| The interaction of femtosecond laser with gas is a basic and crucial process of the material's response to the light exposure, involving energy, medicine, environment and Many other fields that people concerned. Only can we find out the dynamical process of laser and molecular interaction, can we use the external conditions to regulate the internal reaction in molecular and let it develop in the direction of people's subjective. To explore the process of Ultrafast Intense Laser interacting with gas, the most important factors are yield, energy distribution, angular distribution, velocity distribution and so on. While the velocity map imaging(VMI) technology can use a certain technique to explore these interrelated quantities in the process of laser and gas interaction, the experimental detection process is relatively simple and intuitive, which makes it a main technology method to study on micro dynamics.In international, the German Mark Planck Institute, American Kansas State University has been using the designed instrument to carry out related research and have make important achievements. On the domestic only seldom group has carried out the experiment on their designed VMI spectrometer such as Peking University, shanghai Institute of Optics and Fine Mechanics and Wuhan Institute of Physics and Mathematics(Chinese Academy of Sciences). While with the research direction differently and the research problem variously, we urgently need building our own VMI spectrometer system to carry out the research for atomic and molecular ultrafast dynamics detection. In this paper, the main purpose is to measure the energy spectrum. Combining with the experimental requirements we have designed, optimized and set a VMI spectrometer system. On the completed experimental platform we have calibrated the instrument systems through the measurement of Ar and Xe. The research contents and results are as follows:(1) We theoretically investigated the composition system of VMI, such as parameters settings of the vacuum system, the extraction of supersonic pulsed molecular beam system, the parameter determination of the jet valve and Skimmer, the determination of the flight tube length and etc...We have demonstrated the requirements and set the parameters.(2) We have systematically studied the design of the focusing electrode plate device and calculated the proportions and parameter ranges that we should select to design our electrode plate, which includes holes diameter and plate spacing.(3) The influence on the VMI voltage settings and ion, electron focusing condition is investigated, explaining how to achieve the optimum energy detection through making reasonable voltage on the electron plate. We also have studied the device resolution properties and finally obtained the device resolution curve is as a parabola and that each curve has its own optimal energy detection point in the middle. The optimal resolution is 0.35% and as for the worst case resolution also reached 4.4% of the order of magnitude, the optimal energy probe points are 32 e V and 45 e V when we detected electron energies of 0-100 e V. When detecting the ion with one charge and 0-100 e V energy distribution the best resolution is 0.5% with the worst resolution also reaches 2.8% level for the results.(4) Because of the effect of interaction regions on the resolution performance for VMI has' t investigated, we refine the regional model, starting from the influence of different direct region for the VMI resolution performance, and come to the conclusion: along the laser beam direction the length changes have a bigger influence on the resolution when detecting low energy electron, and for the detection of high energy electron this impact is relatively small; while the length changes along the X axis affects the resolution in the opposite way: the impact on detecting the resolution of the low energy electron is lower than on the impact on detecting high-energy electrons. This discovery is of great significance to direct the experiment and obtain high-resolution image.(5) A device of VMI spectrometer to improve its resolution performance is invented. From investigation we find another important reason to restrict the resolution standard: the particle focal plan in the device is not in a vertical plane. A new devices has added to the no-electric filed. This device can compensate the flight distance in z direction at certain extent and make the different velocity of ions focus on a vertical plan eventually, thus improving the speed of the resolution of VMI. The resolution levels of the improved device are below 0.7% in detecting the initial photoelectrons energy of 0-100 e V, which is far better than the common device with a 4% resolution level.(6) Testing on the real detection platform based on our own design, we have found that when the laser pulse energy is 1 m J and it goes through a lens with a focal length of 40 cm Ar+is only ionized form Argon without2Ar+, but it can ionize Xenon with Xe+and2Xe+with the ratio of 1.7:1. Argon is ionized easier than xenon by the laser. We have also clearly detected the two-dimensional energy spectrum of Xeon when it reacts with the laser. This has verified the reliability of the device design. |
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