploits Stark potential of polar molecules in spatially inhomogeneous electric fields generated by a sectional, signally or doubly bent quadrupole electrode, which can be used to efficiently select slow molecules from an original molecular beam. We study the working principle of the velocity filter by theoretical calculation and Monte Carlo simulations. Our study shows that the velocity distribution of the guided molecules as well as their total guiding efficiency depend on the Stark shift of the molecular species, the geometry of the quadrupole electrodes, and the applied guiding voltages.In our early experiment, a cold acetonitrile (CH3CN) molecular beam was produced by using the electrostatic filtering and guiding technique with a sectional bent quadrupole electrode. Afterwards, cold molecular beams of some species (such as acetonitrile (CH3CN), fluoromethane (CH3F), trifluoromethane (CHF3), nitromethane (CH3NO2), and chlorobenzene (C6H5C1), etc.) were generated by using electrostatic bent guiding system with two sectional bent quadrupole electrodes in our experiment. Dependences of the temperature and flux of the filtered molecular beam on the guiding voltage as well as the reservoir pressure are investigated in some detail.The basic principle of "buffer gas cooling" and its technique are introduced as follows:buffer gas (He atoms) and sample molecules are directly loaded into a copper cell by two thin thermal-isolated tubes, which are cryogenically cooled by a pulsed tube refrigerator, and then the sample molecules are sympathetically cooled to a temperature well below their freezing point by efficient elastic collisions between buffer gas (He) and the sample molecules. Based on a simple theoretical model with some calculations, we obtain some knowledge on the collision time to reach fully thermalization, and the extraction process in the buffer gas cell as well as the properties of the extracted molecular beam. Our analysis shows that this general technique could be used to cool a wide variety of molecular species.With the combination of the buffer gas cooling technique and the Stark velocity filtering, cold molecular beams of both CHF3 and CH3F are generated experimentally. Dependences of the temperatures of the filtered cold molecular beam on the guiding voltage, buffer gas density as well as the input flux of the molecular species are investigated. Experimental results agree well with their counterparts from Monte-Carlo simulations. By applying pulsed high voltages on the bent quadrupole electrode, pulsed molecular beams with high repetition rates are generated with ourexperimental system as well.We also propose some novel schemes working with our generated cold molecular beams, including surface Stark velocity filter, surface molecular trap as well as surface molecular storage ring. We numerically calculate the spatial electric fields of the configuration of these schemes, and simulate the trajectories of polar molecules moving in the corresponding in electric fields, confirming their feasibility in experiment.