Theoretical And Experimental Study On Electrical And Optical Velocity Filter

As an experimental technique for generating cold beams of molecules, buffer-gas cooling has been applied on many experiments of cold molecules. This thesis reports the experimentally manipulation of buffer gas cooled polar molecular beams with quadrupole using electrostatic stark effect, by adjusting the different flux of CH3F sample molecules and buffer gas helium, different kind of cold molecular beams can be obtained. When the injected CH3F flux is set to be constant, by increasing the inputted flux of buffer-gas helium gradually, the detected signal of CH3F is increasing then decreasing. A theoretical model for explaining the decreasing of the guided CH3F signal in high buffer-gas density region is proposed. 3D-Monte-Carlo simulation is carried out based on this theoretical model. By comparing the experimental and simulated results, we find that this method can be used to measure the ratio between the translational and average rotational collisional cross sections of polar molecules at low temperture. For molecules that have neither a permanent electric dipole moment nor a magnetic dipole one, a scheme to manipulating I2 molecular beams by using red-detuned cavity-enhanced single frequency single mode laser beams are proposed, and both the dynamic velocity-filtering and beam-splitting processes of the guided cold I2 molecular beam are studied by using 3D Monte-Carlo method. MgF free radicals for laser cooling experiment are produced by laser ablation. The mass spectrum and spectral signals of MgF free radicals are detected separately.First of all, the basic principle of manipulation of cold polar molecular beams using electrostatic stark effect is introduced, the spacial distribution of the electric field in the quadrupole guide is simulated by using finite software. The dynamic process of cold polar molecules in the electrostatic field is simulated by using Monte-Carlo method, and the velocity distribution, spacial distribution and guiding effiency of the cold molecular beam can be obtained. After calibrated by two calibrators, the transverse longitudinal velocity distribution of CH3F and guiding effiency for different rotational states are also studied.The experimental setup is described separately, including vacuum system, quadrupole guiding electrodes, pulsed high voltage switch equipment, cryogenic system and quadrupole mass spectrometer molecular beam detection system. In chapter III, the method of obtaining the longitudinal velocity distribution of cold molecular beams by using high voltage switches is introduced particularly. In our early experiment, because of the unreasonable design of the molecule inlet pipe, vacuum system and the cell structure, the initial experimental results obtained by us is different from the international counterparts, then by investigating vast amount of literature, we found the problem, and we did a series of improvements on cryogenic system, vacuum system and the cell. After that, the CH3F molecular signal intensity and longitudinal velocity distribution are obtained, the experimental results are compared with the former one, and the similarities and differences among the experimental results are analyzed and discussed.For study the decreasing translational and rotational temperature of CH3F caused by collisions with 3.5K buffer-gas helium in the cell, theoretical calculation and 3D Monte-Carlo simulation are carried out, The 3D Monte-Carlo simulation process are described in some detail in chapter IV, by comparing the experimental and simulated results, we find that the ratio between the translational and average rotational collisional cross sections of CHaF-He is γ=σt/σt＝36.49±6.15In chapter V, in order to obtain ultracold molecular beam source, for neutral molecules that have neither electric dipole moment nor magnetic one (such as I2 and CH4 molecules, etc.), a new type of all-optical molecular velocity filter and beam splitter were proposed. The mode of 1064nm red detuned laser in cavities are theoretically calculated based on the theory of ABCD matrix, also based on the current reality of the experimental conditions and the available high power laser that can be used in our scheme, the cavity enhancement factor is theoretically calculated, the current market of high power lasers are also investigated. The spacial intensity distribution of the laser is generated by home-made Labview program, and then import it into C++ program and the dynamic process of molecules in the laser field is simulated by Monte-Carlo simulation. If the intersection angle between the guided oblique beam and the guided straight beam is set to 80 degrees, a cold I2 molecular beam with the full-width at half-maximum longitudinal velocity of lm/s and a translational temperature of 8mK can be generated. When the laser intensity of the two cavities are constant, by adjusting the intersection angle between the two cavities from 80 degrees to 10 degrees, an adjustable splitting ratio from about 1.6% to 98.4% can be obtained. Also, when the intersection angle between the two cavities is constant, by adjusting the laser power difference, a splitting ratio of the two-arm output molecular beams from about 10.3% to 89.7% can be obtained. Our study shows that this new scheme can be used for manipulation of neutral molecules (such as I2 and CH4 molecules, etc.), and it is obvious that the ultracold I2 molecular beam is very useful in the field of optical frequency standard.Laser ablation is a common method for producing beams of molecule from solid processor, in chapter VI, the MgF free radicals, which can be used for laser cooling, are produced by laser ablation of MgF2 solid processor. Mass spectrum signal of MgF is detected by quadrupole mass spectrometer, and fluorescence generated by spontaneous emission of MgF is also detected using photomultiplier tube. If the focus spot of the ablation laser is not moving, the output of molecular beams will decrease quickly in a few minutes, only by changing the laser spot focused on the solid processor constantly if we wanted to obtain a stable signal for a long time. In our experiment, two stepper motors are used for adjusting the knobs on the reflecting mirror, and long-duration pulsed molecular beams of MgF free radicals are produced.In order to realize laser cooling of MgF, the experimental scheme for generating MgF free radicals is introduced in chapter VI, including laser ablation of MgF2 solid processor, the experimental scheme, mass spectrum and spectral results of MgF free radicals and so on. Conclusions and outlook are presented in the final chapter.