Optimal Design Of The Electrostatic Hexapole Device

Electrostatic hexapole technique was developed by using Stark effect of polarmolecules in the the non-uniform gradient electrostatic field. Electrostatic hexapoletechnology was first used in actual scientific experiments in the1960s. Electrostatichexapole devices were first used in selecting different rotational states of symmetrictop molecules, which can be oriented in electrostatic field, and subsequently it wasfound that the electrostatic hexapole device can be applied in focusing some linearand asymmetric top molecules. Therefore the scope of application of the electrostatichexapole device was expanded.This paper studied the optimization of the existing experimental electrostatichexapole setup. By using the existing experimental electrostatic hexapole setup, thefocusing curves of the symmetric top molecules have been obtained. We found thatthe curve of each rotational substantial overlapped, which made it difficult todistinguish the position of each rotational state and single rotational state can not befully selected. Through analyzing the results, we found that, first, the rotationaltemperature of molecular beam was not cooled enough, which caused more rotationalstates participating in the focusing and the distribution weight of rotational statescloser, so the focusing peaks overlapped more. Second, it is due to the geometricalparameters of the existing hexapole device, for example, the radius of the collimatorsbefore and after the end of the hexapole which limit the width of the molecular beam.Based on these analyses, we studied the optimal design of the existing hexapoledevice.In the first chapter of this thesis, we introduced of the history of the developmentof electrostatic hexapole technique and its application value in the field of physics andchemistry. In the second chapter we introduced supersonic molecular beam techniquewhich is important in electrostatic hexapole technique and also we introduced themolecular beam simulation software. Then, in the third chapter, we introduced the principle of the Stark effect and the its application in the focusing of symmetric topmolecules in hexapole device, and then we introduced the factors influencing thehexapole focusing curve and the focusing curve simulation software. In the fourthchapter, we proposed the optimal design of the existing experimental electrostatichexapole setup. We introduced the structure of the existing experimental system andthe related parameters of the hexapole device. By using DS2G software which applysDSMC method, we simulated the molecular beam before the hexapole. Through thesimulation we obtained the number density, axial velocity, radial velocity, andtranslational temperature of the molecular beam. Then we simulated the situation ofadding a skimmer between the existing nozzle and skimmer, and found that we couldobtain the molecular beam of lower translational temperature, which is good for theexperiment. Also, we suggested adding a beamstop between the nozzle and hexapole,which can reduce the width of the focusing curve and make each focusing peak moreseparated, and adding a collimator before the hexapole which can block the moleculesinvoving in the discharging effect. Meanwhile, we analyzed the influence of the thelength of hexapole on the focus curve, we recommended that the hexapole should belonger. In the end, we introduced some methods for cooling the molecular beameffectively.